Fish Fossils Plug Hole in Evolutionary Theory
By Julie Steenhuysen, Reuters
July 9, 2008
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This undated handout image shows an evolutionary tree with the gradual anatomical
changes in the evolution of flatfishes resulting in the origin of their extraordinary, modern
body plan. Some odd-looking fish fossils discovered in the bowels of several European
museums may help solve a lingering question about evolutionary theory, U.S.
researchers said on July 9, 2008. (M. Friedman/The Field Museum/Handout./Reuters)
CHICAGO (Reuters) - Some odd-looking fish fossils discovered in the bowels of several
European museums may help solve a lingering question about evolutionary theory, U.S.
researchers said on Wednesday.
The 50 million-year-old fossils -- which have one eye near the top of their heads -- help
explain how flatfish such as flounder, sole and halibut developed the strange but useful
trait of having both eyes on one side.
For flatfish, which lie on their sides at the bottom of the sea, this arrangement gives them
the use of two watchful eyes.
But the trait has posed a problem for evolutionary biologists because no one had found
any so-called transitional fossils -- fossils showing intermediate steps in the evolution of
this trait.
"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.
This missing link in the evolution of flatfishes has been seen as a hole in the theory of
natural selection.
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.
Biologists have theorized that maybe the changes occurred all at once with a large-scale
mutation. According to this popular "hopeful monster" theory, flatfishes developed this
weird trait, which luckily turned out to be very useful.
Friedman's find now suggests that flatfishes followed a more conventional evolutionary
plan. "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.
EYE MIGRATION
More than 500 species of flatfishes live in fresh and salt water. They are born with eyes
in the normal spot, but one eye gradually migrates to the other side of the head.
"Every flatfish is born symmetrical before it becomes a perversely asymmetrical adult,"
Friedman said.
Friedman examined specimens of two kinds of fossil fishes from the Eocene period in
northern Italy. One was a new genus that Friedman named Heteronectes or "different
swimmer."
"It is a fossil I found unloved in a museum drawer in Vienna covered with about a century
of industrial-era soot."
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.
That inspired him to use computed tomography or CT scans to get a better look at the
skulls of these fossils.
What he found is that one eye had begun migrating, but had not quite crossed the
middle of the head.
"It's not quite in the Cyclops position," Friedman said.
The find raises the question of why this bizarre intermediate form developed. "It turns out
they don't lie flat and completely prone on the sea floor. They actually will prop
themselves up slightly (with their fins)," Friedman said.
Once in that position, having a slightly asymmetrical eye arrangement must have proved
advantageous, he said.
But Friedman cautioned about making too many inferences.
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"Our inability to imagine is what got us into this predicament," he said, referring to the
whole flatfish debate.
(Editing by Maggie Fox)
Flatfish Evolution was Neither Fluke Nor Turbot-Charged
AFP
July 10, 2008
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Graphic on the evolution of flatfish, a group of species with an unusual adaptation
whereby two eyes are on one side of its asymmetrical head. (AFP Graphic)
PARIS (AFP) - Starting with Charles Darwin, evolutionary biologists have fretted and
fought over the origins of flatfish, among the handful of weird, deeply asymmetrical
creatures in Nature's bestiary.
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Did flatfish wind up with two eyes on the same side of a lopsided skull through a few
chance mutations?
Or did this happen gradually, over tens of millions of years?
The answer, in turns out, has been gathering dust for nearly two centuries in museum
drawers, according to a study to be published Thursday in the British journal Nature.
That was where Matt Friedman, a graduate student at the University of Chicago, found
two fossils that clearly show a glacial evolutionary shift from normal upright fish to their
bug-eyed, bottom-hugging descendents.
Each dates from the Eocene epoch, some 45 million years ago, and one -- Heteronectus
chaneti -- is an entirely new genus and species.
The other is from the Amphistium genus, whose earliest known specimens date back
some 200 million years.
Both are true "missing links," with one eye just below the dorsal fin on the side of the fish
closest to the ocean floor.
For reasons unknown to scientists, some species of modern flatfish, such as turbot,
have both eyes on the left side, while other -- halibut and sole, for example -- see from
the right side.
The two fossils "deliver the first clear picture of flatfish origins, a hotly contested issue in
debates on the mode and tempo of evolution," said Friedman.
There can no longer be any doubt, he said in a statement: "The evolution of the profound
cranial asymmetry of extant flatfishes was gradual in nature."
The flatfish controversy has vexed scientists for at least 150 years, and even provided
fodder for creationist challenges to the very notion of Darwinian evolution.
All the great figures of early evolutionary biology weighed in with theories falling roughly
into two schools.
The absence of any fossils showing a halfway point between normal fish and flatfish led
some to believe that change took place in dramatic leaps, a process they called
"saltation".
Others, including Darwin himself, predicted that we would one day find evidence of a
gradual eye migration that mirrors the maturation of living forms.
When flatfish are only days old they are perfectly symmetrical.
But they rapidly metamorphose as they grow, with one eye migrating toward the other.
Neither of the key fossils rediscovered by Friedman had been examined with modern
scientific tools for fear of causing damage.
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But Friedman was finally allowed to treat a single specimen housed in the
Naturhistorisches Museum in Vienna with a weak acid bath, and to carry out computerbased tomography imaging of the skull of another specimens at the Natural History
Museum in London.
Both fossils had been found in limestone quarries in northern Italy.
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