While every part of an animal’s body is a tribute to evolutionary engineering, perhaps none is more fascinating than the eye. Apart from its major function of sight, it conveys moods, emotions and, most of all, life itself — many vegetarians will tell you that they simply cannot eat something which has eyes. The story of how eyes evolved is as fascinating as the end product, which is a marvel of function, design and beauty.
Biologists believe that the whole process began 500 million years ago. A mutation in the skin cells might have caused light sensitivity in some creatures, giving them the ability to sense the presence of a predator as its shadow fell over them. Thus, there would soon be more light-sensitive creatures around because of their greater survivability. The next mutation could have been the formation of a shallow pit containing light-sensitive cells. The creatures would now be able to tell the direction from which the predator (by the shadow) approached. There are fossils that depict this kind of “eye” and they still exist in creatures like mollusks. The pit could then have narrowed, till it was like a pinhead, like in a pinhole camera, giving the first signs of actual image formation. A clear transparent shield over this, for protection from the elements, could have become the lens. And so on, until the eye became the sophisticated tool it is today.
Of course, there are many who believe this whole process is outlandish and would have taken too long: after all, evolution moves glacially. But there is fossil evidence to show that it did take place. Moreover, two Swedish scientists ran a computer simulation to see how long the process would take. On a computer-generated patch of light-sensitive receptors, they imitated the natural process: random changes would be incorporated in these receptors and only those with a slight advantage were carried over to the next “generation” of receptors and so on. Finally, they calculated how many generations it would take for the transformation from simple light-sensitive skin patches to an eye, complete with lens. The figure was 400,000. Assuming these little creatures had a lifespan of one year, this meant that the whole process was done in around 400,000 years, which is much less than the time life has been on earth — 500 million years. What’s more, the eye evolved independently at least 40 times in the animal kingdom.
As myriad creatures took up different lifestyles, they required eyes that were “specialised”. Thus, eagles have eyes that winkle out a rabbit in the grass two miles away: their vision is eight times as good as ours. Owls have forward-facing eyes, with more light sensitive “rods” than we do, giving them binocular vision and night vision.
Predators, like the big cats, have forward-facing eyes to enable them to focus and judge distance with excellent night vision. Their prey — deer, livestock and even goats — have horizontal or oblong-shaped pupils to give them wide-angle vision so they can see a predator approach from behind them (but horses, for example, have a blind spot directly in front of their faces).
Insects usually have compound eyes, wherein a single eye is made up of multiple independent lenses, each forming a small part of the image resulting in a final composite image. The bulbous-eyed dragonfly has around 30,000 such lenses as well as three simple eyes on the top of its head, which enable it to sense and react like lightning to movement.
The mantis shrimp, a predator that resides in shallow warm seas, has the most complex eye. It’s equipped with 12 types of “cones” (which discern colours) as against our three. But this doesn’t enable the mantis shrimp to see many more colours than us. It helps pick up movement, through changes in shades, and react quickly.
We may be outclassed in many departments of vision — in night vision, peripheral vision and telescopic vision. But there is one in which, we and only we excel: We are the only ones who will exchange meaningful looks with a chosen one and then, very slowly, wink!