LITMIR.BIZ

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dark horizontal pupils – like cats’ eyes turned on their side.

      The octopuses watched Matt, and also watched one another. Some started roaming around. They’d haul themselves out of their dens and move over the shell bed in an ambling shuffle. Sometimes this elicited no response from others, but occasionally a pair would dissolve into a multi-armed wrestle. The octopuses seemed to be neither friends nor enemies, but in a state of complicated coexistence. As if the scene were not sufficiently strange, many baby sharks, each just six inches or so long, lay quietly on the shells as the octopuses roamed around them.

      A couple of years before this I was snorkeling in another bay, in Sydney. This site is full of boulders and reefs. I saw something moving under a ledge – something surprisingly large – and went down to look at it. What I found looked like an octopus attached to a turtle. It had a flat body, a prominent head, and eight arms coming straight from the head. The arms were flexible, with suckers – roughly like octopus arms. Its back was fringed with something that looked like a skirt, a few inches wide and moving gently. The animal seemed to be every color at once – red, gray, blue-green. Patterns came and went in a fraction of a second. Amid the patches of color were veins of silver like glowing power lines. The animal hovered a few inches above the sea floor, and then came forward to look at me. As I had suspected from the surface, this creature was big – about three feet long. The arms roved and wandered, the colors came and went, and the animal moved forward and back.

      This animal was a giant cuttlefish. Cuttlefish are relatives of octopuses, but more closely related to squid. Those three – octopuses, cuttlefish, squid – are all members of a group called the cephalopods. The other well-known cephalopods are nautiluses, deep-sea Pacific shellfish which live quite differently from octopuses and their cousins. Octopuses, cuttlefish, and squid have something else in common: their large and complex nervous systems.

      I swam down repeatedly, holding my breath, to watch this animal. Soon I was exhausted, but I was also reluctant to stop, as the creature seemed as interested in me as I was in it (in him? in her?). This was my first experience with an aspect of these animals that has never stopped intriguing me: the sense of mutual engagement that one can have with them. They watch you closely, usually maintaining some distance, but often not very much. Occasionally, when I’ve been very close, a giant cuttlefish has reached an arm out, just a few inches, so it touches mine. It’s usually one touch, then no more. Octopuses show a stronger tactile interest. If you sit in front of their den and reach out a hand, they’ll often send out an arm or two, first to explore you, and then – absurdly – to try to haul you into their lair. Often, no doubt, this is an overambitious attempt to turn you into lunch. But it’s been shown that octopuses are also interested in objects that they pretty clearly know they can’t eat.

      To understand these meetings between people and cephalopods, we have to go back to an event of the opposite kind: a departure, a moving apart. The departure happened quite some time before the meetings – about 600 million years before. Like the meetings, it involved animals in the ocean. No one knows what the animals in question looked like in any detail, but they perhaps had the form of small, flattened worms. They may have been just millimeters long, perhaps a little larger. They might have swum, might have crawled on the sea floor, or both. They might have had simple eyes, or at least light-sensitive patches, on each side. If so, little else may have defined “head” and “tail.” They did have nervous systems. These might have comprised nets of nerves spread throughout the body, or they might have included some clustering into a tiny brain. What these animals ate, how they lived and reproduced – all are unknown. But they had one feature of great interest from an evolutionary point of view, a feature visible only in retrospect. These creatures were the last common ancestors of yourself and an octopus, of mammals and cephalopods. They’re the “last” common ancestors in the sense of most recent, the last in a line.

      The history of animals has the shape of a tree. A single “root” gives rise to a series of branchings as we follow the process forward in time. One species splits into two, and each of those species splits again (if it does not die out first). If a species splits, and both sides survive and split repeatedly, the result may be the evolution of two or more clusters of species, each cluster distinct enough from the others to be picked out with a familiar name – the mammals, the birds. The big differences between animals alive now – between beetles and elephants, for example – originated in tiny insignificant splits of this sort, many millions of years ago. A branching took place and left two new groups of organisms, one on each side, that were initially similar to each other, but evolved independently from that point on.

      You should imagine a tree that has an inverted triangular, or conical, shape from far away, and is very irregular inside – something like this:

      Now imagine sitting on a branch on top of the tree, looking down. You are on the top because you’re alive now (not because you are superior), and around you are all the other organisms alive now. Close to you are your living cousins, such as chimpanzees and cats. Further away, as you look horizontally across the top of the tree, you’ll see animals that are more distantly related. The total “tree of life” also includes plants and bacteria and protozoa, among others, but let’s confine ourselves to the animals. If you now look down the tree, toward the roots, you’ll see your ancestors, both recent ones and those more remote. For any pair of animals alive now (you and a bird, you and a fish, a bird and a fish), we can trace two lines of descent down the tree until they meet in a common ancestor, an ancestor of both. This common ancestor might be encountered just a short way down the tree, or further down. In the case of humans and chimps we reach a common ancestor very quickly, living about six million years ago. For very different pairs of animals – human and beetle – we have to trace the lines further down.

      As you sit in the tree, looking across at your near and distant relatives, consider a particular collection of animals, the ones we usually think of as “smart” – the ones with large brains, who are complex and flexible in their behavior. These will certainly include chimps and dolphins, also dogs and cats, along with humans. All these animals are quite near to you on the tree. They are fairly close cousins, from an evolutionary point of view. If we’re doing this exercise properly we should also add birds. One of the most important developments in animal psychology over the last few decades has been the realization of how smart crows and parrots are. Those are not mammals, but they are vertebrates, and hence they are still fairly close to us, though not nearly as close as chimps. Having collected all these birds and mammals, we can ask: What was their most recent common ancestor like, and when did it live? If we look down the tree to where their lines of ancestry all fuse, what do we find living there?

      The answer is a lizard-like animal. It lived something like 320 million years ago, a bit before the age of the dinosaurs. This animal had a backbone, was of reasonable size, and was adapted to life on land. It had an architecture similar to our own, with four limbs, a head, and a skeleton. It walked around, used senses similar to ours, and had a well-developed central nervous system.

      Now let’s look for the common ancestor that connects this first group of animals, which includes ourselves, to an octopus. To find this animal we have to travel much further down the branches. When we find it, about 600 million years before the present, the animal is that flattened worm-like creature I sketched earlier.

      This step back in time is nearly twice as long as the step we took to find the common ancestor of mammals and birds. The human-octopus ancestor lived at a time when no organisms had made it onto land and the largest animals around it might have been sponges and jellyfish (along with some oddities I’ll discuss in the next chapter).

      Assume we’ve found this animal, and are now watching the departure, the branching, as it happened. In a murky ocean (on the sea floor, or up in the water column) we’re watching a lot of these worms live, die, and reproduce. For an unknown reason, some split off from the others, and through an accumulation of happenstance changes they start to live differently. In time, their descendants evolve different bodies. The two sides split again and again, and before long we are looking not at two collections of worms, but at two enormous branches

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