Origin and evolution of eyes in animals

By | April 15, 2021
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Eyes form a window through which we see the world around us and usually the first feature to get noticed when you look at a person. Eyes are signature of beauty and these beautiful organ forms an excellent example of homologous organ present in wide varieties of animals. Eyes happen to be very complex and delicate organs ,where many functional parts will work together to make sight possible in animals. Evolution of eyes forms a subject of significant and controversial study ever since the days of Charles Darwin.The evolution of a structure as complex as the projecting eye by natural selection has been said to be difficult to explain. Charles Darwin himself wrote, in his Origin of Species, that the evolution of the eye by natural selection at first glance seemed “absurd in the highest possible degree”. However, he went on to explain that despite the difficulty in imagining it, it was perfectly feasible:

…if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real

Today scientists have come up with explanations through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes and complexities to form the human eye.Complex eyes appear to have first evolved within a few million years, in the rapid burst of evolution known as the Cambrian explosion. There is no evidence of eyes before the Cambrian, but a wide range of diversity is evident in the Middle Cambrian Burgess shale. However as mentioned before ever since Darwin’s Origin of Species evolution of eyes remained a matter of immense debate. Scientists and thinkers always argued whether the eyes in animals evolved multiple times independently or it has evolved once once from a common, simple precursor.

If you take a look at different morphology of eyes and photoreceptor cell types observed in various animals gives us an impression that animal eyes evolved multiple times independently. Interestingly ,the entire animal kingdom is dependent on function of pax6 gene (master control gene of eyes) for the development of eyes and not only this, but the genes that interact with pax6 to form the photoreceptor cells are also highly conserved in evolution providing strong evidence supporting one time evolution of animal eyes but modified on many occasion to suit the requirements of the organisms that bear that. Therefore, as it was previously thought that eyes formed independently many different times, we now know that each type of eye is nothing but a variation on a common theme evolved from a common, simple precursor called “the proto-eye”.

The proto-eye can be reconstructed by the structural and molecular comparison of extant eyes such as the insect compound eye, the vertebrate camera eye, and the simple pigment-cup eyes found in many invertebrate groups. Its common to believe that all animals do have eyes but a significant part of animal species lack eyes completely or have rudimentary looking eyes in the form of spots eg: Sea urchins,Sponges, Ctenophores,flatworms etc. Eye spots in simple animals spots can sense whether it’s night or day or whether a shadow is passing closely, but fail to form any kind of image like the ones of jelly fishes and bilaterians. The animal group that exhibit true eyes are a ensemble of multicellular organisms including jellyfish ,arthropods, molluscs, annelid worms, onychophora (velvet worms), and chordates.This list is rather strange (if one look at the phylogeny tree) giving us an impression that eyes appeared in scattered lineages and mixed up with groups that lack them. This was one reason of the hypothesis that animal eyes evolved multiple times but this is not true as we have seen earlier.

One of the important aspect of natural selection is that it favors an organism which has slight advantage over others on the planet ,which tends to survive and produce offspring for the next generation. According to many scientists from the field, the simple light-sensitive spot on the skin of some ancestral creature gave it some tiny survival advantage, could be to evade predators and further subsequent changes by natural selection led to changes then created a depression in the light-sensitive patch, a deepening pit that made vision a little sharper. later, the pit’s opening gradually narrowed, so light entered through a small aperture, like a pinhole camera. every little change counts,where the light-sensitive spot evolved into a retina and eventually a complex human of today.

For more in depth information:
An excellent article by Prof. PZ Myers
Review by Detlev Arendt
Evolution of eyes and photoreceptor cell types
Int J Dev Biol. 2003;47(7-8):563-71
Special Issue: The Evolution of Eyes on Evolution: Education and Outreach
Wikipedia evolution of eyes
Image credit :
Eyes by Mrinmoy Debbarma