Cockeyed Flatfish Ancestor Tells an Evolutionary Tale

An evolutionary puzzle that Charles Darwin mulled over has finally been solved, thanks to a re-examination of some dusty fish fossils that had been lying neglected in a museum archive. Scientists had long wondered how flatfish evolved to their present form, with both eyes on the same side of their heads; now a report in the journal Nature [subscription required] reveals that the trait evolved gradually and in stages.

All living flatfishes, which include halibut, flounder and sole, have a bizarre adaptation: both eyes are on one side of their head [Telegraph]. This allows them to lie flat on the ocean floor while using both eyes to watch for passing prey. Scientists couldn’t figure out how this trait could evolve gradually over time, and wondered whether a fish in the intermediate evolutionary stage would garner any advantage from having one eyeball that was near the top of its head. This caused people to argue that flatfish might be the product of a large and sudden evolutionary leap, and the fish were used as an argument against natural selection. Googling “flatfish creationism” will also reveal that the arguments spilled out of scientific circles as well [Ars Technica].

Evolutionary biologist Matt Friedman scuttled those theories when he rediscovered three fish fossils in natural history museums that represent a new species that lived 45 million years ago; the fish’s asymmetrical eye sockets indicate that they’re a transition species that led to modern flatfish. One eye had moved, but it had not crossed the midline of the fish’s body, as seen in today’s flatfish, Friedman reports [Science News].

The story “peddled for flatfishes,” Friedman said, is that “they arose in a single generation, following the birth of a deformed ‘hopeful monster’ with both eyes on one side of the skull.” The “hopeful monster” refers to the idea that some genetic mutations could give rise to a deformed “freak” that is usually at a disadvantage but every so often the oddball makes sense functionally [LiveScience]. Instead, Friedman says the fish evolved gradually over many generations, and says the cockeyed fish may have found their asymmetrical eyeball useful in scanning for prey, both below and above.

Image: Credit: Naturhistoriches Museum, Vienna; Matt Friedman/Univ. of Chicago