Other Minds: The Octopus and the Evolution of Intelligent Life. Peter Godfrey-Smith

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      Figure by Ainsley Seago.

      As well as zeppelins, a range of cephalopod hovercrafts and tanks probably prowled the sea floor – some of the shells from this time seem too unwieldy to carry in the open water. All these animals are now extinct, with one non-fearsome exception, the nautilus. Many of the losses occurred as part of the mass extinctions that punctuate the history of life, but it’s also likely that some predatory cephalopods were slowly outcompeted by fish, as those fish became larger and better armed. The zeppelins were challenged, and eventually vanquished, by airplanes.

      The nautilus, however, made it through. No one knows why. At the start of this book I cited a Hawaiian creation myth that judges the octopus a “lone survivor” from an earlier world. The real survivor is indeed a cephalopod, but nautilus rather than octopus. Still living in the Pacific, present-day nautiluses are little changed from 200 million years ago. Living in coiled shells, they’re now scavengers. They have simple eyes and a cluster of tentacles, and they move up and down, from the deep sea to shallower water, in a rhythm that’s still being studied. They seem to stay higher in the water at night, deeper in the day.

      Another shift was to come in the evolution of cephalopod bodies. Sometime before the age of the dinosaurs, it seems, some cephalopods began to give up their shells. The protective casings that had become buoyancy devices were abandoned, reduced, or internalized. This enabled more freedom of movement, but at the price of greatly increased vulnerability. It seems quite a gamble, but this was a path taken several times. The last common ancestor of “modern” cephalopods is not known, but at some stage the lineage split into two main branches, an eight-armed group including octopuses and a ten-armed group including cuttlefish and squid. These animals reduced their shells in different ways. In the cuttlefish, a shell was retained internally, and still helps the animal remain buoyant. In squid, a sword-shaped internal structure called a “pen” remains. Octopuses have lost their shell entirely. Many cephalopods began to live as soft-bodied, unprotected animals on reefs in shallow seas.

      The oldest possible octopus fossil dates from 290 million years ago. I emphasize the uncertainty – it’s just one specimen, and little more than a smudge on a rock. After this there is a gap in the record, and then at around 164 million years ago there is a clearer case, a fossil that looks undeniably like an octopus, with eight arms and an octopus-like pose. The fossil record of octopuses remains skimpy because they don’t preserve well. But at some stage they radiated; around 300 species are known at present, including deep-sea as well as reef-dwelling forms. They range from less than an inch in length to the giant Pacific octopus, which weighs in at 100 pounds and spans twenty feet from arm tip to arm tip.

      That’s the journey of the cephalopod body, a path from Ediacaran macaron through limpet-like shellfish to predatory hovercraft and zeppelin. The encumbrance of the external shell is then abandoned, as the shell is brought inside the body or, in an octopus, lost completely. With that step, the octopus loses almost all definite shape.

      To completely forgo both skeleton and shell is an unusual evolutionary move for a creature of this size and complexity. An octopus has almost no hard parts at all – its eyes and beak are the largest – and as a result it can squeeze through a hole about the size of its eyeball and transform its body shape almost indefinitely. The evolution of cephalopods yielded, in the octopus, a body of pure possibility.

      During the time I was writing an early version of this chapter, I spent a few days watching a pair of octopuses in the rocky shallows. I saw them mate once, and then spend much of the next afternoon just sitting, it seemed. The female moved off a little way, but returned to her den as the sun got low. The male had spent the day in a more exposed spot, less than a foot from her den. He was there when she came back.

      I watched them, off and on, for two afternoons, and then storms came. Winds of sixty miles per hour lashed the coast, and waves rolled in from the south. The bay where the octopuses live has some protection from this onslaught, but not much. Waves swept around the entrance and turned the water into a boiling white soup. The shore was beaten by these storms for the next four days. Where do the octopuses go when the waves are pounding their rocks? It was impossible to get into the water to see. The cuttlefish have no problem. They disappear for weeks when the weather is bad. They fire up their jet propulsion and move off to some unknown deeper place. Perhaps the octopuses also move further out to sea, but more likely they climb into a crevice and hang on, for days at a stretch, recalling their ancestors who gripped rocks from inside cap-shaped shells.