On The Evolution Of The Eye

At a discussion group round a friend's house the other night a Christian friend of mine made a comment about the evolution of the eye, doubting whether such a complex and 'well-designed' organism could have arisen through natural selection.

It's true the eye is well designed (although it's far from perfect), and it's also true that eyesight is hugely beneficial to many creatures. But it's not true that that means evolution probably didn't produce it - in fact, it is because eyesight is so beneficial to so many creatures that evolution did bring about the design of eyes over time. Not only did eyesight evolve very slowly over millions of years in stages, the story of eyesight evolution started with something very primitive.

Studies of biology and taxonomy reveal a rich array of organisms alive today that demonstrate the intermediate stages of eyes. We can even sequence their DNA and see which genes give rise to those stages. For example, the first step is something like an amoeba which has a photosensitive area inside it that can detect light. Then evolution brings about a surface patch that improves further the detection of light. Then generations later we find that the surface patch is indented in a cup shape which allows the patch to crudely detect the direction from which the light is coming. So we’ve gone from detection to direction – that’s already an improvement for the organism. 

One can fairly easily imagine some kind of crude skin-form with the kind of accretive layering that could facilitate the emergence of a light-sensitive spot. All that would be needed would be a combination of pigmented cells with light-sensitivity ratcheted onto it, and the formation of translucent cells that could encase the pigmented cells and give the structure a degree of protection. It may well take a few hundred thousand generations for even a crude eye to evolve - but as long as we can conceive of the principle for functional improvement, we can imagine the steps required*. 

Translucent cells subject to mutations would have an index of refraction, because refraction is based on indices where the n of a substance (that is, the optical medium) defines how radiation (in this case, light), propagates through that medium. Once the cells underwent variations in size and density and became more cup-like we would find improved ability to facilitate dispersion, which means varying wavelengths and (in the higher forms) emergence of colour.  Mutations that improve the organism would survive due to evolution's ratchet mechanism, and further improvements would emerge. 

We know quite unequivocally how two light sensitive patches would have evolved, and how tiny improvements in the ability to detect light and then actually discern the movement of a predator or locate food would confer very strong advantages to the organism. 

Later on what’s then facilitated is a strong genetic tendency in the more developed organism for bilateral symmetry (a body divided into a left half and right half). Dividing the organism down the middle from front to back you will find that the two halves are a mirror image of each other on the outside (that’s two sets of legs, two feelers, two sets of fins or something of that kind) – and quite naturally, a pair of photoreceptors or cupped receptors – with this eventually leading to functioning eyes. 

But there is a bigger picture too that mustn't be missed. The eye is but one microcosmic example of the sublime beauty of God's creation through evolution. It truly is a remarkable narrative that every living thing, with such a rich multitude of shapes, sizes and colours - plants, trees, flowers, fruit, vegetables, insects, fish, dinosaurs, lizards, elephants, giraffes, hamsters, whales, sharks, chimpanzees, dogs, cats, lions, tigers and humans - all came about from a common ancestor, and then billions of years of time and adaption. Evolution really is a quite magnificent story of beauty.

* Not only has the eye evolved independently about 40 times - we now have the precise computational facilities to map out a trajectory for the evolution of the eye. This has been done numerous times - most notably by Dan Nilsson and Susanne Pelger, but on other occasions too.