The Octopus

The last common ancestor of humans and octopuses lived about 600 million years ago, and it was probably a flatworm with barely a brain at all. Everything intelligent about an octopus was built since then, from scratch, along a completely separate branch of the tree of life. Our intelligence comes from a backbone and a skull; theirs from the line that produced snails and clams. Meeting an octopus is the nearest thing we have to meeting a mind from another world.

A body like no other

An octopus is a mollusc, a relative of oysters and slugs that has given up its shell entirely. With no skeleton, its body is almost pure muscle, and it can pour itself through any opening wider than its beak, the small hard mouthpart that is its only rigid piece. A 20-kilogram giant Pacific octopus has escaped through a hole the size of a coin.

It has three hearts: two pump blood through the gills and one through the body. Its blood is blue, because it carries oxygen with a copper-based molecule rather than the iron-based one that makes ours red. It breathes water through gills inside its body and can also jet that water out to swim in rapid bursts. Its eyes, which look eerily like ours, evolved independently and are built differently inside, with no blind spot.

Eight thinking arms

Of an octopus's roughly 500 million neurons, about two thirds are not in its head. They are in its arms. Each arm has a chain of nerve clusters that controls its own movement, and an arm can grasp, explore and even pass food toward the mouth without instructions from the brain. Severed arms continue to react to touch for hours. The brain sets goals; the arms work out how to do them. In effect, an octopus has one central mind and eight semi-independent ones.

Every arm carries around 250 suckers, and each sucker can taste as well as touch. An octopus exploring a crevice is tasting its way along, feeling the chemistry of everything it touches.

Masters of disguise

Octopus skin is packed with millions of tiny colour sacs, called chromatophores, each controlled by muscles that stretch it wide to show the pigment or squeeze it shut to hide it. Beneath these lie reflective cells that can produce blues and greens, and the skin itself can be raised into bumps and spikes. The whole system is controlled directly by the nervous system, so an octopus can match the colour and texture of a rock, sand or coral in about a fifth of a second, and pass waves of colour across its body as it moves. It does all this despite being, as far as we can tell, colour-blind; how it chooses the right colours is still a mystery, and one theory is that the skin itself can sense light.

The mimic octopus of Indonesia goes further, shaping and colouring itself to impersonate lionfish, flatfish and venomous sea snakes, choosing a disguise suited to whatever predator is nearby.

Intelligence

Octopuses in laboratories unscrew jars to get at crabs inside, learn to navigate mazes and remember the solutions for months, recognise individual human keepers and squirt water at the ones they dislike. In the wild they carry coconut shells to assemble into shelters later, the first tool use seen in an invertebrate. They play, pushing objects around in currents for no purpose anyone can detect. They appear to sleep and even to dream, cycling through colour patterns as they rest.

All this in an animal that lives, in most species, only a year or two, learns everything alone with no parents to teach it, and dies shortly after reproducing once. A female octopus guards her eggs for weeks or months without eating, dying as they hatch. An intelligence this rich, built on a life this short and solitary, challenges most of what biologists assumed about why big brains evolve.

Why it matters

Intelligence, we used to assume, needed a backbone, a long life, a social group and parents to learn from. The octopus has none of these, and is intelligent anyway. It shows that minds can be built in more than one way, that the route vertebrates took is not the only one, and that if there is life elsewhere in the universe, its thoughts may be organised in ways we have not imagined. The nearest alien is already here, in the rock pool.