How could something as complicated as the human eye come to be? The question plagued Darwin and he called the idea of eye evolution “absurd in the highest possible degree”.
Nevertheless he imagined gradation from simple-eyed animals with pigment-coated nerves to ones with eyes of a “high stage of perfection”, such as our own.
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Subsequent studies show he was right in all but the last point. For human eyes, like those of all vertebrates, aren’t perfect. In common with so many body parts, they carry evolutionary baggage: the blind spot.
Though eyes throughout the animal kingdom use the same family of photoreceptor proteins – opsins – to acquire light signals (suggesting a common ancestry for all life), the make-up of many animal eyes differ. In octopuses, for instance, the eyes are formed by a patch of the external surface pulling in on itself to create a pit, an invagination.
We vertebrates do things differently. Our eyes originate instead as extensions of the brain. Light goes into our eye sockets and signals are sent through the optic nerve at the back of the eye into the brain. Useful, but not perfect. The optic nerve takes up valuable light-gathering space in the eyeball, resulting in a patch that fails to gather light at all: the blind spot. It’s yet another reason why octopuses are truly the greatest of life-forms.
