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 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.
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 computer-based 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.