In an aquarium at night, when the staff have gone home, an octopus sometimes leaves its tank. It hauls itself across the floor, prises open a neighbouring tank, eats the crabs inside, and returns to its own before morning — leaving keepers to puzzle over the shrinking crab population. Stories like this are not aquarium folklore; they are documented behaviour, and they hint at a mind unlike any other on Earth.
The octopus is the closest thing we have to an alien intelligence, and the reason is evolutionary distance. Our last common ancestor with the octopus was a simple, probably worm-like creature that lived more than five hundred million years ago, before nervous systems as we know them had taken shape. Whatever cleverness the octopus has, it evolved entirely separately from ours. Vertebrate intelligence and cephalopod intelligence are two independent experiments in the same problem — a striking case of convergent evolution.
Nine brains, and skin that sees
An octopus has around five hundred million neurons, comparable to a dog. But their arrangement is bizarre. Only a minority sit in the central brain wrapped around the gullet; roughly two-thirds are distributed through the eight arms, in clusters of nerve cells that give each arm a striking degree of autonomy. A severed arm can reach and grasp on its own. This is why the animal is sometimes said to have nine brains — one central and eight peripheral — though the arms and the centre do communicate and coordinate.
Then there is the skin. Octopuses are colour-blind by the usual test — they have a single type of photoreceptor in their eyes — yet they produce some of the most sophisticated camouflage in nature, matching colour and texture to their surroundings in a fraction of a second using pigment sacs called chromatophores. Part of the resolution to that paradox is remarkable: their skin itself contains light-sensitive proteins, so in a real sense an octopus can sense light with its whole body, not just its eyes.
Problem-solvers with no bones
In the laboratory octopuses learn to open screw-top jars to reach food, including unscrewing a lid from the inside. They navigate mazes, use tools — some carry coconut-shell halves to assemble into portable shelters — and appear to recognise and even take a disliking to particular humans. In one famous account an octopus repeatedly aimed jets of water at a specific keeper it seemed to resent. They are escape artists precisely because a boneless body can squeeze through any gap larger than its beak, the only hard part of its anatomy — a talent that has embarrassed many an aquarium and made the sealed lid a standard piece of octopus-keeping equipment.
All this cleverness sits atop a puzzle: octopuses are profoundly antisocial and astonishingly short-lived. Most species live only one or two years. They are semelparous — they breed once and then die. The female stops eating to guard her eggs and typically perishes as they hatch; the male dies not long after mating. An animal this intelligent has almost no opportunity to learn from its elders, because there are no elders. Each octopus, in effect, works out the world largely from scratch, its intelligence written mostly in its genes rather than passed down as culture.
That combination — high intelligence, no social learning, a body that thinks partly with its arms, senses light through its skin, and lives barely long enough to reproduce — makes the octopus a genuine test of what intelligence even is. We tend to assume that a big brain, a long life and a rich social world go together, because in mammals they do. The octopus took a completely different route and arrived at flexible, curious, problem-solving behaviour anyway.
The philosopher Peter Godfrey-Smith, whose book Other Minds drew wide attention to these animals, put the point memorably: meeting an octopus is probably the closest we will come, on this planet, to meeting an intelligent alien. It looks back at you through those strange horizontal-pupilled eyes, changing colour, tasting the glass with suckers that each carry their own cluster of neurons, and you are left with the unsettling sense of a mind organised on principles that are not remotely like your own. The alien intelligence, it turns out, has been living next door in the sea the whole time.
Quiz nuggets
- An octopus has roughly five hundred million neurons, but about two-thirds of them are in its eight arms, not its central brain.
- Octopus and human intelligence evolved independently from a common ancestor more than 500 million years ago — a case of convergent evolution.
- Octopuses are effectively colour-blind, with one photoreceptor type, yet camouflage using pigment sacs called chromatophores; their skin also contains light-sensitive proteins.
- Most octopuses live only one or two years and are semelparous — they breed once and then die.
- Peter Godfrey-Smith's book Other Minds popularised the octopus as the nearest thing to an alien intelligence on Earth.