METAZOA
ANIMAL MINDS AND THE BIRTH OF CONSCIOUSNESS
Peter Godfrey-Smith

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William Collins

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This eBook first published in Great Britain by William Collins in 2020

First published in the United States by Farrar, Straus and Giroux in 2020 as Metazoa: Animal Life and the Birth of the Mind

Copyright © Peter Godfrey-Smith 2020

Extract from Wodwo by Ted Hughes here reproduced with permission from the publisher, Faber & Faber Ltd

Peter Godfrey-Smith asserts the moral right to be identified as the author of this work.

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Source ISBN: 9780008321192

Ebook Edition © September 2018 ISBN: 9780008321215

Version: 2020-10-20

Dedication

Dedicated to all who lost their lives in the Australian bushfires of 2019–20, and to the people who fought the fires

Epigraph

And let me in this place movingly admonish you, ye ship-owners of Nantucket! Beware of enlisting in your vigilant fisheries any lad with lean brow and hollow eye; given to unseasonable meditativeness … “Why, thou monkey,” said a harpooneer to one of these lads, “we’ve been cruising now hard upon three years, and thou hast not raised a whale yet. Whales are scarce as hen’s teeth whenever thou art up here.” Perhaps they were; or perhaps there might have been shoals of them in the far horizon; but lulled into such an opium-like listlessness of vacant, unconscious reverie is this absent-minded youth by the blending cadence of waves with thoughts, that at last he loses his identity; takes the mystic ocean at his feet for the visible image of that deep, blue, bottomless soul, pervading mankind and nature; and every strange, half-seen, gliding, beautiful thing that eludes him; every dimly-discovered, uprising fin of some undiscernible form, seems to him the embodiment of those elusive thoughts that only people the soul by continually flitting through it. In this enchanted mood, thy spirit ebbs away to whence it came; becomes diffused through time and space; like Wickliff’s sprinkled Pantheistic ashes, forming at last a part of every shore the round globe over.

—HERMAN MELVILLE, Moby-Dick; or, The Whale

CONTENTS

COVER

TITLE PAGE

COPYRIGHT

DEDICATION

EPIGRAPH

1. PROTOZOA

Down the Steps * Matter, Life, and Mind * The Gap

2. THE GLASS SPONGE

Towers * Cell and Storm * The Taming of Charge * Metazoa * Light Through Glass

3. THE ASCENT OF SOFT CORAL

Ascent * In Search of the First Animal Actions * The Animal Path * Avalon to Nama * Traction

4. THE ONE-ARMED SHRIMP

Maestro * The Cambrian * Animal Sensing * An Inquisitive Crab * Another Path * The Decorator * Goodbye

5. THE ORIGIN OF SUBJECTS

Subjects, Agents, Selves * Qualia and Other Puzzles * Beyond the Senses * Night Dive

6. THE OCTOPUS

Rampage * When Cephalopods Ruled * Lines of Control * Octopus Watching * Octopus and Shark * Integration and Experience * Down Among the Stars

7. KINGFISH

Power * History of Fishes * Swim * The Presence of Water * Other Fish * Rhythms and Fields * River Riven

8. ON LAND

Hothouse * Leaders Once Again * Sense, Pain, Emotion * Varieties * Plant Life

9. FINS, LEGS, WINGS

Difficult Times * Our Branch of the Tree * The Roles of Land and Sea

10. PUT TOGETHER BY DEGREES

1993 * Elsewhere * By Degrees * Consequences * The Shape of Mind

PICTURE SECTION

FOOTNOTES

NOTES

INDEX

ACKNOWLEDGMENTS

ABOUT THIS BOOK

ABOUT THE AUTHOR

ALSO BY PETER GODFREY-SMITH

ABOUT THE PUBLISHER

PROTOZOA
Down the Steps

You walk ten steps down on a stairway shaped from breakwater rocks straight into the water, which is flat and still, right at the top of the tide. Sound recedes with gravity and light fades to soft green as you dip beneath the surface. All you can hear is your breathing.

Soon you are in a sponge garden, in a jumble of shapes and colors. Some of the sponges have the form of bulbs or fans, growing upward from the seafloor. Others spread sideways over whatever they find, in an irregular encompassing layer. Amid the sponges are what look like ferns and flowers, and also ascidians (with a silent “c”), pale pink spout-like structures with enamel patterns inside. The spouts resemble the downward-curved air funnels on the decks of ships, though these spouts face in every direction. They are covered by all manner of tangled life, often so encrusted that they appear to be part of the physical landscape in which things live rather than organisms in their own right.

But the ascidians make small shifts, as if asleep and half sensing you as you pass. Occasionally, and always startling me a little, an ascidian body half-collapses in place and visibly expels the water held inside the animal, as if with a shrug and sigh. The landscape comes to life and makes its own comment as you go by.

Among the ascidians are anemones and soft corals. Some corals take the form of a cluster of tiny hands. Each hand has the regularity of a flower, but a flower that grasps at the water around it. They clench and slowly open again.

You are swimming through something like a forest, surrounded by life. But in a forest, most of what you encounter is the product of a different evolutionary path: the plant path. In the sponge garden, most of what you see are animals. Most of those animals (all except the sponges themselves) have nervous systems, electrified threads that stretch through the body. These bodies shift and sneeze, reach and hesitate. Some react abruptly as you arrive. Serpulid worms look like tufts of orange feather fixed to the reef, but the feathers are lined with eyes, and they vanish if you come too close. One can imagine being in a green forest, and finding the trees sneezing and coughing, reaching out hands, glimpsing you with invisible eyes.

This slow swim out from shore is showing you remnants and relatives of early forms of animal action. You are not swimming into the past—the sponge, ascidian, and coral are all present-day animals, products of the same span of evolutionary time that produced humans. You are not among ancestors but far-removed cousins, distant living kin. The garden around you is made of the topmost branches of a single family tree.

Farther out and under a ledge is a tangle of feelers and claws: a banded shrimp. Its body, partly transparent, is just a few inches long, but antennae and other appendages extend its presence at least three times as far. This animal is the first I’ve mentioned that might see you as an object, rather than responding to washes of light and looming masses. Then a bit farther still, on top of the reef, an octopus is stretched out like a cat—a very camouflaged cat—with several arms extended and others curled. This animal watches you, too, more overtly than the shrimp, raising its head in attention as you pass.

Matter, Life, and Mind

Something was dredged from the depths of the North Atlantic by HMS Cyclops in 1857. The sample looked like seafloor mud. It was preserved in alcohol and sent to the biologist T. H. Huxley.[1]

The sample was sent to Huxley not because it seemed especially unusual, but because of an interest, both scientific and practical, in seafloors at the time. The practical interest stemmed from the project of laying deep-sea telegraph cables. The first cable to span and send a message across the Atlantic was completed in 1858, though it lasted only three weeks, when the insulation failed and the signal-carrying current leaked away into the sea.

Huxley looked at the mud, noted some single-celled organisms and puzzling round bodies, and stored the sample away for about ten years.

He returned to it then with a better microscope. This time he saw discs and spheres of unknown origin, and also a slime-like substance, a “transparent gelatinous matter,” surrounding them. Huxley suggested that he had found a new kind of organism, of an exceptionally simple form. His cautious interpretation was that the discs and spheres were hard parts produced by the jelly-like matter itself, which was alive. Huxley named the new organism after Ernst Haeckel, a German biologist, illustrator, and philosopher. The new form of life was to be called Bathybius Haeckelii.

Haeckel was delighted with both the discovery and christening. He had been arguing that something like this must exist. Haeckel, like Huxley, was entirely convinced by Darwin’s theory of evolution, unveiled in On the Origin of Species in 1859. Huxley and Haeckel were the leading advocates of Darwinism in their respective countries, England and Germany. Both were also eager to press on to questions that Darwin had been reluctant, beyond a few brief passages, to speculate about: the origin of life and the beginning of the evolutionary process. Did life arise just once on Earth, or several times? Haeckel was convinced that the spontaneous generation of life from inanimate materials was possible, and might be going on continually. He embraced Bathybius as a fundamental form of life, one that might cover large tracts of the deep seafloor; he saw it as a bridge or link between the realm of life and the realm of dead, inorganic matter.

The traditional conception of how life is organized, a picture in place since the ancient Greeks, recognized just two kinds of living things: animals and plants. Everything alive had to fall on one side or the other. When the Swedish botanist Carl Linnaeus devised a new scheme of classification in the eighteenth century, he installed plant and animal kingdoms alongside a third, inanimate realm, the “kingdom of rocks,” or Lapides. This three-way distinction is still seen in the familiar question, “animal, vegetable, or mineral?”

By the time of Linnaeus, microscopic organisms had been observed, perhaps first in the 1670s by the Dutch draper Antonie van Leeuwenhoek, who made the most powerful of the early microscopes. Linnaeus included a fair number of tiny, microscopically observed organisms in his classification of beings, putting them in the category of “worms.” (He concluded the tenth edition of his Systema Naturae, the edition that began the classification of animals as well as plants, with a group he called Monas: “body a mere point.”)

As biology progressed, puzzle cases began to appear, especially at the microscopic scale. The tendency was to try to put them with either plants (algae) or animals (protozoa), on one side of the boundary or the other. But it was often hard to tell where some new creature belonged, and natural to feel that the standard classification was under strain.

In 1860, the British naturalist John Hogg argued that the sensible thing to do was to cease the shoehorning and add a fourth kingdom for the small organisms, increasingly recognized as single-celled, that are neither plants nor animals. These he called Protoctista, and he placed them in a Regnum Primigenum, or “primeval kingdom,” that accompanied animals, plants, and minerals. (Hogg’s term, Protoctista, was later shortened by Haeckel to the more modern Protista.) As Hogg saw it, the boundaries between the different living realms were vague, but the boundary between the mineral kingdom and the living was sharp.

The wrangling of categories I’ve described has so far been concerned with life, not with the mind. But life and the mind have long seemed linked somehow, even if their perceived relationship has not been stable. In the framework of Aristotle, developed over two millennia earlier, soul unifies the living and the mental. Soul, for Aristotle, is a kind of inner form that directs bodily activities, and it exists in different levels or grades in different living things. Plants take in nutrients to keep themselves alive—that shows a kind of soul. Animals do this and can also sense their surroundings and respond—that is another kind of soul. Humans can reason, in addition to the other two capacities, and so have a third kind. For Aristotle, even inanimate objects that lack souls also often behave in accordance with purposes or goals, tending toward their natural place.

The overthrow of Aristotle’s picture in the seventeenth century’s “Scientific Revolution” included a redrawing of these relationships. This involved a hardened conception of the physical—the assertion of a mechanical, push-pull view of matter with little or no role for purpose—and a lifting or etherealization of the soul. The soul, integral to all living nature in Aristotle, became a more rarified, intellectual affair. Souls may also be saved by divine will, permitting a kind of eternal life.

For René Descartes, an especially influential figure in this period, there is a sharp divide between the physical and the mental, and we humans are a combination of both; we are physical and mental beings. We succeed in being both because the two realms make contact in a small organ in our brains. This is Descartes’s “dualism.” Other animals, for Descartes, lack souls and are purely mechanical—a dog is without feeling, no matter what is done to it. The souls that make humans special are no longer present, even in faint forms, in animals and plants.

In the nineteenth century, the time of Darwin, Haeckel, and Huxley, advances in biology and other sciences made dualism of Descartes’s kind look less and less viable. Darwin’s work suggested a picture in which the divide between humans and other animals is not so sharp. Different forms of life along with different mental powers might arise through gradual processes of evolution, especially by adaptation to circumstances and the branchings that originate species. This should suffice to explain both bodies and minds—if you can get things started.

