Small, huge, very sensitive, coarse. The eyes in the animal kingdom are one of the most present and most varied structures, but also among the most mysterious. So much so that even Darwin, the father of evolution, thought they could question his theory (he thought it absurd to think that only natural selection could have built a perfect structure like the eye). But they are also so indispensable that, according to some scholars, life on Earth has "discovered" the organ to capture light rays not once, not twice, but over 40 times.
And indeed a brief look at all species reveals an impressive amount of solutions. If we want to build a scale of complexity (which is not an "evolutionary" scale - every eye has its value and its history, regardless of how it is made), we start with very simple spots on the skin where some cells have assumed the function to perceive light rays. It is obvious that these elementary "eyes" can only perceive the difference between light and shadow, and allow the animal to hide among the ravines of the ground, away from the desiccant light of the Sun and the enemies that can see them.Not to be missed
See the world through the eyes of octopus, jellyfish, dolphins, fish and shellfish: in this multimedia!
Eyes of this type are present in very simple animals, such as flatworms (also called flatworms ) that live in sand or under rocks in fresh water or at sea. To improve the sight starting from there, it is enough to simply bury the sensitive area until it becomes a hollow with a small "entrance", a kind of pupil in which the light rays are concentrated, and allow one (albeit minimal) directionality. Now you can see at least vaguely where the light comes from, and you can escape to the other side. It is an eye, however, without protection and without a lens that focuses: so are those of one of the oldest molluscs, the nautilus . Who, living at great depth, does not have a pressing need for a precise view.
Just the lens, however, is the invention that allowed the eye to make a decisive leap in quality. No longer just differences in light and shadow or vague threatening forms. An eye with crystalline and cornea, a protective layer in front of a "little hole" from which light passes, the pupil, is very similar to that of the animals we know, such as mammals, birds. However, if we look carefully at the true rulers of planet Earth, arthropods (which include insects and crustaceans, among others) we discover that their eyes have a completely different shape. Instead of being formed by a single structure that perceives and "collects" light rays to send them to the brain, flies, bees and crayfish have eyes formed by many tiny parts, each of which is able to function as an eye, and gathers rays of light to leave the brain the task of processing them.
Many eyes look similar, but basically they are extremely different; even if molluscs like octopus and cuttlefish have eyes similar to ours, for example, their organization is completely different (and much better than that of mammals - for example an octopus does not run the risk of detachment of the retina). Thus, according to some evolutionists, this means that life has worked hard to invent different ways to always reach the same organ. But others argue instead that at the base of everything there are light-sensitive molecules (the so-called opsins) and some genes (such as Pax-6) present everywhere, which work in the same way for all animals and always give the same instructions in embryonic development to build a sense organ that captures light. The eye as an ancestral invention, or discovery continues throughout the history of life, then? Some evolutionists, like Russell Fernald, cut off the bull's head by saying that the basic instructions and materials are the same, but how to assemble them is to the evolution and the fantasy of the species.