The earliest fossil evidence of eyes comes from the Cambrian explosion, around 500 million years ago, their appearance probably driven by the rapidly escalating evolutionary arms race between predators and prey at that time.
From simple eyespots, eyes have evolved into organs of such complexity and precision that they have been put forth as evidence of intelligent design, most famously in William Paley’s 1802 watchmaker analogy. Paley argued that someone coming across a watch on the ground would naturally assume that the watch had a maker. Unlike a nearby rock, the watch is too complex to have formed by natural processes. By the same logic, examples of complexity from life, such as the eye, imply a designer – God.
As has been pointed out in rebuttals from Darwin to Dawkins, given enough time, complexity can arise through the natural process of evolution.
Recent discoveries have shown that eyes are even more complex than previously thought. Ironically, at least some of the complexity appears to have arisen due to ‘flaws’ in the structure of the vertebrate eye.
The vertebrate retina is often said to be built backwards. The light-detecting photoreceptors are at the back of the retina, furthest away from the point at which light enters the eye. Between the photoreceptors and the light source is a mess of neurons and blood vessels that actually obscure the light path.
A 2009 study by Solovei et al found that mice have an unusual distribution of DNA within their photoreceptor cells. Normally within a cell, islands of condensed DNA are distributed within a sea of diffuse DNA. In mouse photoreceptors however, the condensed DNA forms a single central blob. This pattern is not seen in cells from any other part of a mouse’s body and is virtually unknown in multicellular animals.
The group looked at photoreceptor cells from other animals and found a strange correlation. The central blob pattern of DNA also occurred in some other mammals but was absent in fish, birds, reptiles and other vertebrates.
Below is a list of the mammals with and those without the unusual pattern. The members of each group are united by a common behavioural characteristic.
|Conventional pattern||Unusual pattern|
|Wild boar||Domestic cat|
|Domestic pig||Red fox|
|Chipmunk||Velvety free-tailed bat|
|Crab-eating macaque||Colugo (flying lemur)|
Only nocturnal and crepuscular (active at dusk and dawn) mammals have the unusual pattern. There were a few exceptions (horses are crepuscular but have the conventional pattern; cows are diurnal and have an intermediate pattern)
Further analyses showed that the central blob pattern minimises scattering of light passing through the photoreceptor cells. It is most likely an evolutionary adaptation that improves the efficiency of the retina in low light conditions.
Early mammals avoided competition with dinosaurs by exploiting a nocturnal niche. As a result, they developed adaptations to nightlife such as endothermy and a heightened sense of hearing. The changes to photoreceptor DNA probably appeared at this time. After the extinction of dinosaurs, some mammals became predominantly diurnal and reverted to the conventional DNA distribution.
The mammalian eye is a work in progress. Unfortunately, much of the architecture is firmly embedded and so, when circumstances change, some aspects become anachronistic. Any adaptations have to work around these inherent imperfections, a problem that an omniscient designer would be unlikely to face.