Metazoa

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.