Arnia Srinivasan at the London Review of Books:
Octopuses do not have any stable colour or texture, changing at will to match their surroundings: a camouflaged octopus can be invisible from just a few feet away. Like humans, they have centralised nervous systems, but in their case there is no clear distinction between brain and body. An octopus’s neurons are dispersed throughout its body, and two-thirds of them are in its arms: each arm can act intelligently on its own, grasping, manipulating and hunting. (Octopuses have arms, not tentacles: tentacles have suckers only at their tips. Squid and cuttlefish have a combination of arms and tentacles.) In evolutionary terms, the intelligence of octopuses is an anomaly. The last common ancestor between octopuses on the one hand, and humans and other intelligent animals (monkeys, dolphins, dogs, crows) on the other, was probably a primitive, blind worm-like creature that existed six hundred million years ago. Other creatures that are so evolutionarily distant from humans – lobsters, snails, slugs, clams – rate pretty low on the cognitive scale. But octopuses – and to some extent their cephalopod cousins, cuttlefish and squid – frustrate the neat evolutionary division between clever vertebrates and simple-minded invertebrates. They are sophisticated problem solvers; they learn, and can use tools; and they show a capacity for mimicry, deception and, some think, humour. Just how refined their abilities are is a matter of scientific debate: their very strangeness makes octopuses hard to study. Their intelligence is like ours, and utterly unlike ours. Octopuses are the closest we can come, on earth, to knowing what it might be like to encounter intelligent aliens.