That was a big if. Haeckel, Huxley, and others approached this part of the problem as follows. They thought there must be a stuff, present in living things, that enables both life and the beginnings of a mind. This stuff would be physical, not supernatural, but quite unlike ordinary matter. If we could isolate it, you could pick up a spoonful of it, and in your spoon it would still be the special stuff. They called it “protoplasm.”

This might seem an odd approach, but it was motivated in part by close inspection of cells and simple organisms. When people looked inside cells, it seemed that not enough organization was present—not enough parts were different from other parts—for cells to do what they are evidently able to do. What they saw seemed to be just a substance, transparent and soft. The English physiologist William Benjamin Carpenter, writing in 1862, marveled at what single-celled organisms could achieve: the “vital operations” that one sees “carried on by an elaborate apparatus” in an animal are instead brought about by “a little particle of apparently homogeneous jelly.” The particle of jelly is seen “laying hold of its food without members, swallowing it without a mouth, digesting it without a stomach,” and “moving from place to place without muscles.” This led Huxley, and others, to think that it could not be an intricate organization of ordinary matter that explains living activity, but a different ingredient, one that was inherently alive: “organization is the result of life, not life the result of organization.”

Against that background, Bathybius seemed extraordinarily promising. It appeared to be a pure sample of the stuff of life, stuff that perhaps arises spontaneously all the time, forming an ever-renewing deep-sea organic carpet. Further samples were examined. Bathybius obtained from the Bay of Biscay was described as being capable of movement. Other biologists were not so sure about this alleged primordial life-form, however, and the growing mass of speculation around it. How was Bathybius staying alive down there? What might it eat?

Then came the Challenger expedition—a four-year project organized by the Royal Society of London in the 1870s that took samples from hundreds of deep-sea sites around the world. The aim was the first comprehensive inventory of life in the deepest waters. The chief scientist on the expedition, Charles Wyville Thomson, was willing to work on the Bathybius question although he was wary of it. No fresh samples were found by the Challenger, and two scientists aboard the ship began, amid some tinkering, to suspect that Bathybius was not alive and not even close to it. With a series of experiments, they showed that Bathybius appeared to be nothing more than the product of a chemical reaction between seawater and the alcohol used to preserve samples, including Huxley’s old sample from the HMS Cyclops.

Bathybius was dead. Huxley acknowledged his error immediately. Haeckel, more committed to Bathybius as a missing link, hung on, unfortunately, for nearly ten more years. But the bridge had failed.

Afterward, some people still held out hope for a bridge of roughly the same kind—a special substance that would link life with matter. But in the years that followed, views of that kind subsided. They were replaced in a slow process of discovery, a process that eventually made living activity no longer mysterious. The resulting explanation of life proceeded in exactly the way that Huxley and Haeckel could not countenance: in terms of the hidden organization of ordinary matter.

That matter is not “ordinary” in every sense, as we will see, but it is ordinary in its basic composition. Living systems are made of the same chemical elements that make up the rest of the universe, running according to physical principles that extend also into the inanimate realm. We don’t presently know how life originated, but its origin is no longer a mystery of a kind that might make us believe that some extra substance generates the living world.

This has been the triumph of a materialist view of life—a view that permits no supernatural intrusions. It was also the triumph of a view that sees the physical world itself as unified in its basic constituents. Living activity is not explained in terms of a mysterious ingredient, but in terms of intricate structure on a tiny scale. That scale is almost inconceivable. To pick just one example, ribosomes are important parts of cells—the stations where protein molecules are assembled—with a rather complex structure of their own. But over 100 million ribosomes could fit on the period printed at the end of this sentence.

Life, then, has fallen into place. In the case of the mind, much less is resolved.

The Gap

From the late nineteenth century onward, with Darwin’s revolution gathering steam, it seemed hard to maintain a dualist view of the mind like Descartes’s. Dualism makes some sense within an overall picture that locates humans as a unique and special part of nature, close to God in some way. Then all the rest, alive or dead, can be purely material, while we have an added ingredient. An evolutionary perspective on humanity, one that sees continuities between ourselves and other animals, makes dualism difficult, though not impossible, to maintain. This motivates the attempt to develop a materialist view of the mind, one that explains thought, experience, and feeling in terms of physical and chemical processes. The fact that life itself succumbed to a materialist treatment of this kind is encouraging, but it is not clear how much it really helps; it’s not clear what relationship the success of materialism in biology has to the puzzles of the mind.

Looking again at the history, we can distinguish two alternative paths that continue through to the present. Aristotle, as we saw, recognized several different grades of soul, linking plants, animals, and ourselves. What we call “mind” is viewed as a natural extension, or version, of living activity. Aristotle’s view was not an evolutionary one, but it is not too hard to recast such a picture in evolutionary terms. The evolution of complex life naturally gives rise to the mind, through the growth of purposeful action and sensitivity to the environment.

Descartes, in contrast, saw life as one thing and mind as entirely another. There is no reason, in this second view, to think that progress in understanding life will make much of a difference to problems about the mind.

Over the last century or so, most views in this area have been materialist, but in one respect they have moved close to Descartes. From the mid-twentieth century onward, theorists shifted away from seeing close connections between the nature of life and the mind. This was encouraged by the advance of computers. Computer technology, as it developed from the middle decades of the last century, promised a different bridge between the mental and the physical, a bridge made of logic rather than life. The new mechanization of reasoning and memory—computation—seemed a better way forward. As artificial intelligence (AI) systems developed, some of them started to seem a bit intelligent, but there was little reason to think of them as alive. Animal bodies, it seemed, did not matter very much—they came to appear entirely optional, in fact. Software was the heart of the matter. The brain runs a program, and that program might run on other machines (or things other than machines) as well.

These years also saw a sharpening of the problem of mental and physical. “The mind” as puzzle was replaced by a more specific conundrum. The new view holds that some of the mind can be fairly readily explained in materialist terms, while another aspect is more resistant. The resistant side is subjective experience, or consciousness. Consider memory, for example. We might find that various kinds of animals have memory; they create traces of the past in their brains, and use those traces later, when working out what to do. It is not too hard to imagine how brains might achieve this. Much of that problem is unsolved, but it certainly looks soluble; we should be able to work out how this side of memory works. But in humans, at least, some kinds of memory also feel like something. As Thomas Nagel put it in 1974, there is something it’s like—something it feels like—to have a mind. There is something it feels like to remember a good experience, or a bad one. The “information-processing” side of memory, the ability to store and retrieve useful information, might either be accompanied by this additional feature or not. The hard part of the mind-body problem is explaining that last side of our mental lives, explaining in biological, physical, or computer-based terms how felt experience can exist in the world.

This problem is often still approached through a range of classic options. The main divide has materialist (or “physicalist”) views on one side, and dualism on the other. More radical possibilities are also entertained. Panpsychism holds that all matter, including the matter in objects like tables, has a mental aspect to it. This is not the idea that the entire universe is made of experience—that is idealism. Instead, a panpsychist accepts the physical layout of the world as it appears, but adds that the material that makes up that world always has a side to it that is faintly mind-like. This mind-like side of matter gives rise to experience and consciousness, once some of that matter is organized into brains. Despite its apparent extravagance, panpsychism has serious defenders. Thomas Nagel, who I mentioned above, argues that panpsychism should be kept on the table as an option, because every view has significant problems and panpsychism’s problems are no worse than others’. Ernst Haeckel, in the post-bathybius years, was also attracted to panpsychism. Huxley was attracted to another unorthodox view. He suspected that conscious experience might be an effect of material processes, but never a cause of them. This is an unusual kind of dualism, and it also has defenders today.

Something that is vivid in the wild sweep of these alternative views of the universe, and visible also in more mundane discussions, is a huge diversity in ideas about where minds are to be found. For some, mind is everywhere, or nearly everywhere. For others, it is confined to humans and perhaps a few animals similar to us. One person will look at a paramecium, a single-celled organism, swimming vigorously through a film of water and say: What is going on in that creature is enough for it to have feelings. The paramecium is responsive, and has goals. On a tiny scale, it has experience. Another person will not merely dismiss the paramecium, but will look at a complex animal, like a fish, and say: There is probably no feeling there at all. The fish has a lot of reflexes and instincts, and some fairly complicated brain activity, but all of this activity is going on “in the dark.” If this second person is wrong, why are they wrong? If panpsychism is also wrong and there is no hint of feeling in a grain of sand, why is that wrong? Might things be that way? There often seems to be a kind of arbitrariness in the situation. People can say whatever they like. If I were to guess where most people stand at the moment, when asked which living things around them have experiences, I would conjecture that a common answer is “yes”for mammals and birds, “perhaps” for fish and reptiles, and “no” for everything else. But if someone insists on pushing further out (to ants, plants, and paramecia) or pulling further in (mammals only), the discussion rapidly gets a bit untethered. How could we possibly work out who is right?

This sense of arbitrariness is related to something the philosopher Joseph Levine has called “the explanatory gap.” Even if we come to be pretty sure that the mind must have a purely physical basis, with nothing added, we would also want to know why this physical setup gives rise to this kind of experience, rather than something else. Why does it feel like this to have a brain of the particular kind you have, going through the processes it is going through right now? Even if the difficulties faced by other views convince us that materialism has to be true, it’s hard to see how it’s true, how things could be this way.

That is the cluster of problems I want to address in this book. The aim is not to answer Levine’s questions about particular experiences—which activities of the brain are involved in seeing color or feeling pain. That is a task for neuroscience. The aim, instead, is to make sense of why it feels like something to be a material being of the kind that we are. That “we” is intended to be rather broad; my main target is not the intricacies of human consciousness, but experience in general, something that might extend to many other animals. I want to address these questions about experience in a way that reduces the sense of arbitrariness I described above—the feeling that you could say yes to bacteria, no to birds, whatever strikes you.

The approach I take to the mind-body problem is biological, and one that fits into a materialist picture of the world. The word “materialism” to many suggests a hard-headed, tough-minded view: the world is smaller than you thought, less special or less sacred, just atoms bumping into each other. Atoms bumping into each other are indeed quite important, but I do not want to get the story moving with a mood of toughness and restriction in the air. The “physical” or “material” world is more than a world of thudding collisions or dry structure. It is a world of energy and fields and hidden influences. We should be ready for ongoing surprises about what it contains.

The approach taken in this book is a biological materialism, but in many ways the heart of my outlook is a broader position, sometimes called monism. Monism is a commitment to an underlying unity in nature, a unity at the most basic levels. Materialism is one kind of monism, as it is committed to the idea that mental phenomena, including subjective experience, are manifestations of more basic activities described in biology, chemistry, and physics. Idealism, the idea that everything is mental, is another kind of monism—it is a different assertion of unity. (An idealist must explain how what seem to be physical objects and goings-on are really manifestations of mind or spirit.) Yet another way of being a monist is to think that both what we call the “physical” and what we call the “mental” are manifestations of something else that is basic; this view is called neutral monism. Rather than explaining the mental in physical terms or explaining the physical in mental terms, we explain both the physical and mental in terms of something else. That “something else” tends to remain rather mysterious. If I was not a materialist I’d be a neutral monist, but that is an outside possibility for me. The way I will proceed is by starting with life—understood in a materialist way—and trying to show how the evolutionary development of living systems can give rise to minds. I want to close—partially, at least—the explanatory gap between mental and physical.

Before we proceed, however, let’s take a closer look at the mental side of the puzzle, and the words we use to describe it. The side of the mind that Nagel tried to point to by saying “there is something it’s like …” is now often called consciousness. (Nagel himself calls it that.) You are conscious, in this sense, if there is something it feels like to be you. But the term “consciousness” is often misleading here, as it tends to suggest something quite sophisticated. That phrase “something it’s like …” is supposed to include the presence of feelings of any kind. There is something it feels like to be you—or a fish, or a moth—if the vaguest, dimmest washes of sensation are part of your life. The fact that the word “consciousness” suggests more than this tends to cause trouble.

