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STANFORD - Eyes, the premier outpost of the brain, are so important for survival they have evolved 60 to 80 times in the history of the earth.
Given this record of optical innovation, biologists were surprised recently to find some elements of the eye have been preserved throughout evolution. Fruitflies and cows, octopuses and humans, all use the same type of enzyme to begin the process of seeing, Stanford University neurobiologist Russell Fernald and Michael Land of the University of Sussex in Brighton, England, have reported from work of scientists using the tools of molecular biology.
These enzymes are conjugated proteins called opsins. They are components of the visual pigment that catches photons of light, Fernald said. Despite the existence of at least 10 optically distinct eyes, when researchers compare opsins from various species, these molecules all are related, as are the chromophore molecules they use, which are descendants of one of four close relatives of vitamin A.
The chromophore accepts the photon of light, flipping the opsin molecule and triggering a biochemical cascade that ultimately excites the receptor cell, Fernald said.
What happens next depends on the optical structure or organ the species has invented to aid its particular vision needs.
"One of the nice things about studying the eye is that there are a limited number of ways you can deal with the physics of light - you've got to collect it, and bend it and focus it," Fernald said. "And, except for the chambered nautilus, no animal has survived without inventing a lens to do that."
The chambered nautilus of the South Seas sees poorly through a pinhole, similar to the simple cameras children make in basic science class.
What there is for the lenses of other animals to see is in the eye of the beholder.
Horizontal, ribbon-like scanning retinas help many sea birds watch a few degrees around and below the horizon.
One surface-feeding fish has two horizontal scanning streaks separated by about 40 degrees, allowing it to view the surface from above and below. Fish that live in holes and cracks in a reef have a more circular field of vision.
Land-walkers, such as humans, had to deal with danger and opportunity lurking at their sides - hence the evolutionary success of those with elliptical-shaped peripheral vision.
At the molecular level, however, the photon-capturing opsins all consist of seven transmembrane helices with short loops on each side, Fernald said. The loop between helices 1 and 2 and the attachment location of the chromophore in helix 7 are similar from all examined species of vertebrates, insects and octopuses, whose ancestors diverged 500 million years ago.
"Based on the degree of similarity in their DNA, they must share a common ancestry," he said.
Opsins are not the only visual proteins with an interesting history, however. And some others - those that form lenses in different species - leave Fernald and Land with lingering doubts about a shared ancestry for eyes.
The lenses in vertebrates are formed from cells that contain high concentrations of soluble proteins known as crystallins, and the distribution of these proteins is responsible for the gradient refractive index of their lenses.
Insects, fish and octopuses borrowed differing proteins from various organs to make their lenses, but all somehow arranged those proteins to have the necessary gradient refractive index to bring light beams into focus, he said.
"They all converged on a nearly optimal lens protein distribution, which is remarkable," he said, given the many choices available.
"If eyes contain an opsin that's clearly been conserved and lens proteins that are radically different among species, there are two ways to think about it," he said.
Conventional evolutionary science would assume "all eyes came from the same origin even though their lens materials are so different," he said. Each surviving species kept a few useful constituents in rebuilding its optical structure again and again.
"Another possibility is that the opsins were 'rediscovered' again and again," Fernald said. "Although that seems somewhat improbable, it would not be so improbable if it turned out there were only a limited number of ways to capture light."
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