The rise of eyes began with just one

Even Charles Darwin was puzzled by the evolution of the vertebrate eye. New research suggests that it traces back to a cyclopean invertebrate with a single eye atop the head

- CARL ZIMMER

LOOK AT JUST about any vertebrate and you'll see two eyes looking back at you. Falcons circling overhead have two eyes, just like hammerhead sharks roving through the ocean.

Scientists have long puzzled over how the vertebrate eye first evolved. A pair of new studies suggests a strange beginning: Our invertebrate ancestors 560 million years ago were cyclopes, with a single eye at the top of their head, scientists now propose, that only later split in two.

Charles Darwin fretted a lot about the exquisite complexity and sophistication of the vertebrate eye as he developed his theory of evolution. “The eye to this day gives me a cold shudder,’ he confided to his friend, the American botanist Asa Gray, in 1860. Somehow evolution had produced the eye from many parts, such as the lens and retina, through tiny changes through the generations.

Yet opponents of evolution continued to cast doubt on the idea that eyes could evolve. Even in the 1990s, creationists claimed that natural selection would need many billions of years to produce an eye — far more time than life has existed on Earth. Dan-E Nilsson, a neurobiologist at Lund University in Sweden, grew so annoyed by these claims that he estimated how long it would actually take for a patch of light-sensitive cells to evolve into an image-forming eye. In 1994 he and Susanne Pelger, a colleague at Lund, concluded that an image-forming eye could evolve in just a few hundred thousand years. “It’s not precise in any way at all, but it goes to show that there is plenty of time for eyes to evolve,’ Dr Nilsson said.

The model only addressed how the shape of eyes evolved. In actuality, many other changes occurred along the way. New proteins had to emerge that could bend light in the lens, for example, while others absorbed light in the retina.