For example, neuroscientists often say that consciousness depends on the cerebral cortex, the folded part at the top of our brains, something found only in mammals and some other vertebrates. Here is a quote from the physician and essayist Oliver Sacks, talking about a patient who had, as a result of a brain infection, lost all ability to hold new events in memory. Sacks asked: “What is the relationship of action patterns and procedural memories, which are associated with relatively primitive portions of the nervous system, to consciousness and sensibility, which depend on the cerebral cortex?” Sacks is asking a question here, but also stating an assumption: that consciousness and sensibility depend on the cerebral cortex. Does Sacks mean that if someone or something lacks a cerebral cortex, they will lack consciousness in its here-I-am richness, but might still have some feelings? Or does he think that without a cortex the lights are completely off, and any such being would have no experience at all, even if it could manage some behaviors? Most animals, especially most of the animals in this book, do not have a cerebral cortex. Do they have experience of a different kind from us, or no experience at all?

Some people do think that without a cortex there can be no experience at all. Perhaps we will be pushed to a view like this in the end, but I doubt it. We need to continually avoid falling into the habit of thinking that all forms of experience must be human-like in various ways. When the word “consciousness” is used for the very broad idea of felt experience, it is easy to go astray. But many people do now use the word “consciousness,” or some modification of it (“phenomenal consciousness”), in this very broad manner. I am not going to be fussy about the words, and no terminologies are perfect. In some ways, “sentience” is a good term for the broader concept. We can ask: Which animals are sentient? This is, or might be, different from asking which ones are conscious. But “sentience” is often used for particular kinds of experience—for pleasure and pain and related experiences that include a valuation, good or bad. Those experiences are certainly important, and it probably makes sense to think that they can exist without sophisticated kinds of consciousness. But these may not be the only kinds of basic or simple experience. In a later chapter, I will look at the possibility that sensory and evaluative sides of experience are somewhat distinct—registering what is going on might be distinct from evaluating whether it is good or bad. “Sentience” is not usually used for the sensory side of this distinction.

Another term is the unwieldy “subjective experience.” The term looks redundant (is there another kind of experience?) and it has no easy adjective, like “conscious” or “sentient.” But “subjective experience” points in a good direction, by calling up the idea of a subject. In some ways this book is about the evolution of subjectivity—what subjectivity is and how it came to be. Subjects are the home of experience, where experience lives.

I will also talk sometimes just about the mind, as I think that is what we come to understand through this story: the evolution of the mind and how it fits into the world. I’ll move between terminologies without laying down the law. Our present understanding is not good enough to insist on one language or another.

The project I am trying to advance can be described in a number of different ways, but it is difficult no matter how we look at it. This project is to show that somehow a universe of processes that are not themselves mental, or conscious, can organize themselves in a way that gives rise to felt experience. Somehow, a part of the world’s often-mindless activity folded itself into minds.

Dualism and panpsychism and various other views think that this cannot happen; you can’t make a mind—not wholly, anyway—from something else, from entirely non-mental ingredients. Mind must be present in everything, or it has to be added “on top”—not literally on top, but added to a physical system that would be complete, in principle, without it. I think, instead, that you can—or evolution can—build a mind from something else. Given some arrangements of things that are not themselves mental, a mind comes to exist. Minds are evolutionary products, brought into being by the organization of other, non-mental ingredients in nature. That coming into being is the topic of the book.

I said that mind is an evolutionary product and something built, but I want to prevent a common mistake from arising right away. A materialist view does not claim that the mind is an effect of physical processes in our brains, a consequence or product of them. (Huxley seems to have thought that.) The idea, instead, is that experiences and other mental goings-on are biological, and hence physical, processes of a certain kind. Our minds are arrangements and activities in matter and energy. Those arrangements are evolutionary products; they are slowly brought into being. But those arrangements, once they exist, are not causes of minds; they are minds. Brain processes are not causes of thoughts and experiences; they are thoughts and experiences.

That is the biological materialist project as I see it—showing that such a position makes sense, and is, most likely, how things actually are. The aim of this book is to work as far as I can down this path. I don’t think a solution to the problem will be revealed in a single stroke of the pen, in a move that pulls a rabbit from a hat. It will be more cumulative. As this book moves along, I will develop a positive view, a sketch of a solution that combines roughly three elements in a picture that I think makes sense. But not every question will be answered, and many puzzles will remain. The way I think things will go is vividly expressed in a passage that through years of drafts I had as an epigraph to this book. The passage is by Alexander Grothendieck, a mathematician.

The sea advances insensibly and in silence, nothing seems to happen and nothing is disturbed … But it finally surrounds the stubborn substance, which little by little becomes a peninsula, then an island, then an islet, which itself becomes submerged, as if dissolved by the ocean stretching away as far as the eye can see.

Grothendieck worked on very abstract problems—abstract even by the standards of pure mathematics. The quote describes his approach to problems in his field. A puzzle in front of us seems to resist the usual methods. What we should do in response is build knowledge around it, expecting that as we do this, the puzzle will transform and disappear. The situation becomes reshaped and eventually comprehensible. The image he used for this process is the submerging of an object, a mass, in water.

I have had that image in my mind for a long while now. I don’t think, as some philosophers do, that the puzzles in this area are mere illusions that we can overcome if we just talk a bit differently. New things have to be learned. But as they are learned, the problem itself changes shape and fades.

Grothendieck’s image seemed so apt that I once used it to head the book. But the image has new connotations now, at a time when the melting polar ice of a rapidly warming Earth is leading to the loss of precious Pacific islands. Given these new associations, it seemed wrong to begin the book that way. But Grothendieck’s metaphor does still guide my thinking, and the perspective expressed there guides how the book will work. Metazoa approaches the puzzles of mind and body by exploring the nature of life, the history of animals, and the different ways of being an animal that surround us now. By exploring animal life, we build around the problem and see it transform and subside.

This book is a continuation of a project that began in another, called Other Minds. That book was an exploration of evolution and the mind guided by a particular group of animals: cephalopods, the group that includes the octopus. Other Minds began with encounters with these animals in the water, scuba diving and snorkeling. Encountering them there, in their protean, color-warping complexity, led to an attempt to understand what might be going on inside them. That led, in turn, to a tracing of their evolutionary path, a path taking us to a pivotal event in the history of animals, an ancient fork in the genealogical tree. That fork, over half a billion years ago, led on one branch to the octopus (among others), and on the other branch to us.

Some ideas about minds, bodies, and experience were sketched in that book, guided by the animals I was following. Here, those ideas are developed and augmented. That development comes as a result of a closer look at the philosophical side, an exploration of further branches of the tree, and watery hours spent with more of our animal relatives. Whereas in Other Minds I kept coming back to octopuses, my aim in this book is to move along with many kinds of animals, both closer and further from us on the evolutionary tree. For some of those animals I, too, was a being they could observe and encounter; for others, a presence in less than a dream. Toward the end of the book, we begin to approach nearer relatives, with bodies and minds more like our own. But the historical story is weighted to the earlier evolutionary stages, and its goal is to make sense of how experience came to exist on Earth at all, first in its waters, later on land.

That, then, is the path of the book. We walk—crawl, grow, swim—through the story of animal life from its beginnings, guided by a collection of present-day creatures. We learn from each animal—from its body, how it senses and acts, how it engages with the world. With their aid, we try to discern not just the history, but the different forms of subjectivity around us now. My goal is not encyclopedic, trying to cover every variety of animal. I concentrate on those that mark transitions in the evolution of the mind, especially the stages by which it came to be. Most of these are marine animals, living in the sea. Let’s walk down the steps.

THE GLASS SPONGE
Towers

A sponge garden often begins just below the layers of water that sunlight penetrates well, especially in places where currents flow. There, as light falls away, you may find a landscape of motionless animal bodies. They have the appearence of cups, bulbs, grails, or branching trees. Sometimes they look like hands in thick mittens, as if something huge beneath the seafloor was trying to reach upward with soft, half-made limbs.

While in this shallow zone, look out and imagine a sea much colder, the scene now a blackness with a thin fall of particles from above. On the ocean floor, 3,000 feet below the surface, a pale tower, about a foot tall and cylindrical, sits in a cluster of other towers, each held fast at the bottom and a little broader, partly open, at the top. Within its soft exterior is a lattice of tiny hard parts. The smallest of these are stars, hooks, or slender crosses, with angles skewed to knit into that tower-like form. The towers are held to the seafloor by delicate anchors. The anchors and crosses are made of silicon dioxide, the main constituent of glass.

A sponge, on a temperate reef or a deep ocean moonscape, looks dead and inert, but if you look closely, it is not. It is a silent pump, drawing water through itself. As it does so, it senses and responds. The deep-sea tower, the glass sponge, has a body that also conducts light and charge, like an electric light bulb (“think!”) at the bottom of the sea.

Cell and Storm

The background to the evolution of the mind is life itself—not everything about life, not DNA and its workings, but other features. The start is the cell.

Early life, before animals and plants, was single-celled. Animals and plants are huge collaborations of cells. Before those collaborations arose, cells were probably not entirely solitary, but often lived in colonies and clumps. Still, a cell then was a tiny self of its own.

Cells are bounded, with an inside and outside. The border is a membrane; it partially seals the cell but has channels and ports embedded in it. A continual to-and-fro runs across that border, and inside is a frenzy of activity.

A cell is composed of matter, a collection of molecules. I don’t know exactly what comes into your head when I say “matter,” but the word often brings to mind an inert, ponderous way of being, with weighty objects needing to be pushed into motion. That picture of matter is guided by how things work on dry land and at the scale of midsize objects like tables and chairs. When we think of the material of a cell, we need to think differently.

Inside a cell, events occur on the nanoscale, the scale at which objects are measured in millionths of a millimeter, and the medium in which things happen is one of water. Matter in this environment behaves differently from anything in our midsize, dry-land world. At this scale, activity arises spontaneously, without having to be made to happen. In a phrase due to the biophysicist Peter Hoffmann, within any cell is a “molecular storm,” a ceaseless turmoil of collisions, attractions, and repulsions.

If we imagine a cell full of intricate apparatus, parts with jobs to do, these devices are continually being bombarded by water molecules. An object in a cell has a fast-moving water molecule collide with it about every ten trillionth of a second. That’s not a typo; the scale of events in a cell is almost impossible to think about in an intuitive way. Those collisions are not trivial; each has a force that dwarfs the forces those devices can themselves exert. What the apparatus inside a cell can do is nudge events in one direction rather than another, bringing some coherence to the storm.

The medium of water is important in maintaining this storm. Many objects at this spatial scale would stick together and seize up in a clump if they were out on dry land, but do not seize up in water; instead they’re kept in motion, and this makes the cell a realm of self-generated activity. We often think of “matter” as inactive and inert, I said. The problem cells have to deal with, though, is not getting things to happen, but creating order, instituting some rhyme and reason in the spontaneous flow of events. Matter, in these circumstances, does not sit there doing nothing, but risks doing too much; the problem is getting organization out of chaos.

Nearly all the associations we habitually bring to bear when we think about matter are misleading when we consider life and how it might arise. If life had to evolve on dry land out of table-and-chair-sized ingredients, it could not happen. But life did not have to do this; it evolved in water—perhaps in thin films of water on a surface, but in water—through the emergence of order in a molecular storm.

The origin of life occurred fairly early in the history of Earth, perhaps around 3.8 billion years ago, on a planet now about 4.5 billion years old. The first life may not have been cell-shaped, but there must have been some initial way for a special set of chemical processes to be contained, marked off, and prevented from diffusing away. Then at some stage there were cells, presumably leaky and tenuous at first, but eventually arriving at something like bacteria, cells that consistently maintain their organization and reproduce.

As cells acquired the power to keep themselves going—transforming materials, imparting order, bringing method to madness—a central achievement was gaining control over charge.

