Other Minds: The Octopus and the Evolution of Intelligent Life. Peter Godfrey-Smith
in the animal’s body. Imagine a jellyfish-like animal shaped like a dome, with a mouth underneath. One nervous system evolves on the top, and tracks light, but not as a guide to action. Instead it uses light to control bodily rhythms and regulate hormones. Another nervous system evolves to control movement, initially just the movement of the mouth. And at some stage, the two systems begin to move within the body, coming into new relations with each other. Arendt sees this as one of the crucial events that took bilaterians forward in the Cambrian. A part of the body-controlling system moved up toward the top of the animal, where the light-sensitive system sat. This light-sensitive system, again, was only guiding chemical changes and cycles, not behavior. But the joining of the two nervous systems gave them a new role.
What an amazing image: in a long evolutionary process, a motion-controlling brain marches up through your head to meet there some light-sensitive organs, which become eyes.
~ The Fork
The bilaterian body plan arose before the Cambrian, in some small and unremarkable form, but it became the bodily scaffold on which a long series of increases in behavioral complexity was laid down. Early bilaterians also have another role in this book. Sometime soon after they appeared, probably still in the Ediacaran, there was a branching, one of the countless evolutionary forks that take place as the millennia pass. A population of these animals split into two. The animals who initially wandered off down the two paths might have looked like small flattened worms. They had neurons, and perhaps very simple eyes, but little of the complexity that was to come. Their scale was measured perhaps in millimeters.
After this innocuous split, the animals on each side diverged, and each became ancestor to a huge and persisting branch of the tree of life. One side led to a group that includes vertebrates, along with some surprising companions such as starfish, while the second side led to a huge range of other invertebrate animals. The point just before this split is the last point at which an evolutionary history is shared between ourselves and the big group of invertebrates that includes beetles, lobsters, slugs, ants, and moths.
Here is a diagram of this part of the tree of life. Lots of groups are omitted from the picture, both outside and inside the branches shown. The moment we’re talking about is labeled “the fork.”
On each path downstream of the fork, more branchings occurred. One side eventually sees fish appear, then dinosaurs and mammals. This is our side. On the other side, further branchings give rise to arthropods, mollusks, and others. On both sides, passing from the Ediacaran into the Cambrian and beyond, lives become entangled, the senses open, and nervous systems expand. Until, in one tiny example of this sensory and behavioral entangling, a rubber-encased mammal and a color-changing cephalopod find themselves staring at each other in the Pacific Ocean.
* If you’ve seen the word “sensorimotor” instead, please treat this as the same.
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Mischief and craft are plainly seen to be characteristics of this creature.
– Claudius Aelianus, third century A.D.,
writing about the octopus
In a Sponge Garden
Someone is watching you, intently, but you can’t see them. Then you notice, drawn somehow by their eyes.
You’re amid a sponge garden, the sea floor scattered with shrub-like clumps of bright orange sponge. Tangled in one of these sponges, and the gray-green seaweed around it, is an animal about the size of a cat. Its body, though, seems to be everywhere and nowhere. Much of the animal seems to have no definite shape at all. The only parts you can keep a fix on are a small head and the two eyes. As you make your way around the sponge, so too do those eyes, keeping their distance, keeping part of the sponge between the two of you. Its color matches – exactly, perfectly – the seaweed around it, except that some of its skin is folded into tiny tower-like peaks, and the tips of these peaks match – nearly as exactly – the orange of the sponge. You keep coming round its side of the sponge, and eventually it raises its head high, then rockets away under jet propulsion.
A second meeting with an octopus: this one is in a den. Shells are strewn in front, arranged with some pieces of old glass. You stop in front of its house and the two of you look at each other. This one is small, about the size of a tennis ball. You reach forward a hand and stretch out one finger, and one octopus arm slowly uncoils and comes out to touch you. The suckers grab your skin, and the hold is disconcertingly tight. Having attached the suckers, it tugs your finger, pulling you gently in. The arm is packed with sensors, hundreds of them in each of the dozens of suckers. It’s tasting your finger as it draws it in. The arm itself is alive with neurons, a nest of nervous activity. Behind the arm, large round eyes watch you the whole time. Hundreds of millions of years on from the events of chapter 2, this is one place the evolution of animals has landed.
~ Evolution of the Cephalopods
Octopuses and other cephalopods are mollusks – they belong to a large group of animals which also includes clams, oysters, and snails. Part of the story of the octopus, then, is the evolutionary history of mollusks. In the previous chapter we reached the Cambrian, the period in the history of life when a great range of animal body plans appear in the fossil record. Many of these animal groups, including mollusks, must pre-date the Cambrian, but in the Cambrian mollusks become noticeable, because of their shells.
Shells were the mollusks’ response to what looks like an abrupt change in the lives of animals: the invention of predation. There are various ways of dealing with the fact that you are suddenly surrounded by creatures who can see and would like to eat you, but one way, a molluscan specialty, is to grow a hard shell and live within or beneath it. The cephalopod line probably goes back to an early mollusk of this kind, crawling along the bottom of the sea under a hard shell peaked like a cap. This animal looked a bit like a limpet, one of those plain, cup-like shellfish that grip rocks in tide pools today. The cap grew, Pinocchio-like, over evolutionary time, slowly taking the shape of a horn. These animals were small – the “horn” was less than an inch long. Beneath the shell, as with other mollusks, a muscular “foot” anchored the animal and enabled it to crawl along the sea floor.
Then, at a stage later in the Cambrian, some of these animals rose from the sea floor and entered the water column. On dry land, no effortless move up into the air is possible for an animal; such a move requires the expense of wings or something similar. In the sea you can lift off easily, be carried, and see where you end up.
An upward-pointing shell which protects can be made into a buoyancy device, by filling it with gas. Early cephalopods seem to have done just that. Making the shell buoyant may have initially enabled easier crawling, and many of the old cephalopods might have moved by engaging in a half-crawl, half-swim on the bottom of the sea. Some, though, rose higher, and found a world of opportunity above. A small amount of gas, held within the shell, will turn a limpet into a zeppelin.
Once aloft, the “foot” is no use for crawling, so the zeppelin-cephalopods invented jet propulsion, by directing water through a tube-like siphon which could be pointed in several directions. The foot itself was freed up for grasping and manipulating objects, and part of it flowered into a cluster of tentacles. Talk of “flowering” would sound inappropriate, though, to the animals on the other end of these tentacles – the animals being grasped – as some of the tentacles sprouted dozens of sharp hooks. The opportunity the cephalopods were seizing by rising up into the water was the opportunity to feed on other animals, to become predators themselves. This they did with great evolutionary enthusiasm. Many forms appeared, with straight shells and coiled, and the largest reached sizes of eighteen feet or more. Beginning as diminutive limpets, cephalopods had become the most fearsome predators in the sea.