Evolution and Intelligent Design

 The eye, as we all know, is the sensory organ responsible for perception of light. In the higher organisms especially, the eye is an optical system so complex that even Charles Darwin quoted that “The evolution of eye by natural selection at a first glance seems absurd in the highest possible degree.” However, he went on to quote if the evolution of eye had occurred in steps- from a simple, imperfect design, which nonetheless conferred some advantage to the organism- its evolution seems perfectly feasible. This is exactly what happened millions of years ago!

In the beginning, life was blind. Then one day, about a 500 million years ago, there occurred a mutation in the DNA of a microscopic bacterium, which led to the translation of a photosensitive protein. This protein helped the bacterium to distinguish light and darkness, which was sufficient for synchronisation of the circadian rhythm. This bacterium then propagated its DNA through the process of replication and over time the photosensitive proteins became concentrated at a region forming the eyespot. This eyespot can be seen even today in a unicellular eukaryote, Euglena. The eyespot allowed Euglena to respond to the light, move towards it for photosynthesis.

Next, the flat eyespot deepened to form a cup- the cup allowed the organism to sense the direction from which light was coming, because unlike a flat surface, in a cup, the photosensitive pigments that are stimulated would depend on the angle of the incident light. These ‘pit eyes’ are seen in invertebrates like Planaria. This significant change in the design of eye, allowed the organism to sense in a very crude manner in which direction a predator is present and evade it, giving it the evolutionary advantage for survival.

Consequently, to reduce the interference between the light waves, the opening invaginated and over several generations, it formed a pinhole to achieve better imaging. Though the image obtained was very dim, it gave an image that was in focus.

However, the key structure to the development of the eye as we know it today, is the formation of lens. This is assumed to have evolved from a transparent protective layer which helps prevent infection- what we call today as the cornea. Soon, a fluid began accumulating within the chamber. This ‘transparent humor’ allowed the eye for optimisations in filtering, blocking UV radiation. Near the opening, crystalline proteins formed a structure- the lens, which helped in focusing light at a single point in the photosensitive layer- which developed to form the retina. The lens provided with both brightness and sharpness. Further, it gave the ability to see near and far objects through the process of accommodation with the development of ciliary muscles. However, it is important to note that such an evolution of eye would not have been possible without the development of the nervous system to process the information perceived by the eye. Thus, the photosensitive layer developed to form the retina with several layers, each of which help in processing the light energy to electrical energy to give the input to the optic nerve.

Albeit, this seems to be the general scheme of evolution, the eye appears to have evolved convergently, i.e. it has evolved independently in different species to give analogous eyes. This explains the reason for existence of several different types of eye- like the compound eyes seen in insects, reflector eyes seen in cats and dogs, the vertebrate eye (includes human eye) and the cephalopods’ eye, to name a few.

Human Beings are among the younger species, comparable to a newly formed branch in the Tree of Life. However, the human eye is far from perfect. It is in fact an example for a clunky design performing well in function. The retina of the vertebrate eye in fact is one of the best examples for a poor design. The retina is made of several layers- starting with the photosensitive layer to the nerve fibre layer. However, in the vertebrate eye, the photosensitive layer away from the direction of light with the nerve fibre layer lying inward. As a result, the photons of light must travel around the bulk of retinal layers to hit the receptors at the back. This is in fact comparable to a microphone held in the wrong direction. It would still work, yes but the efficiency is highly decreased.

Since the nerve fibre layer is facing inward, their axons converge at a point in the centre of the retina. At this point, there are no photoreceptive cells and hence it cannot perceive light. This is what we call the blind spot! Since we have two eyes to compensate and a brain to fill in the details of the picture, we don’t really notice them.

However, when we take the example of a cephalopod, the eye of an octopus, for instance, its retina is ‘straight’ unlike our ‘inverted’ retina. This also gives it the advantage of the absence of blind spot in its eye.

It would seem that only octopuses have better eyes than humans, right? But no, apparently even dogs, cats, cattle, deer and several nocturnal animals have better vision than ours. These animals possess reflector eyes. In their eye, behind the retina, there exists a reflecting layer called ‘tapetum lucidum’. This layer reflects the light back to the retina, thereby increasing the light available to the photoreceptor cells. This provides these organisms with a night vision far superior to ours. And it is also this layer that makes the eyes of these organisms, glow in the dark!

Birds too in general have better visual acuity (ability to see things from a distance without blurriness), can see colours on an even broader spectrum with a wider field of view. Some birds have an additional translucent eyelid, that allows them to look at the sun, at length without damaging its retina.

Another interesting design of the eye is the compound eyes which insects possess. Almost all of us have the experience of trying to swat a fly, right? But how many of us were really successful? I would say very few, almost none. So, how is it that the fly is always able to get the better of us? The flies have compound eyes, where in the eye is divided into smaller subunits called ommatidia. Each ommatidium functions as an independent eye, with its own cornea, lens and photoreceptor cells. The image is formed by each subunit and the brain forms an image from these independent images. Since, each ommatidium is oriented in different directions, the fly is capable of sensing even a small flicker of light and evades its predator.

So, if given the chance to design the human eye, would you do it differently? This is what doctors and biomedical engineers today are trying to do by comparing different eye structures. And perhaps, one day, the engineered eye, might prove to be better than the evolved one.