Seeing The Light

Critics of Darwin like to ask how something so complex and as apparently perfect as the eye could possibly have evolved gradually, and the man himself devoted several pages of later editions of On the Origin of Species to refuting their arguments. Perhaps he need not have worried. Eyes are complex, but their structure reveals the unplanned nature of evolution.

The most famous flaw is found in vertebrate eyes. Their light-sensing structure, the retina, is wired up back-to-front, with the light-sensitive cells behind the nerves and blood vessels that support it. Not only does light have to pass through this layer first, obscuring the image, but the nerves and blood vessels have to dive through the retina, creating a blind spot in each eye.

In cephalopods, such as squid and octopuses, the eyes are built the "right" way around, so why not in vertebrates too? The answer is that when eyes first evolved in the ancestors of modern vertebrates, the retina arose from an infolding of the developing brain, and the cells that could form light receptors happened to end up on the inside of this fold. "Once you have done something like this it's very hard to change," says Michael Land, a specialist in eye physiology at the University of Sussex, UK. ¹

IT LOOKS wrong, but the strange, "backwards" structure of the vertebrate retina actually improves vision. ²

However, Kenneth Miller, a biologist at Brown University in Providence, Rhode Island cautions that this doesn't mean that the backwards retina itself helps us to see. Rather, it emphasises the extent to which evolution has coped with the flawed layout. "The shape, orientation and structure of the Müller cells help the retina to overcome one of the principal shortcomings of its inside-out wiring," says Miller. ⁴

THE eye has long been an evolutionary battleground. Ever since William Paley came up with the watchmaker analogy in 1802 - that something as complex as a watch must have a maker - creationists have used it to make the "argument from design". Eyes are so intricate, they say, that it strains belief to suggest they evolved through the selection and accumulation of random mutations.

Recently, evolutionary biologists have turned this argument on its head. They say that the "inside out" vertebrate retina - curiously structured so that its wiring obscures the light sensors and leaves us with a blind spot - can be described as one of evolution's "greatest mistakes".

The anatomy of the retina is indeed good evidence that eyes were cobbled together bit by bit. Surely a creator would never have chosen to construct an eye in this way. In return, creationists have argued that the backwards retina clearly has no problems providing vertebrates with excellent vision - and even that its structure enhances vision.

This week, a study by (non-creationist) neurophysicists in Israel has found just that (see "Optical fibre cells transform our weird, 'backward' retinas"). Their simulations showed that Müller cells, which support and nourish the neurons overlying the retina's light-sensitive layer, also collect, filter and refocus light, before delivering it to the light sensors to make images clearer. ⁵

The new understanding of the role of Müller cells might find applications in more successful eye transplants and better camera designs, says Ribak. ⁶

“The findings do not mean the creationists have a point - although they may well quote the study” ⁷