Genetic Discovery Suggests Humans Were Once Nocturnal

Forget Fido. Bats, opossums, and raccoons may have once been man's best friend.

Biologists say the discovery of a new photoreceptor gene in fish, birds, and amphibians implies that humans may have gone through a nocturnal phase in the course of evolution.

The recently discovered gene codes for melanopsin, a pigment that makes cells in the eye responsive to light and helps regulate circadian rhythms. The gene, found in non-mammalian vertebrates, matches the mammalian melanopsin gene. That match led researchers to conclude that a melanopsin gene previously found in frogs is missing from the mammalian genome. The paper, which suggests that mammals may have lost their second melanopsin gene over the course of evolution, was published in the August issue of the journal PLoS Biology.