We all distinguish between plants and animals. We understand that plants, in general, are immobile, rooted in the ground; they spread their green leaves to the heavens and feed on sunlight and soil. We understand that animals, in contrast, are mobile, moving from place to place, foraging or hunting for food; they have easily recognized behaviors of various sorts. Plants and animals have evolved along two profoundly different paths (fungi have yet another), and they are wholly different in their forms and modes of life. And yet, Darwin insisted, they were closer than one might think.
Charles Darwin’s last book, published in 1881, was a study of the humble earthworm. His main theme—expressed in the title, The Formation of Vegetable Mould through the Action of Worms—was the immense power of worms, in vast numbers and over millions of years, to till the soil and change the face of the earth. But his opening chapters are devoted more simply to the “habits” of worms.
Worms can distinguish between light and dark, and they generally stay underground, safe from predators, during daylight hours. They have no ears, but if they are deaf to aerial vibration, they are exceedingly sensitive to vibrations conducted through the earth, as might be generated by the footsteps of approaching animals. All of these sensations, Darwin noted, are transmitted to collections of nerve cells (he called them “the cerebral ganglia”) in the worm’s head.
“When a worm is suddenly illuminated,” Darwin wrote, it “dashes like a rabbit into its burrow.” He noted that he was “at first led to look at the action as a reflex one,” but then observed that this behavior could be modified—for instance, when a worm was otherwise engaged, it showed no withdrawal with sudden exposure to light.
For Darwin, the ability to modulate responses indicated “the presence of a mind of some kind.” He also wrote of the “mental qualities” of worms in relation to their plugging up their burrows, noting that “if worms are able to judge…having drawn an object close to the mouths of their burrows, how best to drag it in, they must acquire some notion of its general shape.” This moved him to argue that worms “deserve to be called intelligent, for they then act in nearly the same manner as a man under similar circumstances.”
As a boy, I played with the earthworms in our garden (and later used them in research projects), but my true love was for the seashore, and especially tidal pools, for we nearly always took our summer holidays at the seaside. This early, lyrical feeling for the beauty of simple sea creatures became more scientific under the influence of a biology teacher at school and our annual visits with him to the Marine Station at Millport in southwest Scotland, where we could investigate the immense range of invertebrate animals on the seashores of Cumbrae. I was so excited by these Millport visits that I thought I would like to become a marine biologist myself.
If Darwin’s book on earthworms was a favorite of mine, so too was George John Romanes’s 1885 book Jelly-Fish, Star-Fish, and Sea-Urchins: Being a Research on Primitive Nervous Systems, with its simple, fascinating experiments and beautiful illustrations. For Romanes, Darwin’s young friend and student, the seashore and its fauna were to be passionate and lifelong interests, and his aim above all was to investigate what he regarded as the behavioral manifestations of “mind” in these creatures.
I was charmed by Romanes’s personal style. (His studies of invertebrate minds and nervous systems were most happily pursued, he wrote, in “a laboratory set up upon the sea-beach…a neat little wooden workshop thrown open to the sea-breezes.”) But it was clear that correlating the neural and the behavioral was at the heart of Romanes’s enterprise. He spoke of his work as “comparative psychology,” and saw it as analogous to comparative anatomy.
Louis Agassiz had shown, as early as 1850, that the jellyfish Bougainvillea had a substantial nervous system, and by 1883 Romanes demonstrated its individual nerve cells (there are about a thousand). By simple experiments—cutting certain nerves, making incisions in the bell, or looking at isolated slices of tissue—he showed that jellyfish employed both autonomous, local mechanisms (dependent on nerve “nets”) and centrally coordinated activities through the circular “brain” that ran along the margins of the bell.
By 1883, Romanes was able to include drawings of individual nerve cells and clusters of nerve cells, or ganglia, in his book Mental Evolution in Animals. “Throughout the animal kingdom,” Romanes wrote,
nerve tissue is invariably present in all species whose zoological position is not below that of the Hydrozoa. The lowest animals in which it has hitherto been detected are the Medusae, or jelly-fishes, and from them upwards its occurrence is, as I have said, invariable. Wherever it does occur its fundamental structure is very much the same, so that whether we meet with nerve-tissue in a jelly-fish, an oyster, an insect, a bird, or a man, we have no difficulty in recognizing its structural units as everywhere more or less similar.
At the same time that Romanes was vivisecting jellyfish and starfish in his seaside laboratory, the young Sigmund Freud, already a passionate Darwinian, was working in the lab of Ernst Brücke, a physiologist in Vienna. His special concern was to compare the nerve cells of vertebrates and invertebrates, in particular those of a very primitive vertebrate (Petromyzon, a lamprey) with those of an invertebrate (a crayfish). While it was widely held at the time that the nerve elements in invertebrate nervous systems were radically different from those of vertebrate ones, Freud was able to show and illustrate, in meticulous, beautiful drawings, that the nerve cells in crayfish were basically similar to those of lampreys—or human beings.
And he grasped, as no one had before, that the nerve cell body and its processes—dendrites and axons—constituted the basic building blocks and the signaling units of the nervous system. Eric Kandel, in his book In Search of Memory: The Emergence of a New Science of Mind (2006), speculates that if Freud had stayed in basic research instead of going into medicine, perhaps he would be known today as “a co-founder of the neuron doctrine, instead of as the father of psychoanalysis.”
Although neurons may differ in shape and size, they are essentially the same from the most primitive animal life to the most advanced. It is their number and organization that differ: we have a hundred billion nerve cells, while a jellyfish has a thousand. But their status as cells capable of rapid and repetitive firingis essentially the same.
