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Evolution of Flatfish Eyes a Puzzle No More

July 11, 2008

By WILLIAM MULLEN

By William Mullen

Chicago Tribune

CHICAGO

Some dusty fossil fish spotted by a sharp-eyed University of Chicago doctoral student as he rummaged through forgotten corners of museum collections in Europe have solved a question that has long vexed scientists.

The puzzling question was: How did flatfish, a bizarre, highly specialized group of bottom-feeding fish – sole, plaice, turbot, flounder and halibut among them – end up with both of their eyes on one side of their faces?

Scientists have until now largely assumed the asymmetrical, one- sided eye arrangement was a trait that must have arisen suddenly in flatfish, because they could not see a benefit for the fish if it took millions of years for an eye to migrate from one side to the other. Even Charles Darwin had trouble answering critics who used flatfish and their strange eyes as an argument against his evolutionary theory after he published it in 1859.

Fossils of two long-extinct flatfish species found in European museum collections by Matt Friedman, 28, a doctoral candidate at Chicago, should change some minds. Friedman spotlighted the two species in his article, “The Evolutionary Origin of Flatfish Asymmetry,” which appeared in Thursday’s edition of the science journal Nature.

The evidence that Friedman has gathered proves, he said, that it indeed took millions of years for the flatfish to evolve their look.

His findings may not be earth-shattering, but they seem to solve one of the oldest puzzles of evolutionary biology.

“Matt’s article is extremely significant,” said Thomas J. Near, a Yale University evolutionary biologist who also studies fish.

“The obvious is that Matt’s work shows how the flatfish eye got to the other side of the head,” Near said, “and that it was a gradual change rather than a single, mutational big jump.

“It’s also another piece of knowledge and evidence that we didn’t have before. The creationism/intelligent design movement has always pointed out that there was nothing in the fossil record that showed the (movement of the flatfish eye) in transition. The real scientific impact from Matt’s work is that there is now an intermediate form.”

Friedman made the discoveries while researching his doctoral thesis, which is a study of the evolution of modern diversity of spiny finned fishes.

“This work on the flatfish is only one chapter in the overall thesis,” he said, “a case study of a very strange body plan for a fish.”

It is a body plan that has utility. Flatfish spend their entire lives lying on one side, feeding on food they find on the seafloor. Rather than have one eye uselessly buried in the sand, both eyes are on top , where they better spot predators.

Though Friedman has done paleontological field work, excavating fossils from rock formations, he has found better luck searching through tens of thousands of fossil fish excavated by earlier generations of paleontologists, who stored them in museum collections where they were seldom or never looked at.

He decided to try to track down very early flatfish fossils to see whether he could find any specimens in an intermediate state in which the eye had started to move.

“I was looking through a strange, obscure book from Germany with photos of fossil fishes,” Friedman said, “when I came on a photo that looked suspiciously like a flatfish but was classified as something else.”

It was a specimen from the genus Amphistium, a 50-million-year- old fossil first known from a woodcut made in the late 1700s of a fossil recovered from a limestone quarry near Verona, Italy.

His first “transitional” fossil was an adult specimen he stumbled on in the Paris Museum, part of a collection that had been given to Napoleon as tribute as he fought in Italy 200 years ago. It clearly had an eye socket near the top of the skull, he said, in transit to the other side of the face.

In London, Friedman found a second fossil species of Amphistium, slightly younger than the one recovered in Italy. In London he also borrowed two Amphistium specimens still deeply embedded in Italian sandstone, which he analyzed with CT scans, also revealing eye sockets in transition.

In Vienna, he found misclassified fossils that turned out to be an entirely new flatfish genus he named Heteronectes (“different swimmer”) which he said clearly is in the transitional stage.

“There is a broad implication for the tempo and mode of evolution here,” Friedman said.

After publishing his evolutionary theory, “On Origin of the Species,” Charles Darwin and the theory came under furious attack by religious leaders. In 1871, St. George Mivart, a Catholic lawyer and zoologist, published “On Genesis of the Species,” challenging Darwin, prominently using the example of flatfish and their eyes in his argument.

“Darwin feebly responded with a scenario that relied on evolution of inherited traits,” Friedman said. The flatfish argument has been an arrow in the quiver of anti-evolutionists ever since, cited as recently as 2003 in pro-creationist James Best’s online book “God and Fallacy in the Theory of Evolution.”

puzzling placement

Flatfish feed off organisms and nutrients that settle to the bottom of water bodies. The difficulty in understanding why flatfish have both eyes on one side of their head was used as an argument against Charles Darwin’s theory of evolution.

Originally published by BY WILLIAM MULLEN.

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