Carl Zimmer has an interesting article over at the New York Times. He discusses the evolution of eye… but how many eyes? Some of our early ancestors, about 560 million years ago, might have had only one. During the Cambrian Period, 518 million years ago, some early vertebrates from China may have had two pairs of eyes, one on each side and a second pair on the top of their heads.

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 suggest 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. Darwin couldn’t say for sure what that sequence of changes was.