Long after The Origin of Species was published Charles Darwin confessed, in a letter to an American colleague: ‘The eye, to this day, gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shoulder.’
It wouldn’t be an exaggeration to say that our eyes are nothing short of magic. As clear from the above remark, even the greatest of scientists marvel at the complexity and beauty that the eye possesses. How could an organ so complex, so sophisticated evolve through natural selection? But, it has. That’s the point. And that’s what Richard Dawkins explains to us in his book Climbing Mount Improbable. Let’s go through this journey consisting of millions of years of evolution in a matter of minutes.
The First Signs
In the beginning, there was no eye. But there was the sun and there was the earth, and the life had barely begun on latter’s surface. Since there was no eye, no life-form could see anything. And when the sunlight, containing a stream of photons, fell on a life-form, it led to the release of energy. In green plants and green bacteria, this energy was used to build food molecules, a process which we call photosynthesis.
So the earliest life-forms, much like the ones that followed them, responded to the exposure to sunlight. That means, they could distinguish between the dark and the light. They couldn’t tell where this light was coming from, or any other fact for that matter, but they could sense if there was any light or not.
Reading and Interpreting the Signs
Photons arrive at random times, like raindrops. When rain starts, how do we know the exact moment when it has begun? We feel a single drop and look up apprehensively, unconvinced until a second or third drop arrives. When rain is spitting infrequently like this, one person may say that it is raining while his companion denies it.
Similarly detecting photons for the earliest of life-forms wouldn’t have been easy. They would have missed many. More importantly, they would have competed against each other and developed more layers of pigment to improve their chances of catching a photon.
The next step of improvement was about the acquisition of some rudimentary sensitivity to direction of light and direction of movement of , say, a menacing shadow. The minimal way of achieving this is to back the photocells with a dark screen on one side only. A transparent photocell without a dark screen receives light from all directions and cannot tell where light is coming from. An animal with only one photocell in its head can steer towards, or away from, light, provided the photocell is backed by a screen. A simple recipe for doing this is to swing the head like a pendulum from side to side.
A slightly more evolved way to detect the direction of light came with having more than one photocell, pointing in different directions, and each one backed by a dark screen. Then by comparing the rates of photon rain on the two cells, one could make inferences about the direction of light. If you have a whole carpet of photocells, a better way is to bend the carpet, with its backing screen, into a curve, so that the photocells on different parts of the curve are pointing in systematically different directions. That’s how the light-sensing organ of these earlier life-forms started bending to form curves.
Image Formation
So far the eye (if at all it was there) was only capturing photons and telling its direction. It wasn’t forming any image, which would be a huge mountain to climb for the earlier species.
Why is there no image-formation?
The simple answer is, the eye is seeing too much. Every bit of an object would send a ray to every other direction at the same time. So the eye was seeing an infinity of objects, which means it wasn’t seeing anything clearly. The obvious solution to this problem was to subtract: to cut out every other image except one. This happened gradually as the aperture had narrowed to a pinhole. As the pinhole became extremely small, the blurring disappeared and a single, sharp picture of an object remained.
At the same time, the eye needed to come with some kind of lens so that it could focus well. That took quite a lot of time. About 364,000 generations to evolve a good fish eye with lens. Wait, there is more. In order to focus rays that are coming from a very distant target, you need a weaker lens than to focus rays that are coming from a close target. So we needed more advancements, which happened, once again, over a long period of time. Mammals and birds started doing it by means of muscles that pull on the lens and change its shape a little. Chameleons, snakes, fishes and frogs would do it in the same way a camera does, by pulling the lens a little way forwards or backwards.
There’s More
The above is only a simplified version of the evolution of human eye. Also, one must remember that eyes have evolved over forty times independently in various parts of the animal kingdom. In some cases these eyes use radically different principles. The important thing to note here is that although we may consider one eye more evolved than the other, but for a particular animal, its own eye would be ideal given the way it survives and procreates. For instance, snails might have a lot of advantage if they had eyes like ours, but how would they carry them around?
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