Evolution: convergence of eyes (Introduction)

by David Turell @, Monday, April 01, 2024, 16:16 (20 days ago) @ David Turell

Adapted to lifestyle:


"... different organisms have evolved to view the world differently, with eye structures and configurations optimized for various kinds of existence.

"...the horizontal pupils of herbivores give them a panoramic view of their surroundings, which helps both to see predators coming, and to avoid obstacles as the animals make an escape. Meanwhile, nocturnal predators have vertical pupils to maximize their night vision.

"No other animal has a pupil quite like the cuttlefish. It's shaped like a W, a trait biologists have determined helps the animals balance a vertically uneven field of light, which is common in the watery depths they inhabit. But that's just the start.

"Cuttlefish only have one type of photoreceptor, which should mean they can only see in monochrome. Yet those strange, wide pupils of cuttlefish and other cephalopods could facilitate an entirely different way of seeing color – by using the way light passing through a prism splits into a rainbow.


"Unlike other cephalopods, though, cuttlefish eyes can swivel, allowing them to see the world in 3D as well; recently, scientists found these swivelly eyes result in stereoscopic vision, giving cuttlefish yet another advantage in their environment.


"Cephalopods only have one photoreceptor type, as we have established. Humans have four, three cones and a rod, which means we have color sensitivities at three peak wavelengths, what we call trichromatic vision. (The rod is for low-light vision.)

"Birds have six – four cones giving tetrachromatic vision, a rod, and an unusual double cone for non-colored motion perception.

"In addition, a protein in their eyes could allow them to see magnetic fields. Migratory birds can navigate extraordinarily well; scientists narrowed it down to a class of proteins called cryptochromes, which are sensitive to blue light.

"Birds' magnetoreception – that is, their ability to perceive magnetic fields – seems dependent on blue light, suggesting that the sense may be vision-based. There's the distinct possibility that this magnetic filter for the color blue is the result of a quantum quirk. More recent lab studies have shown how a magnetic field affects a quantum property of cryptochromes, governing their electrons.

"Behold the largescale four-eyes (Anableps anableps), of the four-eyed fish genus.

"This fascinating beastie doesn't actually have four eyes – but its two eyes have evolved an incredible adaptation. Their ecological niche is the surface of the water, where they spend the majority of their time, preying on insects that hover around aquatic ecosystems.

"Their eyes are situated on top of their heads, all the better to see the flying bugs in an aerial environment. But a portion of their optic organ sits below the surface of the water, and this is where things get interesting: each pupil is divided into two halves, one of which sits above the waterline (dorsal), while the other sits below (ventral), pointing downwards into the murky depths.

"In this way, the fish can simultaneously see above and below the water – environments through which light propagates differently – to watch for both predators and prey. The thickness of the lens varies too, to accommodate the different refractive indices of aerial and aquatic media, as does the thickness of the corneal epithelium.

"And the proteins in the retinal photoreceptor cells are slightly different as well – more sensitive to green light in the dorsal retina, and more sensitive to yellow light in the ventral retina. Since the fish often live in muddy environments, like mangroves, this is thought to improve vision in murky waters.


"Mantis shrimps of the order Stomatopoda, have 16 in their compound peepers. What do they do with these photoreceptors? They see. They see all of the things...They can see five different ultraviolet frequency bands.

"In addition, mantis shrimps can see polarized light; that is, the orientation of the oscillations of the wave of propagating light. Many animals can see linearly polarized light, including cuttlefish. Mantis shrimps are the only animals that can see circularly polarized light that we know of.

"Each eye is mounted on a stalk, and can be moved independently. And each eye has the ability to perceive depth. Humans rely on binocular vision for depth perception. Mantis shrimps only need one.

"Chitons have eyes, but they're embedded in their armor, and made of mineral; more specifically, a type of calcium carbonate known as aragonite.

"The simple eyes of chitons, which litter the surface of their shells alongside hundreds of sensory organs known as aesthetes, consist of an aragonite lens covered by a cornea, and some sort of retina; to the surprise of scientists, these tiny primitive organs can actually resolve images.

"Trilobites, for example, also had mineral eyes, with lenses made of calcite. These extinct creatures had the first truly complex eyes that we know of, so understanding them can tell us a lot about how vision evolved on Earth in all its dazzling complexity."

Comment: see the illustrations. Adaptations with a purpose. Designed.

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