The Taming of Charge

The taming of electric charge was a pivotal event in recent human history. In the nineteenth century, electricity went from being a mysterious, often dangerous force—encountered most directly in lightning—to an element in technologies that soon formed the modern world. If you are reading this book under electric light or on a computer, the act of reading is electrically sustained. This modernizing electrical advance was the second of two. Charge was also tamed billions of years before, during early stages of life’s evolution. In cells and organisms, electricity is the means by which much of what happens is done. It is the basis for brain activity—our brains are electrical systems—and also a great deal else.

What is electricity? Even many physicists find this question elusive. Electric charge is a basic feature of matter. Charge can be positive or negative. Objects with the same charge (positive and positive, for example) repel, and those with unlike charges (positive and negative) attract. The stuff of ordinary objects contains both. Any atom is a combination of even smaller particles, some that are positive (protons), others that are negative (electrons), and in most cases, other particles (neutrons) with no charge. Usually, an atom will contain the same number of electrons as it has protons, so the atom itself will have no net charge, as the positive and negative charges within it are exactly balanced.

The electrical tendency to attract and repel is strong. Here is the inimitable Richard Feynman, in his Lectures on Physics.

Matter is a mixture of positive protons and negative electrons which are attracting and repelling with this great force. So perfect is the balance, however, that when you stand near someone else you don’t feel any force at all. If there were even a little bit of unbalance you would know it. If you were standing at arm’s length from someone and each of you had one percent more electrons than protons, the repelling force would be incredible. How great? Enough to lift the Empire State Building? No! To lift Mount Everest? No! The repulsion would be enough to lift a “weight” equal to that of the entire earth!

In the mix of charged parts that comprises ordinary matter, electrons, the negative particles, are on the outside of atoms, while protons (along with neutrons) are on the inside. Electrons on the outside can sometimes be gained or lost, resulting in an ion. An ion is an atom (or sometimes a molecule that combines a few atoms) that has unbalanced its charged parts through such a loss or gain, and hence has an overall charge of its own. When many chemicals dissolve in water, they produce ions that then drift around. Salt water is water with dissolved ions. Any droplet of seawater will contain countless ions, interacting with each other and with the water molecules, attracting and repelling.

An electric current is a movement of charged particles, either positive or negative. In a metal wire, a current takes the form of a movement of electrons, with the rest of each atom making up the wire remaining in place. The electric currents used in technology (lights, motors, computers) mostly work in this way. But a current can also be a movement of whole ions. If some positive or negative ions in water, for example, can be induced to move in a consistent direction, that is an electric current. It does not make a current flow; it is one. Any container of salt water can contain such a current, if you can somehow get an overall pattern of movement of ions of the right kind to occur. In living systems, unlike human inventions, most currents take this form.

Charge is not life-like or mental in itself. It produces much of what happens in the inanimate world as well as the animate. But living activity runs on charge, especially by the corralling, pumping, herding, and unleashing of ions.

A cell’s membrane keeps many things either outside or inside, but it contains channels that selectively let some material through. Many of these are ion channels. Sometimes a channel will passively allow ions to move from one side to the other, perhaps under specific circumstances; in other cases, the cell pumps the ions across the membrane.

Ion channels are shared, with variations, across all kinds of cellular life, including bacteria. The reasons for bacteria to build elaborate ports and passageways for ions are often not entirely clear. Channels may have arisen initially just to enable cells to adjust their overall charge in relation to the outside—tuning as well as taming their charge. Whenever there is traffic across a living system’s boundaries, though, it tends to take on further roles. A flow of ions can function as a minimal form of sensing, for example: suppose contact with a particular external chemical opens a channel and lets in ions. Those charged particles can set new events in the cell in motion.

The next consequence of these ion flows is related to that to-and-fro traffic, but is a larger, more wholesale change to the cell. This next step is excitability. Channels control the flow of charged particles, and these channels can themselves be controlled: they can be opened or closed. This can happen through chemistry, or physical impact, but it can also involve charge itself. Voltage-gated ion channels are channels that open as a response to electrical events that they, the channels, are exposed to. This makes possible a chain reaction; a flow of current creates a greater flow of current, one that spreads over the cell membrane.

This might not seem much of a step, and it has less of an obvious ring of usefulness than the arrangement I described above, where the flow of ions is sensitive to chemicals the cell encounters in its travels. But voltage-gated ion channels are the basis for another innovation, the action potential. This is a moving chain reaction of changes to the membrane of a cell, especially in our brains. Positive ions flow into the cell at one point, affecting ion channels at adjacent points, which open and allow more ions to come in, and so on. A wave of electrical disruption travels along the membrane like a pulse. An action potential is the zap-like event that is described as a brain cell “firing.” That zap happens by means of voltage-gated ion channels.

In a voltage-gated ion channel, a controller of current is affected by charges it is exposed to; the flow of current is electrically controlled. This is the principle of a transistor. At the start of this section I mentioned the nineteenth-century advances that brought electricity into the realm of human technology. Another such advance occurred in the twentieth century, with the invention of the transistor. The silicon chips in computers and smartphones are collections of tiny electrical switches of this kind. The transistor was invented around 1947 at Bell Laboratories in the United States—or invented once then, anyway. The first Bell Lab transistor was an inch or so in size, and it has been continually refined and shrunk since then. The same device was invented billions of years ago in the evolution of bacteria.

If bacteria invented transistors, what were they doing with them? Why did they need to control electricity with electricity? As far as I can tell, no answer to this question is widely agreed on. Bacteria might have been using them as part of the electrochemical upkeep of the cell. They might have been used in the control of swimming. Channels that sense external chemicals may be incidentally sensitive to charge, and bacteria that form colonies in “biofilms” signal from cell to cell using ions. But bacteria don’t have action potentials—the zap-like chain reactions in our brains—and the situation does seem quite odd to me. Several billion years ago, nature invented the fundamental hardware device in computer technology—a complicated and costly device, too—and did so in bacteria, but bacteria do not seem to have been doing much computing with it.

Regardless of why it arose, the voltage-gated ion channel was a landmark in the taming of charge. These channels do not have a single obvious use, I said above. In a sense, neither does a transistor, and in both cases, that is part of their importance. A transistor is a general means for control, a device for making events here affect events there in a reliable, rapid way. The events controlled can be multifarious—whatever might be useful. When they enable action potentials, voltage-gated ion channels also make it possible for a cell’s activity to have a “digital” quality; a neuron either fires or not, yes or no. Not all animals have neurons with these zap-like firings, and nervous systems can work with milder kinds of excitability, but this digital feature is certainly a useful one. It is remarkable that this control device was invented so far back, when most of the uses it has now were not even glints in evolution’s eye.

In the days of ubiquitous computers and AI, it is natural, almost inevitable, to ask about the relationships between living systems and these artifacts. Do organisms and computers do essentially the same thing with different materials? Similarities between the two do arise, often unexpectedly, but it’s also important to recognize dissimilarities. One difference is that much of what a cell does, its main business, is something a computer never has to do. A great deal of the activity in a cell is concerned with maintaining itself, keeping energy coming in, keeping a pattern of activity going despite decay and turnover in materials. Within living systems, the activities that look like the things computers also do—electrical switching and “information processing”—are always embedded within a sea, a mini-ecology, of other chemical processes. In cells, everything that happens takes place in a liquid medium, subject to the vicissitudes of the molecular storm and all the chemical digressions that living systems engage in. When we build a computer, we build something whose operation is more regular and uniform; we build something that will be distracted as little as possible by the undirected ruminations of its chemistry.

This relates to a broader point. Often in these early chapters, I’m trying to describe a tangle of parts and processes inside cells and simple organisms. A natural word to use at many stages is “machinery”—we’re looking at the machinery of sensing, the machinery of excitability. I type the word “machinery” and am never sure whether to delete it. In a broad sense of the term, yes, voltage-dependent ion channels are bits of machinery, and so are nerves and brains. To deny this is to suggest a move toward dualistic (soul + body) or vitalistic (“life force”) views. So, I say to myself, don’t delete the word. But the contrasts between machines and living systems are also important. In cells, the processes of life involve imparting order upon a molecular storm and the imperfect herding of ions. This is nothing like what goes on in any machine we’ve built. We generally build machines to be predictable and restricted in their activities, even if we might then use them to simulate more chaotic goings-on. To describe the intricate materials in cells as “machinery” is right in some ways and wrong in others.

There is one more thing I want to emphasize in this inventory of features of life that were in place before animals. This one has been touched on a few times above, but I now want to put it at center stage for a moment. That feature is traffic, a to-and-fro between living systems and their surroundings. This traffic includes the flow of ions described above, also the taking in of raw materials and elimination of waste. Cells are bounded, but they are not closed to the world. I am emphasizing here the windowed character of cellular life.

This traffic has a metabolic side—a side that involves getting energy and using it to stay alive—and also an informational side. Some incoming influences are important in their own right (as food, for example), while others are important for what they predict and portend, for what they indicate about something else. The metabolic side of this to-and-fro is unavoidable if life is to continue. Living activity itself is a pattern that exists embedded in an energetic flow, one that begins and ends outside of the organism. My colleague Maureen O’Malley expressed this well; combining some chemical jargon with an image from a different source, she said that being alive requires learning how to exist “on a redox rollercoaster, perpetually giving and receiving.” (A redox reaction is one involving a transfer of electrons between two molecules.) A consequence of this, and part of what O’Malley wanted to emphasize, is that living systems are inherently sensitive to changes and events outside. They don’t have the option of being windowless, but are open to the world out of energetic necessity. Once open to their world in this way, they will be affected by what goes on. And once they are affected by those goings-on, evolution will tend to put this sensitivity to use—the organism will often find a way to react to events in a manner that furthers its projects, simple as these might be. All known cellular life, including tiny bacteria, has some ability to sense the world and respond to it. Sensing, in at least the most basic forms, is ancient and everywhere.

Metazoa

Those ideas complete one of the two themes of this chapter. Living cells are physical objects, but unlike any other object we are familiar with. They build membranes to contain and shape storms of activity. They are bounded, but forever dependent on traffic across those boundaries. Self-defining, self-maintaining, cells are selves. The next step in the story takes us to a new kind of unit, a new kind of self: animals.

When we think of animals, we usually first think of animals like us—other mammals, dogs and cats, perhaps birds. But animals extend much further than this. Animals—the Metazoa—form one large branch of the total tree of life, the genealogical network that links all life on Earth. The term “Metazoa” was introduced in the late nineteenth century by Ernst Haeckel, the German biologist of Chapter 1. He contrasted Metazoa, multi-celled animals, with Protozoa, single-celled animals (with “zoa” as in “zoo” and “zoology”). The Greek prefix “meta” originally had meanings like after and beside, then took on the connotation of higher, and now often means about—looking down on. Haeckel probably had in mind some mix of higher and later. But protozoa are no longer considered animals at all, so the “zoa” part of their name becomes misleading. The animals are now just the Metazoa.

Animals are made up of many cells living as a unit; beyond that, they live in a huge variety of ways. They include corals as well as giraffes, wasps that are smaller than some single cells as well as whales at fifty tons. Some look almost entirely like plants. In biology now, the word “animal” refers to any organism found on a particular branch of the genealogical tree, regardless of how it lives or what it looks like. A coral is as much an animal as a wolf is. This is not the only meaningful way the term “animal” might be used, but it is unambiguous and clear, unlike various other uses.

Animals do not form a scale from “lower” to “higher,” though the habit of talking about them in this way seems hard to break. On the genealogical tree, some animals are lower in the sense of earlier, but insects that are alive now are not lower than us; everything alive now is at the top of the tree. So there’s no sense in talk of an evolutionary “scale” or “ladder”; animal life has a different shape from that. Some animals are more complicated than others, in various ways (more parts, wider range of behaviors, more complicated life cycle …), but biology has no room for an overall scale from lower to higher, of the kind that seemed natural before Darwin.

