Nascent Drama

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"The important thing about this study is it delivers evidence of those intermediates," said Matt Friedman of The Field Museum and the University of Chicago, whose study appears in the journal Nature.

 

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This missing link in the evolution of flatfishes has been seen as a hole in the theory of natural selection.

 

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The argument is that intermediate forms of these fish could not exist because there would be no survival benefit from having one eye that was slightly off center, but still on the opposite side of the head.

 

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Biologists have theorized that maybe the changes occurred all at once with a large-scale mutation.

 

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According to this popular "hopeful monster" theory, flatfishes developed this weird trait, which luckily turned out to be very useful.

 

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Friedman's find now suggests that flatfishes followed a more conventional evolutionary plan.

 

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"There was no macromutation that all of a sudden gave them both eyes on the same side of the head," he said in a telephone interview.

 

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EYE MIGRATION

 

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More than 500 species of flatfishes live in fresh and salt water.

 

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They are born with eyes in the normal spot, but one eye gradually migrates to the other side of the head.

 

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"Every flatfish is born symmetrical before it becomes a perversely asymmetrical adult," Friedman said.

 

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Friedman examined specimens of two kinds of fossil fishes from the Eocene period in northern Italy.

 

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One was a new genus that Friedman named Heteronectes or "different swimmer."

 

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"It is a fossil I found unloved in a museum drawer in Vienna covered with about a century of industrial-era soot."

 

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The other fossil, Amphistium, has been incorrectly assumed to have a symmetrical skull, but Friedman noticed that in some fossils, the eye was slightly out of place.

 

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That inspired him to use computed tomography or CT scans to get a better look at the skulls of these fossils.

 

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What he found is that one eye had begun migrating, but had not quite crossed the middle of the head.

 

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"It's not quite in the Cyclops position," Friedman said.

 

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The find raises the question of why this bizarre intermediate form developed.

 

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"It turns out they don't lie flat and completely prone on the sea floor.