The crucial role of synapses—the junctions between neurons where nerve impulses can be modulated, giving organisms flexibility and a whole range of behaviors—was clarified only at the close of the nineteenth century by the great Spanish anatomist Santiago Ramón y Cajal, who looked at the nervous systems of many vertebrates and invertebrates, and by C.S. Sherrington in England (it was Sherrington who coined the word “synapse” and showed that synapses could be excitatory or inhibitory in function).
In the 1880s, however, despite Agassiz’s and Romanes’s work, there was still a general feeling that jellyfish were little more than passively floating masses of tentacles ready to sting and ingest whatever came their way, little more than a sort of floating marine sundew.
But jellyfish are hardly passive. They pulsate rhythmically, contracting every part of their bell simultaneously, and this requires a central pacemaker system that sets off each pulse. Jellyfish can change direction and depth, and many have a “fishing” behavior that involves turning upside down for a minute, spreading their tentacles like a net, and then righting themselves, which they do by virtue of eight gravity-sensing balance organs. (If these are removed, the jellyfish is disoriented and can no longer control its position in the water.) If bitten by a fish, or otherwise threatened, jellyfish have an escape strategy—a series of rapid, powerful pulsations of the bell—that shoots them out of harm’s way; special, oversized (and therefore rapidly responding) neurons are activated at such times.
Of special interest and infamous reputation among divers is the box jellyfish (Cubomedusae)—one of the most primitive animals to have fully developed image-forming eyes, not so different from our own. The biologist Tim Flannery, in an article in these pages, writes of box jellyfish:
They are active hunters of medium-sized fish and crustaceans, and can move at up to twenty-one feet per minute. They are also the only jellyfish with eyes that are quite sophisticated, containing retinas, corneas, and lenses. And they have brains, which are capable of learning, memory, and guiding complex behaviors.1
We and all higher animals are bilaterally symmetrical, have a front end (a head) containing a brain, and a preferred direction of movement (forward). The jellyfish nervous system, like the animal itself, is radially symmetrical and may seem less sophisticated than a mammalian brain, but it has every right to be considered a brain, generating, as it does, complex adaptive behaviors and coordinating all the animal’s sensory and motor mechanisms. Whether we can speak of a “mind” here (as Darwin does in regard to earthworms) depends on how one defines “mind.”
We all distinguish between plants and animals. We understand that plants, in general, are immobile, rooted in the ground; they spread their green leaves to the heavens and feed on sunlight and soil. We understand that animals, in contrast, are mobile, moving from place to place, foraging or hunting for food; they have easily recognized behaviors of various sorts. Plants and animals have evolved along two profoundly different paths (fungi have yet another), and they are wholly different in their forms and modes of life.
And yet, Darwin insisted, they were closer than one might think. He wrote a series of botanical books, culminating in The Power of Movement in Plants (1880), just before his book on earthworms. He thought the powers of movement, and especially of detecting and catching prey, in the insectivorous plants so remarkable that, in a letter to the botanist Asa Gray, he referred to Drosera, the sundew, only half-jokingly as not only a wonderful plant but “a most sagacious animal.”
Darwin was reinforced in this notion by the demonstration that insect-eating plants made use of electrical currents to move, just as animals did—that there was “plant electricity” as well as “animal electricity.” But “plant electricity” moves slowly, roughly an inch a second, as one can see by watching the leaflets of the sensitive plant (Mimosa pudica) closing one by one along a leaf that is touched. “Animal electricity,” conducted by nerves, moves roughly a thousand times faster.2
Signaling between cells depends on electrochemical changes, the flow of electrically charged atoms (ions), in and out of cells via special, highly selective molecular pores or “channels.” These ion flows cause electrical currents, impulses—action potentials—that are transmitted (directly or indirectly) from one cell to another, in both plants and animals.
Plants depend largely on calcium ion channels, which suit their relatively slow lives perfectly. As Daniel Chamovitz argues in his book What a Plant Knows (2012), plants are capable of registering what we would call sights, sounds, tactile signals, and much more. Plants know what to do, and they “remember.” But without neurons, plants do not learn in the same way that animals do; instead they rely on a vast arsenal of different chemicals and what Darwin termed “devices.” The blueprints for these must all be encoded in the plant’s genome, and indeed plant genomes are often larger than our own.
The calcium ion channels that plants rely on do not support rapid or repetitive signaling between cells; once a plant action potential is generated, it cannot be repeated at a fast enough rate to allow, for example, the speed with which a worm “dashes…into its burrow.” Speed requires ions and ion channels that can open and close in a matter of milliseconds, allowing hundreds of action potentials to be generated in a second. The magic ions, here, are sodium and potassium ions, which enabled the development of rapidly reacting muscle cells, nerve cells, and neuromodulation at synapses. These made possible organisms that could learn, profit by experience, judge, act, and finally think.
This new form of life—animal life—emerging perhaps 600 million years ago conferred great advantages, and transformed populations rapidly. In the so-called Cambrian explosion (datable with remarkable precision to 542 million years ago), a dozen or more new phyla, each with very different body plans, arose within the space of a million years or less—a geological eye-blink. The once peaceful pre-Cambrian seas were transformed into a jungle of hunters and hunted, newly mobile. And while some animals (like sponges) lost their nerve cells and regressed to a vegetative life, others, especially predators, evolved increasingly sophisticated sense organs, memories, and minds.