The genealogical network that animals are part of—the “tree of life”—is not always tree-shaped; in many places it is more tangled than that. For simplicity, I will keep referring to it as a tree. This tree links all known life on Earth by relations of ancestry and descent. It is old now, but continues to grow. This growth occurs through evolutionary processes operating over huge spans of time. Populations or species occasionally split into two. Each side then evolves on its own path and acquires its own peculiarities. Extinction is always likely, but any segment—a new species—that doesn’t go extinct may later split again. From a single initial fork, we then have several branches, each with a collection of species rather than just one.

Many years ago, when the tree was younger and smaller, an outgrowth protruded: a new twig. The twig survived, branched repeatedly, and became particularly far-flung and diverse. The organisms on that part of the genealogical tree are the animals. Evolution is open-ended, and there is no telling where future branches might extend, both within and outside the animal part of the tree. But though animals have lived in a great variety of ways, there is a general style of living seen in animals, a way of life invented on the animal branch of the tree.

Animals arose from a particular kind of single-celled organism, larger and more internally complex than bacteria. These cells, eukaryotes, have special devices for handling energy—mitochondria—and an elaborate internal skeleton (the cytoskeleton). This is an internal network of filaments and tubes that can move in relation to each other, enabling the cell to control its shape and motion.

Well before animals arose, the cytoskeleton had initiated a new regime of mobility in single-celled organisms, including active hunting. This apparatus made possible a shift from an existence based primarily on chemical processing, as seen in bacteria, to one based partially on behavior: motion and manipulation. All of these sound like animal characteristics, but we are still talking about single-celled organisms—protists. Some of them have grown large. Members of the genus Chaos, for example, hunt not only bacteria but, in some cases, small invertebrate animals.

Plants are another branch of the genealogical tree, another long-term multicellular experiment, and they too are collections of eukaryotic cells. So are fungi. A recurring theme in evolution is the formation of new and larger units by the collaboration of smaller ones. The eukaryotic cell itself came to exist in this way, through the engulfing of one simpler cell by another. The engulfed cell gave rise to the mitochondria that eukaryotes use as powerhouses.

In the events that gave rise, separately, to animals and plants, another kind of coming-together occurred, this one not an engulfing but a juxtaposition. Suppose a single cell divides, and rather than going their own way, the two daughter cells instead stay stuck together, as the result of a mutation that affects their chemistry. When those cells divide, their daughters will stick together as well. The initial result is just a bigger living object. This object cannot act as a whole, and has no obvious way to reproduce, as opposed to getting bigger. But this is a step toward a new kind of life.

Multi-celled beings of this kind have evolved from one-celled forms repeatedly. On the animal line, this might have happened about 800 million years ago (with a good 100 million years of uncertainty). There are no fossils of the earliest forms, but we can picture the first stages: a ball of cells in the sea, formed by a succession of cells refusing to separate from their sisters.

Where to from there? One tradition of speculation imagines a cup, or a hollow sphere with an opening, as a likely next stage. The ball of cells folds in on itself and becomes hollow. This possibility was also first sketched by Ernst Haeckel.

One reason the cup hypothesis is tempting is that this form is seen in the early stages of individual development—development from egg to adult—in a wide range of animals. The hollow form is a gastrula. It is a mistake to think that something seen early in individual development must also have been there early in evolution (as Haeckel did suppose), but the cup form seems so ancient and widespread that it might be a clue. Haeckel christened this hypothetical animal the “Gastraea.”

The Bathybius affair of Chapter 1 was not Haeckel’s finest hour; the Gastraea was better. It is still a live possibility for a very early animal form. The open sphere might have been the beginnings of a gut; the first animal might have come to exist by forming around its stomach. In that enclosed environment, it could trap food and release digestive enzymes, without having them drift away.

A human gut holds our food. In addition, our guts contain countless living bacteria, from which we benefit greatly as long as things stay in balance. This kind of collaboration is extremely common in animals. It may have also been part of the early stages in animal evolution. That idea was not part of Haeckel’s original version of the view, nor part of most thinking since then. It is a newer idea, informed by the realization that normal animal bodies are homes for large colonies of bacteria that help them process food as well as playing other roles. The recognition of ubiquitous tight associations between our bodies and accompanying microbes has been a significant shift in how biologists think of animal life, and perhaps these associations go very far back. Remember also those engulfing events in the history of cells, the events that produced mitochondria, and also chloroplasts within plants. In those meetings, a metabolic ally was brought inside a cell—or first brought in and then tamed. Here, in comparison, we build a home for collaborating microorganisms without having them enter our cell bodies; instead we build a pen for them. A diverse digestive ecology might be at the beginning of animal life.

This open-sphere idea, with or without collaborating microbes inside, is like a second iteration of the evolution of cells. In the first case, we had the formation of a boundary, with channels across it, forming a unit that controls chemical reactions. Here, we have many cells and they form a hollow sphere, another object with an inside and an outside. Individual cells are each now pieces of the sphere, and they control traffic in and out of this larger unit.

From there—or somewhere—early animal bodies gained more shape. For those trying to work out the next steps, the fossil record is still frustratingly silent as I write these words. But we do have clues in some present-day animals. These clues are easy to misread; the present-day animals are not preserved ancestors, but distant cousins. They have been through as many years of evolution as we have. But some of them might have stayed in a form that resembles old forms, in some ways, or at least indicates something about them.

The animals that contain these clues are a trio: sponges, comb jellies, and placozoans. They are quite different. A sponge, once it has settled, does not move as an adult. It lives fixed in place like a plant. Some sponges also grow very large. Placozoans, in contrast, are tiny, flat, crawling creatures with little definite shape. You need a microscope to see one clearly. Both sponges and placozoans have no nervous system. Comb jellies, as the name suggests, resemble jellyfish, but may be a considerable evolutionary distance from them. They do have nervous systems, and they swim using cilia, tiny hairs along the side of the body that beat in rhythm. So of these clues, one is a piece of motionless undersea furniture, one crawls nervelessly and microscopically, and the third is transparent and swims.

Why are these, among animals, the clues about early forms? First, they are simple animals in various ways. They have few parts and not many kinds of cells. Second, they are genetically very far from us. On the genealogical tree, they are on lines that branched off from our line very early.

It is worth pausing to think about that combination of features—being simple and being far from us. There is no general reason why these should be associated. We might have found, on Earth today, an extremely complicated animal whose evolutionary path turned away from ours very early. All the time we have had to evolve our complex bodies and brains is time that this other animal would have had, too. The best partial example of this combination—complicated, far from us—is the octopus, waiting in a later chapter of this book. But octopuses are not nearly as far from us as sponges and the other animals we are talking about now.

It has often been tempting to tell a story in which some of our earliest animal ancestors looked like a sponge, then later ancestors looked like a jellyfish, and so on. That sequence is not impossible, but it can’t just be read off the evolutionary tree. To do this would be to treat a set of cousins as if they were grandparents—or to treat distant cousins as if they looked more like grandparents than other cousins do. Once we put the point in terms of cousins and grandparents, it is clear that this reasoning would not make sense. However, there can be other reasons that some particular distant cousins might contain clues.

We, in our bodies, possess various evolutionary inventions (brains, hearts, backbones, and so on) that had to come into existence somehow. Sponges and jellyfish live without those inventions, though they do share ancestors with us. So these animals show, first of all, what you might be like if you had to live without them. Further, these animals are not part of evolutionary lines that had these features at some stage but lost them; it is pretty clear they never had them. The inventions they lack include more than mere accessories, too. The left-right body layout that we have is an invention. The intricate folds of tissue that make up our internal organs are an invention. Looking at distant animals who lack these inventions, along with genetic evidence and fossils, we can start to get a sense of what ancestors far below us on the tree might have looked like.

Light Through Glass

Historically, sponges have been seen as the most important of the living clues to very early animals. Sponges have a decent fossil record and are the best known. So without making assumptions about whether the sponges around now look much like any of our ancestors, instead approaching them just as their unusual selves, let’s take a closer look.

Sponges extend widely through the seas, from fingers and trees in temperate waters and huge funnels on tropical reefs to the freezing deep-sea towers that head this chapter. Some encrust themselves on other organisms rather than growing a shape of their own. Their modus vivendi in many cases is that water is drawn into their lower parts, passed upward through the body, and exits at the top. Food, mainly bacteria, is taken from the water as it passes through. A few sponges have a more ambitious diet; the deep sea has some predatory sponges that trap and consume small animals.

A sponge has a body, but one with a big difference from bodies like ours. Most of its cells are in direct contact with the water moving through it. The sponge body is a maze of fine passageways, lined with microbial partners, and the environment suffuses that body.

A sponge has no brain or other nervous system. The larvae (immature forms), which look like tiny fat cigars, can swim, and they have some sensing structures that resemble parts of a nervous system. These sensing mechanisms are facing out to the world, not toward other cells in the same body. The larva settles and the adult grows up in place. Though a sponge has no nervous system, it is not inert. Inside each cell is the storm described earlier in this chapter. A sponge as a whole seems a good deal quieter, but it has something of an active side.

Water moving through the sponge’s body is actively pumped by cells with little tails (flagella). This pumping can be modified or stopped, especially if the water is dirty and the sponge’s channels risk becoming clogged. For a collection of cells with no nervous system, this is not easily achieved. There appear to be specialized sensory cells along the chimneys through which water passes, and these cells signal to others. Given what cells are, it is a significant task for one to influence another. The usual way this is done is with the release of small molecules from one cell, taken up by others. The result is to contract, squeeze down, the canals. This process is slow, but there is little need for it to be fast. In some cases, the sponge expands a little first and then contracts, in a drowsy and invisible “sneeze.”

All this is a reminder of both the opportunites and difficulties of multicellular life. A cell within a sponge is at little risk of being eaten by a larger cell, as it would be if it floated freely in the water. If the cell was merely locked in place with others, though, it could easily starve. In a sponge, a filigree of channels and chimneys keeps most cells in contact with water. But if something then has to be done, it is hard to achieve coordination, especially coordinated motion. All in all, the result looks a fair bit like a plant. Most sponges are entirely content with that, and have been doing what they do for many more years than us. Still, a few tried something different.

The Hexactinellida, or glass sponges, explore in their bodies this chapter’s themes of unity and selfhood in a unique way. A glass sponge is a multicellular organism, like other animals, but as it grows up, most of its cells fuse, losing their boundaries. They don’t abandon boundaries with the outside world, but with neighboring cells. The body eventually forms a single connected net, often described as a “three-dimensional cobweb,” draped over harder elements that support it.

Those harder elements are made of glass. Variously, according to species, they resemble daggers, stars, or snowflakes. These are grouped together to form flowers, vine clusters, and—finally, in combination—the skeleton that holds up the tower. (The drawings of these tiny parts below, by Rebecca Gelernter, reproduce engravings made from specimens collected on the nineteenth-century Challenger expedition, the voyage that doomed Bathybius.)

Like other sponges, the Hexactinellida live in rich associations with other kinds of life. The Venus’s flower basket is a glass sponge usually found with a pair of small shrimp living inside its body. These shrimp enter the sponge when they are very small, and grow up there, never leaving. Eventually they become too big to slip out through any opening. The shrimp eventually reproduce together. They keep the sponge clean, and benefit in turn from both the protection of the sponge’s skeleton and the food in the water the sponge draws through its body.

Glass sponges have no nervous system, but they are not electrically inert, and here the taming of charge takes an unusual form. With that fine living cobweb draped over a skeleton, they, uniquely among sponges, have electrical signaling and a kind of “action potential.” A glass sponge usually moves water through its body in a continual flow. In response to some stimuli, though, such as the plucking of a single glass element from its body, the pumping quickly ceases. This is done by sending an electrical pulse through the body. Electrically, a glass sponge behaves like a single enormous cell; a pulse can run uninterrupted over the whole. A glass sponge achieves coordination not by coordinating signals between cells, but by being, in large part, post-cellular. It is a product of the animal evolutionary path, but one that has partially abandoned the multicellular form of life, living by means of a different kind of unity.

