Why do octopus eyes look like ours?

Octopus eyes look like ours because evolution built the same kind of camera twice. Look an octopus in the eye and something feels familiar: an iris, a lens and a light-sensitive retina, much like a fish's eye, or yours. But octopuses are molluscs, and our last common ancestor had at most a simple light-sensitive spot. The octopus eye and the human eye were built separately.

This is convergent evolution: unrelated lineages independently evolving similar features. Living similar lives creates similar problems, and similar problems can lead to similar solutions. Flight is the classic case: insects, birds, pterosaurs and bats all evolved it on their own.

The camera eye is one of the best-known examples, and the two versions are not identical. The octopus eye is wired the other way round. In our eyes, the nerve fibres run across the front of the retina and have to exit through it, leaving a patch with no light detectors: the blind spot. Your brain fills in the hole, so you never notice. In the octopus eye, the nerves leave from behind the retina, so there is no gap and no blind spot.

Convergence shows up everywhere once you look. Herrings, dolphins and extinct ichthyosaurs share the same streamlined shape. Bats and dolphins both evolved echolocation. Even intelligence evolved separately in octopuses and in animals like us.

Evolution has no plan, yet it keeps rediscovering the same good ideas.