Depth is a trade against field of view

Two forward-facing eyes overlap, and the disparity between the images gives stereoscopic depth. Two side-facing eyes barely overlap, and give an almost complete circle of coverage with poor depth.

Predators face forward. Prey faces sideways. Humans have roughly 120° of binocular overlap inside a field of about 200°. A horse sees roughly 350° — almost everything, at once — with only a narrow forward overlap and two blind spots: directly in front of its own forehead, and directly behind. That is why a horse turns its head to look at something close, and why approaching one from straight behind is how people get kicked.

The compound eye makes the opposite trade again: near-spherical coverage and extraordinary motion detection, at low spatial resolution.

Colour has a number of dimensions, and it is not three for everyone

Colour vision is dimensional in a literal sense: each class of photoreceptor is an axis, and the brain locates a colour by comparing across them.

• Most mammals — two. Dogs, cats and horses are dichromats. A horse cannot separate red from green.
• Humans and other primates — three. Trichromacy is a re-acquisition; the mammal line lost photoreceptor classes early during a long nocturnal phase, and primates got one back.
• Birds, many reptiles, many fish — four, the fourth reaching into ultraviolet. Kestrels track voles by UV-visible urine trails. Many birds that look monochrome to us are patterned in UV, including to each other.
• Mantis shrimp — twelve to sixteen photoreceptor types, and here the obvious inference is wrong. Behavioural testing published in 2014 found they discriminate colours worse than humans do. They appear to recognise colour directly per receptor rather than by comparing across channels — a scanning system, faster and coarser. More channels did not produce finer colour.

Humans are not the reference standard on this axis. We are mid-range mammals who recovered a third channel, looking at a world most birds see one more dimension of.

The same eye, built twice

Eyes of some form have arisen independently dozens of times. The full camera eye — lens, iris, retina — at least half a dozen, and the sharpest case is the octopus, whose eye matches ours closely and was built entirely separately.

It is also wired better. The vertebrate retina is inverted: nerves and vessels sit in front of the photoreceptors, and the bundle punches through to exit, which is why every vertebrate has a blind spot. The octopus retina faces the light and its nerves leave from behind. It has no blind spot. Whatever conclusion is drawn from that, the fact is not in dispute.

Every creature on this axis

Related

• Other senses Open page: Other senses
• Octopus Open page: Octopus
• Horse Open page: Horse