I have been talking about charge, communication, and coordination in these creatures. But this is also an animal made largely of glass—not the charge-conducting cobweb, but the skeleton beneath. A notable feature of glass, of course, is how it handles light. Parts of the skeleton of some glass sponges resemble fiber-optic cable, along which light is conducted and filtered.

Is the sponge doing anything biologically significant with light, or is this conduction just a consequence of employing glass as a building material? Is light used, or just channeled around by happenstance? A wide and rather wonderful range of possibilities has been raised and discussed for different sponge species. Light, which except in the shallowest cases will derive mostly from bioluminescence of one kind or another, might be a further means for communication within the animal. It might also be feeding some of the microorganisms within: tiny diatoms and other creatures that live on light huddle deep inside some sponges, perhaps so deep that they could not get enough light to stay alive if the sponge did not channel rays to them. Light in these cases is even ducted a little way into the seafloor itself. The Venus’s flower basket emits light into the surrounding water, perhaps as a faint lamp in the ocean that attracts the shrimp who come to live inside. These possibilities are unresolved, and some biologists think the light in and around the sponges will be too weak to do much. Either by design or by accident, a glass sponge is a collector and curator of biological light.

THE ASCENT OF SOFT CORAL
Ascent

In a bay north of Sydney, Australia, a short way from the stairs we walked down in Chapter 1, is a sandy underwater plain. The bay is formed where a river winding down from eucalyptus forests inland reaches the Pacific.

The underwater plain experiences a strong tidal flow. Water pushes hard up the river from the ocean as the tide rises, and runs back out to sea as it falls. This flow of water attracts a rich range of animals, but also means that the area can only be visited by scuba divers for a couple of hours each day, around the periods when the water comes to rest at the turn of the tide. There is about an hour at each pause. You dive on top of the high, until the water starts to run.

The turn of the tide comes on fast—a tug and then you are moving. Soon it is not possible to swim against the flow. If you stay too long, you can quickly be pulled out to sea.

A few areas on the plain have fields of purple and white soft coral. This coral is soft and wispy, not jagged and mineralized like tropical “hard” corals. The corals form trees that have cauliflower shapes, though a comparison to cauliflower does them no justice at all. From a distance they look like puffs of white and purple cloud; up close one sees delicate veins and filaments, with cowrie shells and crabs living among the branches.

If you approach on a slight current, perhaps riding the end of the incoming tide, it is like coming in on a glider, silently approaching clouds, and finding that the clouds are growing from the earth on stout pale stems. These trees are not single organisms, but colonies of many small animals, coral polyps. From Chapter 2 we know there is an unending frenzy of microscopic activity within them. But the corals seem to sit motionless while more active animals crawl among their branches.

A few years ago, a local diver and researcher, Tom Davis, who had dived in the bay countless times on the still of the high tide, wondered: What do the soft corals do when no one is looking? Through most of any day the tidal flow is too strong for a diver to be down there with them, but he could put cameras down, and take time-lapse footage of what goes on when the water is fast and no humans are around to see.

Diving with his wife, Nicola, he dropped cameras in various sites where the coral is found. When the cameras were retrieved and they watched the results, they found that as the waters sped up with each change of tide, the soft corals slowly rose, by inflating their bodies, until they were up to three times their still-water size. Very likely, they were stretching out to take in the extra food swept past on the tidal flow. The corals subsided when the waters slowed, and sat lower during the hour or so when humans might return.

In Search of the First Animal Actions

Corals are cnidarians (again with a silent “c,” so “nye-dairians”), the same group of animals that includes jellyfish and anemones. This group diverged from our own evolutionary line at a time still early in the history of animal life. A coral might have last shared an ancestor with you 650 or 700 million years ago. The date is unclear, but it was certainly more recent than the time you last shared an ancestor with a sponge.

A cnidarian body is soft, radial in organization—organized around a disc or cup—and often fringed with tentacles. These can be long streamers or short fingers. Inside the body is muscle, and the electrified threads of a nervous system.

Many cnidarians have a complicated life cycle, proceeding through a series of different bodily forms. These transitions are a bit like metamorphosis, as in caterpillar to butterfly, but not quite the same, as the bodies not only transform but multiply at more than one step; it is as if a caterpillar could make many butterflies, as well as a butterfly many caterpillars. The two adult forms a cnidarian can have are the polyp and the medusa. A polyp is usually fixed to a surface, often shaped like a cup. The medusa is the familiar jellyfish body, swimming in open water with streaming tentacles. Many cnidarians cycle between these two bodies. Corals and anemones only live as polyps.

A little way from the cloud-like trees on the plain, another kind of soft coral lives on a tangled reef. These corals sometimes form shrubs but also cluster in irregular masses. Each polyp looks like a white flower with eight long, finger-like tentacles. Each finger, in turn, has smaller rays extending from it sideways. Those are called pinnules. Fingers on fingers. A coral colony is often partly covered by an orange sponge, growing as a blanket and allowing the flower-like parts of the polyps to reach through.

Having eight of the finger-like tentacles, these animals are called “octocorals.” A mass of them together forms a forest of tiny hands. As you watch, if you are patient, often you will see a slow opening of a polyp like a reach, or a closing of the fingers, a clench.

Occasionally a single tentacle will curl and close while others stay extended. At other times the whole hand will close. On a larger scale, you might come across zones or outcrops where every hand is closed, while a neighboring zone has most hands open. They look like they are reaching and grasping for things, but for a while it was unclear what, if anything, they were catching. John Lewis, a Canadian biologist, looked at thirty octocoral species and found that some were indeed catching not only plankton, but tiny invertebrate animals. This talk of reaching and seizing suggests a momentary action that a person might make, but in general, the process occurs in a graceful slow motion: faster than a plant, but slower than the scale of familiar, busy animal behaviors. In these unfurlings and clenches, there are hints, indicators, echoes of the beginnings and simplest forms of animal action.

Why might this be? First, cnidarians are old animal forms, with body designs that are very possibly part of our past. It is not clear that any modern-day cnidarian—anemone, coral, jellyfish—closely resembles an ancestor of ours, but that radial layout probably does resemble the layout of some of our predecessors.

Second, they can act. Action itself was not invented by cnidarians. Many single-celled organisms can swim, with propeller-like flagella or hair-like cilia. Some can envelop prey and transform their shape. Stirrings into motion can be seen across all the candidates for early animal forms. In the previous chapter we saw the controlled pumping of water through sponges. This is akin to an action and it might be very old.

Evolution is full of gray areas and partial cases—the first of something is rarely clear. Evolution also sometimes sees the rediscovery of something old at a new level or scale. In single-celled life, action exists; there is swimming, seizing, and engulfing. These actions may have been an important spur to the evolution of multicellularity itself. The pre-animal world was a world of single-celled predators and prey, and one option for those wanting to avoid ending up as prey is to become too big to comfortably engulf. Then when cells have come together to make animals, action has to be reinvented at the larger scale. New kinds of coordination are needed. The fitful sponges are on the edge of this rediscovery, a partial case. In cnidarians, action exists again unambiguously, with motion and rearrangements on the grand scale of the animal body.

Those reaches and clenchings are not all that cnidarians can do. Another important capacity, an old action of a different kind, is the firing of their stinging cells, nematocytes. All or just about all cnidarians have stinging cells. Some of the stings, especially on anemones, are so feeble they are hardly noticed by a person. Others, on box jellyfish, can kill us outright. The stingers vary but are similar enough in form that they probably came from a single invention early in the cnidarian part of the tree, then passed down many branches.

In the dramatic and sometimes dangerous cases, a harpoon is coiled inside a cell. The cells with harpoons are surrounded by sensing cells and other controllers in a “battery” (this seems to be an artillery metaphor, and an apt one). The harpoon, when triggered, reaches extraordinary accelerations and high speeds over a tiny distance. In a harpoon release, the behavior itself—the motion executed—is performed by a single cell. That cell has helpers around it, sensors and the like, but no coordination between cells is needed for the act itself. Compare this to the soft coral’s reach. This is not the action of a cell, but a combination of countless contractions by different cells, motions that have to occur together in a concerted way. What I am highlighting here, marking as a special invention, is action that involves coordination across vast scales from a cell’s point of view. The origin of this is what we find echoed in the soft coral’s reach.

Even if these cnidarian behaviors are echoes of early animal actions, why do I choose this one? What about the coordinated swimming of a medusa? The medusa stage is often seen as a later addition to the cnidarian way of life, with the polyp stage evolving earlier. But a more important point is that if you look at a jellyfish swimming and a soft coral reaching, these are in a sense the same motions. Whether swimming or grasping, in a bell or a cup, both motions involve contraction around a radial form. A polyp and medusa can look very different, but in basic layout, a medusa is a polyp upside down. In the medusa, a radial contraction results in a swimming motion; in a polyp, the animal is stationary and the result is a grasp.

If we are looking for the “first actions,” another question is why we should focus on movement, as opposed to the other main thing living organisms do, which is making chemicals. Moving parts of your body and brewing chemicals are both ways of achieving effects that help you make your way. That is true, but the advent of controlled motion on the scale of the animal body was still a landmark. Though action was not invented from scratch by cnidarians, we see in these animals action of a different kind and on a different scale. The bodies that enable these actions were a new kind of object in the world, and a new factor in making things happen.

The Animal Path

The outgrowth of the tree of life that became the animals fairly quickly accumulated a suite of innovations. Perhaps the most important was the nervous system.

Of the animals we have looked at so far, nervous systems are present in cnidarians and comb jellies, but not in sponges or placozoans. Nervous systems evolved early, perhaps just once, perhaps a couple of times. The basis for what nervous systems do is a pair of features that were present long before animals. These are the electrical “excitability” of cells—the capacity for a rapid change in electrical properties, discussed in Chapter 2—and chemical signaling between one cell and another. Nervous systems put those two ancient capacities together. When a cell excites—a sudden shift in its electrical properties—this event is usually confined to that one cell. It is restricted by the boundaries that mark off the cell as a unit. But one thing such a spasm can do is trigger the release of a chemical at the cell’s boundary, a chemical that can be picked up by another nearby cell. This in turn may make the second cell more (or less) likely to go through its own electrical changes. Chemical signaling together with excitability are central to how a nervous system works.

Nervous systems are full of cells that specialize in these sorts of interactions. These cells take a tree-like shape, featuring fine projections that bring one cell into chemical contact with a selected group of others. Nervous systems are usually said to be found only in animals (and most, not all animals), though cells with a combination of excitability and chemical signaling are found in other organisms as well. What makes nervous systems in the full animal sense special is those cells with the branching shape: neurons. These are found nowhere outside of animals. Having such cells changes the way influence works within a body. They enable fast and targeted interactions, in contrast to more diffuse patterns of influence in which a cell sends chemicals out to whoever is listening. A nervous system ties the body together in new ways. Lars Chittka, a biologist who works on bees, has an effective way of illustrating their power. A bee, he says, has a brain that is a cubic millimeter in size. That seems tiny. But, he adds, a single bee neuron can have the complexity of a full-grown oak tree, with respect to its branching. Each neuron can connect to ten thousand others.

The nervous system is a reworking of capacities seen over much of life, but in animals these capacities have been extended and made vastly more powerful. A useful reminder of how much these systems do for us is seen in the fact that “nerve toxins” are the main kind of fast-acting toxin, both in animals like snakes and in human misdeeds—notorious weapons like sarin, VX, and Novichok are nerve toxins. On first hearing about nerve toxins when I was young, I thought: It prevents you from feeling anything? You go numb? You can’t think? But nerve toxins shut down more than that. Death occurs often by asphyxiation or cardiac arrest. Our vulnerability to these chemicals—which are not intrinsically destructive, which do not lay waste to tissue, but instead interfere with tiny sites of interaction between cells—is indicative of how a nervous system ties the animal body together. A way to kill that body is by going after the messengers, hence going after the coordination of parts.

