Inspired by the intricate wonders of the natural world, leading science writer Richard Dawkins argues that evolution is the only known possible theory that could solve the mystery of our existence. In this extract from Climbing Mount Improbable, published by Penguin in 1997, Dawkins considers the human eye, revealing how the slow but re-morseless process of natural selection has
led to incredible complexity and diversity.

Accepting the limitations of the metaphor of Mount Improbable, let's go right down to the bottom of the vision slopes. Here we find eyes so simple that they scarcely deserve to be recognised as eyes at all. It is better to say that the general body surface is slightly sensitive to light. This is true of some single-celled organisms, some jellyfish, starfish, leeches and various other kinds of worms. Such animals are incapable of forming an image, or even of telling the direction from which light comes. All that they can sense (dimly) is the presence of (bright) light, somewhere in the vicinity. Weirdly, there is good evidence of cells that respond to light in the genitals of both male and female butterflies. These are not image-forming eyes but they can tell the difference between light and dark and they may represent the kind of starting point that we are talking about when we speak of the remote evolutionary origins of eyes. Nobody seems to know how the butterflies use them, not even William Eberhard, whose diverting book, Sexual Selection and Animal Genitalia, is my source for this information.

If we think of the plain below Mount Improbable as peopled by ancestral animals that were totally unaffected by light, the non-directional light-sensitive skins of starfish and leeches (and butterfly genitals) are just a little way up the lower slopes, where the mountain path begins. It is not difficult to find the path. Indeed it may be that the 'plain' of total insensitivity to light has always been small. It might be that living cells are more or less bound to be somewhat affected by light - a possibility that makes the butterfly's light-sensitive genitals seem less strange. A light ray consists of a straight stream of photons. When a photon hits a molecule of some coloured substance it may be stopped in its tracks and the molecule changed into a different form of the same molecule. When this happens some energy is released. In green plants and green bacteria, this energy is used to build food molecules, in the set of processes called 'photosynthesis.' In animals the energy may trigger a reaction in a nerve, and this constitutes the first step in the process called seeing, even in animals lacking organs that we would recognize as eyes. Any of a wide variety of coloured pigments will do, in a rudimentary way. Such pigments abound, for all sorts of purposes other than trapping light. The first faltering steps up the slopes of Mount Improbable would have consisted in the gradual improvement of pigment molecules. There is a shallow, continuous ramp of improvement - easy to climb in small steps.

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