Scientists track early evolution of sight

This member of the hydra family is a distant descendant of the very first animal to gain the ability to detect light.
This member of the hydra family is a distant descendant of the very first animal to gain the ability to detect light. David Plachetzki / University of California - San Diego/

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 simple trick 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 at 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.

The discovery also helps to counter one of the principal arguments used by anti-evolutionists to discredit Charles Darwin's theory and to support their belief in ``intelligent design.''

The skeptics contend that it would have been impossible for an eye to form in a series of small steps, by a purely natural process, as Darwin suggested almost 150 years ago. Therefore, they conclude that a supernatural ``designer,'' presumably God, must have done the job.

Like most modern biologists, Oakley vigorously disagrees that a designer is necessary to explain evolution. ``There is no doubt whatsoever in the ability of evolutionary processes to produce all the diversity of life we see,'' he said in an e-mail.

In their research, Oakley and Plachetzki discovered that a gene called 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 off from the line that led to sponges.

Sponges are the most primitive group of all animals. Hydras belong to the next oldest group — called ``cnidarians'' — which includes jellyfish, corals and sea anemones.

``We chronicled when and how animals went from lacking opsin genes to possessing multiple opsin genes with different functions,'' Oakley said. ``We put a timeline on the origin of light-sensitive opsin genes.''

Opsin genes direct the production of light-sensitive proteins, also called opsins, that coat the surface of a hydra, especially around the mouth area. 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.

According to Carroll, the opsin genes would have been created by evolutionary ``tinkering'' and didn't ``require the intervention of a 'designer' to craft specific protein interactions.''

Cook, the NSF program director, said Oakley's work ``shows how simple genetic changes can produce visual pigments that begin the pathway to the evolution of sight.''


More information on hydras. An explanation of how opsins work.