Another feature closely associated with the nervous system in evolutionary terms is muscle. A cnidarian’s actions, unlike the fainter motions of a sponge, are based in muscle. The previous chapter noted the invention of the cytoskeleton, an internal skeleton of mobile filaments inside some single-celled organisms. In animals, the coordination of those internal skeletons across many connected cells generates the innovation of muscle. It enables the coordinated contraction, and relaxation, of massive sheets of cells.

Animals can achieve some action without muscle. In comb jellies, the body is lined with bands of the hair-like cilia seen also in many single-celled organisms. The cilia are lined up vertically in what looks like a comb (hence the animal’s common name). Their motions enable the comb jelly, like its single-celled counterparts, to swim. (Comb jellies do also have muscle, which they use for steering.) Cilia are employed for small-scale motions in many other animals. But all the more extensive motions—the octocoral reach, the medusa’s swim, and other motions to come—are achieved through muscle.

In discussing the innovations that enabled animals to eventually play such an unusual role on Earth, I have been emphasizing new capacities on the side of action. Another animal feature, one I’ve not been talking much about in this chapter, is sensing. Sensing is always present, not just in animals but in all known cellular life. But some of the clues we have suggest that in these early stages in animal evolution, the conspicuous, unprecedented innovation was the creation of action on a new scale. This was the transforming factor.

Cnidarians today have a range of senses, and the same is true of likely ancestral animals at all historical stages. But cnidarians are “skinnier,” so to speak, on the sensory side than they are on the side of action. No eyes at all are found in corals or anemones, and only rather rudimentary eyes in most other cnidarians. (The big exception, box jellyfish, are thought to be a later product.) A polyp’s reach for food, a colony’s ascent and descent, and the firing of stinging cells are all tied to stimuli of various kinds, and cnidarians probably also invented a sense of balance, or a gravity sense. A medusa orients itself in the water by means of organs containing small crystals, called statocysts. The crystals are heavier than water, and when they move in response to a shift in the animal’s position, those motions can be detected. There may be other subtle forms of sensing, too, but in cnidarians, sensing is not what might be called their particular richness, their step into the unknown, the thing they do differently. Instead, the innovation was a new kind of action: large-scale, muscularly controlled motion.

With this transformation of animal life in our sights, let’s think for a moment about the mind-body problem in the background. Ordinary ways of thinking furnish us with a number of concepts that help us get a handle on what minds do. One is the concept of subjectivity. This concept arrives in a complementary pair with another: agency. Subjectivity is a matter of seeming, of for-me-ness. It points toward experience as something that happens to a person. Agency is a matter of doing, trying, initiating. Agency is by-me-ness; it is being a source of action and its effects. It points toward the things a person makes happen. Interestingly, the word “subject” (though not “subjectivity”) also has another set of connotations, in which a subject is a doer or initiator: subject as opposed to object. This is not the last time that these concepts will become entangled.

As everyday concepts, subjectivity and agency gesture toward different aspects of a person or animal, a more sensory side and a more active side. From an evolutionary point of view, however, these are closely tied together. Sensing has its raison d’être in the control of action. Nothing is gained biologically from taking in information that is not put to use. The evolution of the mind includes the coupled evolution of agency and subjectivity. But not everything has to develop in lockstep. There might be, at some stage, a breakthrough in the particular realm of action. A new kind of agency might come into being alongside simpler sensory capacities.

All through this part of the book, I am influenced by the thinking of the Dutch psychologist and philosopher Fred Keijzer, and his emphasis on the shaping of action as a central concern in the early evolution of nervous systems. The discussions in this chapter of the formation of action on a multicellular scale, the size and importance of that achievement, and its relationship to the animal body have all been influenced by him. In the area of the relations between sensing and action in very early animals, Keijzer has an interesting suggestion. He thinks that some new kinds of sensing may come “for free,” or almost for free, as a consequence of the shaping of complex action. Suppose you build an elaborate system for producing some sort of coordinated, choreographed motion. Often, to do this you will need some parts of the system to be sensitive to what other parts of the same system are up to. But then if something external affects the system, especially by touch, this event will automatically be registered to some extent, as it will disrupt the pattern of activity going on among those parts. The internal sensing within the system will—or could easily come to—register that something external has happened. Even if a nervous system were to be looking entirely inward (Keijzer is not suggesting that things are ever like that, but if it were), that system would be responsive to things on the outside. Such a system can’t help seeing outward, to some extent. New and expansive actions carry an expansion of sensitivity along with them.

The appearance of an asymmetry between complex action and simpler sensing at some early stage in animal evolution might be entirely an illusion. Complex sensing can be well hidden. But when thinking about the first forms of experience, or about what exists in animals before experience, it is interesting to imagine an animal whose motions outrun its senses, and ask whether, as Keijzer thinks, sensing would tend to automatically catch up.

Let’s step back from these speculations to the main themes emerging at this stage of the story. All living beings do things. They adjust their activities, and affect what is around them. But in animals, this took a new form. On the animal outgrowth of the tree of life, one thing produced was a multicellular self. In addition, the evolution of animals produced multicellular action, action achieved through sheets of cells that contract, twist, and grasp. Nerves and muscle made this possible; a sponge can do nothing like it. Action of this kind was a transformative invention in evolution; it changed everything.

It changed everything—eventually. When did this transformation begin, and what sort of animal set things rolling? Was it an animal that looked like a cnidarian, or something else, earlier? As we’ll now see, the engine of animal action, this engineer of Earth, had perhaps a fitful start.

Avalon to Nama

In the previous chapter we looked for clues about early forms of animal life, mostly by looking at animals alive now, but far from us. The outer limbs of the animal part of the tree of life, from our point of view, are very unclear. Things get clearer a few steps closer to us. Drawing evolutionary lines with time running up the page, the shape of some of the relationships looks like this:

Nervous systems evolved somewhere below the split on the right that leads toward mammals and cephalopods on one side and cnidarians on the other. They may have evolved twice; this depends on unresolved questions about the tree, indicated by broken lines in the drawing.

All the branchings and evolutionary inventions discussed so far occurred before any fossil record of animals appears. The first period of time from which we have definite fossil evidence of animals is the Ediacaran, which began about 635 million years ago. The slow opening of the curtain on an animal fossil record reveals a scene very different from life around us now.

The setting is the seafloor, sometimes shallow, sometimes deeper, populated by various soft-bodied creatures, some tiny but some up to several feet across. Despite their soft bodies, a number of them left traces. Those traces show enigmatic forms—flower-like designs, whorls and discs, spirals and fractal branchings.

What reason is there to believe they were animals at all? In some cases this is indeed uncertain, and some remains may represent a wholly lost multicellular experiment, or experiments, far from the animals. But in at least some cases, they are animals. This was confirmed in 2018 by a student named Ilya Bobrovskiy, rappelling down a cliff in a remote part of Russia where unusually large and well-preserved remains of a famous Ediacaran organism, Dickinsonia, are found. As Bobrovskiy suspected, the rocks contained not just ordinary fossils, but remains that had been naturally mummified and preserved for over half a billion years. The mummified bodies contain cholesterol, a chemical made only by animals. Dickinsonia was flat, lived almost certainly on the seafloor, and looks like a bath mat, up to a meter long. It has no sign of eyes, limbs, or other familiar animal parts, and that is how it is with Ediacarans. They often have definite body forms—fronds and wheels, three-sided and five-sided—but no legs, fins, or claws, and no sign of complex senses like eyes.

No clear cases of cnidarians or sponges, the animals I’ve been discussing as clues, are known from the Ediacaran, either. But in each case there are possibilities. Some Ediacaran organisms look a fair bit like present-day “sea pens.” These well-named organisms are in the same group as the soft corals we visited at the start of this chapter. But rather than trees, each looks like an old plumed pen with its nib stuck in the seafloor and feathers spreading on a stem above.

Whether any Ediacarans are closely related to sea pens remains controversial, as differences appear once one looks closely. Other Ediacaran organisms have branched fronds that also suggest cnidarian forms, but those similarities, too, may be misleading. A lot of Ediacarans were initially called “jellyfish,” by Reg Sprigg, who first uncovered Ediacaran fossils while inspecting disused mines in South Australia in 1946. Most of those fossils have now been interpreted differently, but up in the water were real jellyfish, most likely, their bodies scrunching into incomprehensibility when they fell.

The Ediacaran in the biological imagination has often seemed a quiet and placid time, a period with little interaction between organisms. There are almost no signs of predation—no half-eaten individuals, no sign of the built-in weapons, offensive and defensive, that animals tend to have now. There are no claws or spines. No signs are known of sexual specialization, either, though it is hard to tell, and no Ediacaran has ever been assigned to one sex or another. Sex was almost certainly present, though it probably coexisted with various kinds of asexual reproduction (as in cnidarians and sponges today). Densities were often high; one can see slabs of rock with a jumble of dozens or hundreds of organisms of several species. But even in these Hieronymous Bosch–like scenes, the animals are not obviously having much to do with each other. There may have been hidden interaction by means of lost soft parts, but much of the familiar machinery of contact between animals seems to be missing.

That placid image of the time is fine as far as it goes. Recent years, though, have seen the emergence of more detail, and the quiet Ediacaran has begun to resolve into something with a little more drama, certainly with transitions and changes.

As it looks now, there were three stages. These were distinguished by a young biologist, Ben Waggoner, about twenty years ago, and have stood up well as more data has come in. The stages have lovely names (thanks to Waggoner, with help from the geography). I say “stages,” but technically each is an “assemblage” (not as lovely a word); an assemblage is a collection of species as fossils, all laid down at roughly the same time.

The first of these assemblages is the Avalon, dating from about 575 million years ago. Even this first stage is fairly late in the Ediacaran as a whole. That period is bounded at its far end, about 635 million years ago, by the easing of an ice age, a massive glaciation that may have covered the whole Earth in ice. Conditions were quiet for a time, another ice age passed, and then the fossils appear soon after that. Oxygen levels may have increased significantly after this second ice age. Through all these early stages, however, one should imagine a low-oxygen world. This would have limited animal activity, to the extent that activity was even an option.

The Avalon assemblage, named after a place in Canada, features what seem to be stationary, plant-like, frond-ish organisms. (In a fortunate etymological confluence, “Avalon” in ancient Welsh means “the island of fruit trees.”) A lot of these creatures look superficially like a single large leaf, or a cluster of leaves, stuck in the seafloor. Inspected closely, each leaf is a sheaf of intricate branching elements.

The Avalon assemblage also has a possible sponge—conical, with the right sort of shape, though not reminiscent of any modern sponge group. Sponges in general are puzzling. Chemical evidence along with genetics suggests that sponges were present, even common, at this time, but fossils so far give us just this one conical candidate and one later possibility, recently discovered, that looks like an old downturned TV antenna, with thin rods coming out from its center.

The Avalon organisms seem to have lived in quite deep seas, too dark for photosynthesis, hundreds and perhaps thousands of meters down. That is today a sparsely populated and challenging zone, but it may have been a cradle for slow but innovative life back then. These creatures might have lived on tiny dissolved particles of organic carbon—their branching-upon-branching design has a “fractal” organization that maximizes surface area, allowing a steady uptake of this organic mist, along with oxygen to burn it.

Next came what looks like a shift. The White Sea assemblage, named for a location in Russia, dates from around 560 million years ago. These fossils show more diverse body plans. There are still no fins or legs, but in a few cases, the body plan and fossil traces strongly suggest that the animal could move.

