Algae have eyes.

They don't have eyes exactly the way humans do, but, as a team of scientists discovered, there is a common one-celled alga with a visual system that works much like the human eye's retina to convert light into electrical signals that govern the organism's behavior.

It had long been known that the alga, a pond-dwelling species called Chlamydomonas, has a light-sensitive "eyespot." The discovery is that the eyespot uses the light-sensitive chemical -- a visual pigment called rhodopsin -- that is in the retinas of animals, including humans.

Although it is a green plant, Chlamydomonas has two threadlike flagella that lash about to pull it through the water much as do the arms of a human swimmer. The organism senses the amount of light coming through the water and steers itself up or down to find the optimal level of light needed for photosynthesis, the solar-powered process that all green plants use to manufacture sugar.

Because algae are among the earliest forms of life to appear on the Earth, the discovery suggests that rhodopsin was "invented" quite early in the process of evolution. The eyes of higher organisms simply are more versatile ways of using rhodopsin's remarkable ability to produce an electrical signal when struck by light.

The eyes of humans and other animals have millions of light-sensitive cells, each sending a different signal to the brain where the pattern of simultaneous signals is interpreted as a picture. Chlamydomonas, with just one light-sensitive spot and no brain, sees no picture but nevertheless uses the signal to guide its movement.

The algal eye can detect light that is only 1 percent as strong as needed to carry out photosynthesis. This directs the organism to swim toward the light. But if the light is too bright, photosynthesis stops and the alga swims toward a more optimal level.

Nobody knows exactly how it happens, but the signal -- a burst of electrically charged atoms -- apparently sets in motion a chain of chemical reactions that determines which of the two flagella strokes more strongly through the water. The choice determines whether the little cell swims up or down.

The discovery was made by Kenneth W. Foster, now at Syracuse University, and a team of six others at Columbia University and the City University of New York, Foster's former school.

"In a way," Foster said, "this sort of vindicates Gottfried Ehrenberg, who named Chlamydomonas back in 1831. He wrote that they had an eye and could see, but everybody thought he was crazy."

Foster said that although the rhodopsin of humans and algae appear similar in molecular structure, it is likely that slight differences will be found. "In any case," he said, "it seems quite likely that we share a common ancestry with Chlamydomonas. We both inherited versions of the rhodopsin molecule."

Foster established the existence of rhodopsin in the alga by doing experiments with a special strain of blind Chlamydomonas. The strain was produced some time ago by other researchers who exposed normal forms to chemicals that cause mutations. The algae were unable to find proper light levels and survived only because researchers provided it in aquariums.

Foster had reason to believe that the algae were blind because the mutation rendered them unable to manufacture an essential component of the rhodopsin molecule, a smaller molecule called retinal. This is the substance, derived from vitamin A, whose absence in the diet can cause blindness in humans. When retinal was added to the water, the algae quickly acquired the ability to swim to optimal light.

Foster concluded that the algae had absorbed the retinal and completed their manufacture of rhodopsin. As Foster put it, "they were cured. They could see again."