Scientists track early evolution of sight to hydras

WASHINGTON — Scientists have traced the origin of eyes back to a transparent blob of living jelly floating in the sea about 600 million years ago.

That creature, the distant ancestor of a modern freshwater animal known as a hydra, could only distinguish light from dark.

But that was such an advantage that it was passed on from generation to generation of the hydra's cousins and their myriad descendants. It was the precursor of the wildly different, ever more complex eyes of fish, ants, flies, giraffes and people.

The hydra work was reported last month in the journal PLoS ONE by biologists David Plachetzki and Todd Oakley, of the University of California, Santa Barbara.

It helps solve one of the puzzles of Darwinian evolution, the process by which a complex organ such as an eye could arise by random genetic mutations and natural selection.

"These results are significant in advancing our understanding of the early evolution of sight in animals," said Jerry Cook, a program director at the National Science Foundation, which financed the work.

In their research, Oakley and Plachetzki discovered that a gene, opsin — after the Greek word "ops," meaning "eye" — exists in hydras but not in sponges, an even more primitive animal.

The scientists calculated that opsin genes appeared about 600 million years ago, because that's when the evolutionary branch that led to modern hydras split from the line that led to sponges.

Opsin genes direct the production of light-sensitive proteins, also called opsins, that coat the surface of a hydra, especially around the mouth. The opsin proteins would help these simple animals tell night from day and perhaps help them find food.

"Hydra probably uses its light sensitivity to find prey," Oakley said.

According to Oakley, the opsin proteins must have evolved from earlier "signaling" proteins that send chemical messages to other proteins. Signaling proteins exist in all living creatures, from single-celled bacteria to humans.

Other biologists commended Oakley's work. "It makes sense that oceangoing animals such as [the hydra's ancestors] would use light detection to orient themselves or regulate a body clock," said Sean Carroll, an evolutionary geneticist at the University of Wisconsin, Madison.