The creatures of the White Sea assemblage lived on shallow seafloors, not the depths as in Avalon. These are seafloors that were, in a sense, alive. They are sometimes referred to as “microbial mats,” but Mary Droser of the University of California, Riverside, who has become a central figure in work on this stage, calls them “textured organic surfaces.” These surfaces contained more than bacteria and the like, probably including algae-like organisms and small embedded animals. The fossil record preserves the textures themselves—“wavy-crinkly lamination, elephant-skin textures …” A tangle of living and dead organisms at different scales made up a mostly two-dimensional surface, a flatland in the sea.

In this setting, new bodies and lifestyles are seen. There are still stationary organisms, raised up and sea-pen-like, but also a range of flatter forms, oriented to grazing on the mat. Some of these could move. Dickinsonia (with its mummified Russian cholesterol) seemed to graze in one place, then move on, leaving a series of faint whole-body footprints behind. Two others were more active, it seems. Kimberella is seen as a likely relative of molluscs. It looked like a crawling macaron, one able to scrape the mat’s surface with an extendable spade-like part.

There is also an enigma called Helminthoidichnites. This fossil, christened with that unbelievably difficult name (Helmin-thoid-ich-night-ies) in the nineteenth century, had initially been found in less ancient rocks, and interpreted as the tracks of a small burrowing animal—perhaps a worm or crustacean. Similar traces were eventually discovered in Ediacaran rocks, and have now been analyzed in detail by Mary Droser and Jim Gehling, in South Australia, near where the very first Ediacaran fossils were found.

This work has used a new kind of excavation, enabling the undersides of huge rock slabs to be studied as wholes. Scrutinized closely, some slabs show traces of complex patterns of movement. The living animal moved through different layers of the undersea mat, piling up levees as it went. The tracks home in on the bodies of other animals, including Dickinsonia. This is the first fossil evidence of scavenging, of an animal consuming the dead. It is also the first physical trace of targeted motion, movement aimed at a goal that is sensed. Initially the targets were dead bodies, but scavenging has natural transitions to predation, especially when the prey is stationary or slow.

I said that Helminthoidichnites was an enigma. All Ediacaran organisms are enigmatic to some degree, but in this case the enigma is extreme. For a long time, all we had was the trail, and no remains of the animal itself. Just as this book was being finalized, a candidate appeared, a tiny bean-shaped creature that might be the maker of the Helminthoidichnites traces. It was found in South Australia, the original Ediacaran home.

The period of the White Sea fossils, then, is marked by a shift: new bodies, new behavioral capacities, a different environment. A couple of other animals from this period also seem to have been mobile. Spriggina has a body with movement written over its form, superficially resembling a scurrying trilobite. There are no known traces of Spriggina movement, but that is not a surprise, as an animal must burrow or scrape in order to leave a mark. If you just glide over the top of the mat, no trace will come down to us, many millions of years later.

This was a time when oxygen levels continued to rise, slowly and erratically. Perhaps the sequence is something like this: with more oxygen, textured living surfaces developed. They became a resource for grazing, encouraging slow movement along the mat. Grazing leads to a concentration of resources in animal bodies, which then die. This makes the environment more patchy—there is lots of food here, less there. Motion becomes more valuable, as does the ability to follow scents in the sea, to track things down.

The third named stage, after Avalon and White Sea, is called “Nama,” after a site in Namibia, Africa. This is the youngest, leading up to the end of the Ediacaran. Given what had been going on just before, we might expect that the Nama phase would have more of this crawling complexity. But instead, these rocks are quieter. The crawling forms, surprisingly, are gone. The Helminthoidichnites traces are still there, and one interpretation of this period, called “Wormworld,” holds that a profusion of small tunnelers and burrowers were major players in this third stage. But those larger mobile animals that look at least dimly reminscent of molluscs and the like seem to have disappeared. Other than the burrowers, life in Nama moved back to swaying frond-like forms (though with fronds mostly different from before). No one knows why this happened. And the Nama assemblage seems to represent the stage leading up to the Ediacaran’s end.

Three stages of the Ediacaran. Organisms include A: Charnia; B: Thectardis (a sponge?); C: Fractofusus; D: Dickinsonia; E: Arborea; F: Coroncollina (also a sponge?); G: Spriggina; H: Helminthoidichnites; I: Kimberella; J: Swarpuntia; K: Cloudina; and L: Rangea. Charnia and Rangea are some of the organisms that have been compared to sea pens.

How does this fit in with the themes in this chapter, with the attempt to discern clues about the evolution of animal action? We have a stage with stationary plant-like forms—Avalon, in the deeper sea—and then a transition to mobile creatures in shallower seas. Genetic evidence suggests that nervous systems evolved before any of these fossils were laid down, or perhaps during the first stage; the genetic dating is rough. Then there seem to be the beginnings of a regime in which new kinds of sensing and acting come online—White Sea. In Nama, this appears to fade.

If the Avalonians we know of were animals with nervous systems (not counting the possible sponge), what were they doing with them? It is tempting to say they might have been coordinating a reach or clasp, as in soft corals today. But in some cases where these organisms left unusually detailed fossils, no evidence of body openings has been found—unlike a soft coral, there is no mouth to bring food to. Instead, their whole body surfaces may have absorbed food—this, again, would make sense of a body design that maximizes surface area.

The frond-ish Avalonian organisms may not have been animals at all. Even if so, nervous systems probably did exist in some form before the crawling animals seen later in the Ediacaran. The evidence is good that nervous systems evolved in a body with something like a radial design. They might have been hidden in some of the flower-like bodies, but we might also need to look up.

I have been focusing on these seafloor forms because that is where fossil evidence was left. But all sorts of additional life may have been aloft in the water column—soft-bodied swimmers, akin to jellyfish and comb jellies. Some early stages in the evolution of nervous systems may have occurred up there. Filmy early swimmers will often be invisible to paleontology, as their delicate bodies are unlikely to fossilize into visible forms when they die. It has been suggested that early crawling animals might have evolved from a larva (an immature form) of a cnidarian-like animal, a larva that reaches the seafloor and starts to move along it. From there one can envision a sequence from mat-grazing and movement to behavior directed on other animals.

Discussions of the Ediacaran sometimes seem to show an understandable preference for making the story work using just the cast of characters present in fossils, and hence living on the seafloor. I often wonder whether, with respect to the evolution of behavior and interaction between animals, the scene on the ocean floor is just the tip of an iceberg, and a lot of what matters took place up in the water column, with animals that left little or no trace. If that is true, it’s hard to work out how the missing pieces of the puzzle might be filled in. But in evolutionary biology, it’s remarkable how often the apparently unknowable becomes knowable, as a result of a sudden technical advance, a mummified Dickinsonia, or a new theoretical idea.

This talk of a transition from swimming to crawling also brings up a stage that has gone unremarked upon so far, but a development we should look at closely. Peering back into the undersea fog of those times, one of the events that had enormous downstream consequences was the evolution of a new kind of body, called the bilaterian, or bilaterally symmetrical, body. These are bodies with a left-right axis, as well as a top and bottom. Our bodies are bilaterian bodies, along with those of ants, snails, and seahorses. We have arms and legs on each side, also eyes and ears; there is a left-right pairing of many of the body’s parts. Most animals now are bilaterians (that is true however you calculate “most”). So are some of the ancients—Kimberella, Spriggina, and others. Cnidarians like corals and jellyfish are not like this, and neither are comb jellies, sponges, or placozoans.

This body form evolved before the Ediacaran’s White Sea stage. It had to be as early as this, because some form of this design had to be in place before the branchings that sent different bilaterian animals down their various paths, and there are at least a handful of different bilaterians in the White Sea. The last common ancestor of you and a butterfly, also the last common ancestor of you and an octopus, lived at least this long ago.

Traction

The bilaterian body, with its left-right symmetry, was an innovation especially in the realm of action. The bilaterian body is set up to go somewhere. There are no non-bilaterian animals at all on dry land—no crawling or walking jellyfish, no anemones with their fingers out in the air (though there are some that live in the intertidal). Bilaterian bodies seem to have begun on a marine version of the land: the seafloor. They are bodies made for crawling over surfaces, with direction and traction.

The very first bilaterians were probably simpler than the fossilizing White Sea Ediacarans, and possibly small, but we don’t know much more than that. Once again there is a clue living today—another clue that might echo old forms. The clue in this case is flatworms. These are small, simple animals whose name does indeed capture how they look.

How useful a clue are flatworms? Perhaps not very useful. Flatworms today, no matter how simple, have been going about their business for a long while. The flatworm body form seems to have arisen, in some approximation, a couple of distinct times, and it makes sense that it would. Many flatworms today are parasites on other organisms, and parasitism often makes for simplicity. Flatworms may not, then, be especially good models for early bilaterians. But with all this in mind, let’s take a closer look at them, as animals in their own right and as possible echoes of something older.

You can find marine flatworms on reefs and on fields of underwater debris. The ones that are noticeable are not the simplest ones—acoel (pronounced “a-seel”) flatworms, which have been seen as the best clues for an early bilaterian form—but another kind, called polyclad flatworms.

They do look simple. They are about a centimeter long in many cases, though sometimes a good deal larger or smaller. They are often oval-shaped, flat with ruffled edges, and very thin. They look like a bit of tissue paper.

If you pause, though, after a few moments it is clear that they have a lot going on. They move faster than a lot of other animals down there. Some flatworms can swim, but even those that crawl often do so briskly and purposefully. “I might look like a bit of tissue paper, but right now I really have something to do.”

Their bodies achieve a lot with a little. There is no “through gut”—what goes in one end goes out the same end—and no circulatory system. A cyclops-like eye cluster sometimes sits in the middle of the back, though eyes, of a very simple kind, seem to pop up just about anywhere on these animals, including on tiny pinches of the tissue-like body, raised a little above the plane of the rest.

Flatworm sex lives are more complicated than one might expect. They are generally hermaphrodites, and some engage in “penis fencing,” trying to dart sperm into the body of another, while the partner does the same thing. It also comes as a surprise that they are often rather exquisite creatures, with bright colors and patterns. Initially, it is not at all clear why this is; they can’t see each other with their simple eyes. But quite a few flatworms are mimics, especially of other small crawling animals, called nudibranchs.

Nudibranchs (“noodi-branks”) are slugs, hence molluscs. They are closely related to slugs and snails on land, but can be astoundingly beautiful, with an endless range of colors and patterns. They often feed on chemically challenging food such as sponges, making them unpalatable to fish and other predators. The bright colors probably advertise this fact.

Two of the main groups are the dorids and the aeolids. The former are helpfully slug-shaped, from the point of view of a flatworm attempting mimicry, while the latter are named for appendages all over their bodies that look like streamers, continually moving underwater as if caught by a breeze. (In Greek mythology, Aeolus was the keeper of the winds.) Each spring, on a reef near the site of this chapter’s opening, tiny aeolids appear. They are found especially on bryozoans, shrub-like animals made from tangles of strands. Aeolids appear on them, almost impossible to see. They are like tiny jeweled birds, millimeters long, in trees with branches like fine hair.

Near to aeolids on the evolutionary tree is Tritonia. This group of species has some large and well-studied animals, but also an elusive form that lives in this chapter’s soft corals and sponges. They are very small, pearl white, with pointed outgrowths from their bodies like spires. On these extended outgrowths are further outgrowths, spires on spires. They look like they were designed by a miniature architect, a tiny Antoni Gaudí.

Those white Tritonias are hard to see, I said, and this is partly because they blend in with soft coral polyps, with their pearl-white spire-on-spire extensions. It took me a while to realize that this, too, is a plausible example of mimicry. I once saw a Tritonia moving about very slowly near a soft coral colony. For some reason I found myself thinking not of mimicry or camouflage, but of homage. The two had taken on the same shape, and stood near each other in a sort of harmony of form, with the coral reaching and the Tritonia appearing to follow: “My body,” it seemed to say, “pays homage to the first animal actions.”

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