How the Flatfish Got Its Weird Body — and Why It Stumped Darwin's Critics
Picture a fish that starts life looking completely normal, then gradually has one of its eyes creep across its own skull to join the other eye on the same side of its head. That is not science fiction — it is what actually happens to every flounder, halibut, and sole alive today. Flatfish are some of the strangest animals on the planet, and for a long time, their weird bodies were used as an argument that the theory of evolution could not possibly be true. Now scientists are using fossils and DNA to finally figure out how this seemingly impossible transformation happened.
Flatfish spend their adult lives lying flat on the ocean floor, both eyes pointing upward, waiting to ambush smaller fish that swim past. One species, the Pacific halibut, can grow as large as a barn door. There are more than 800 species of flatfish in total, and they all share this same strange body plan. But when researchers started comparing flatfish DNA — the chemical instructions found inside every living cell — to the DNA of other fish, they got a surprise: the closest relatives of flatfish are not slow, bottom-dwelling fish at all. They are powerful open-ocean swimmers like tunas, barracudas, and marlins. That discovery left scientists with a big question: how does a fast, streamlined ocean fish end up with descendants that lie flat on the seafloor with both eyes crammed onto one side of their heads?
The answer starts with a catastrophe. About 66 million years ago, a six-mile-wide asteroid slammed into Earth and blocked out sunlight for a long time, wiping out more than half of all species — including the non-bird dinosaurs. That mass extinction, meaning the widespread dying-off of many species at once, opened up empty niches, or roles in the environment that living things can fill. Scientists believe that one group of fish split apart during this period. Some individuals moved into the open ocean, eventually becoming the ancestors of tunas and marlins. Others found food and shelter on the ocean floor, setting off the chain of gradual changes that eventually produced flatfish.
The idea that this transformation happened gradually, one small step at a time, actually goes back to Charles Darwin, who proposed his theory of natural selection in 1859. Natural selection is the process by which traits that help an organism survive and reproduce become more common over generations. Darwin argued that flatfish evolved through thousands of tiny mutations, each one slightly more useful than the last. A British biologist named St. George Jackson Mivart pushed back hard, arguing that a fish with one eye only partway across its skull would not gain any advantage — so natural selection could never push the eye all the way to the other side. He called the whole idea unreasonable. Darwin's theory survived the criticism, but the question lingered for more than a century.
The strongest evidence for Darwin's side came from fossils. In 2008, a paleontologist — a scientist who studies ancient life through fossils — named Matt Friedman found fossils of early flatfish species that had eyes on both sides of their heads, but with one eye positioned close to the top of the skull. These were exactly the kind of in-between forms that Darwin had predicted. They showed that the eye really did migrate gradually, and that even a partly-moved eye could still be useful to the fish. Today, scientists can also watch this migration happen in real time. Baby flatfish, called larvae, hatch with one eye on each side of their head, just like ordinary fish. Hormones from the thyroid gland — a small structure in the throat — then trigger a metamorphosis, the process of dramatic physical change, causing the skull to reshape itself and one eye to shift position.
Even with so much new evidence, one big debate is still unresolved: did the flatfish body evolve once, or twice? In 2021, a team of Chinese researchers analyzed DNA from 13 flatfish species and concluded that the flat body shape evolved independently in two separate lineages, meaning two different branches of the family tree each developed it separately. Other scientists, including Dr. Friedman and marine biologist Ricardo Betancur-R. from the Scripps Institution of Oceanography, are skeptical. The changes required to produce a flatfish — from skull reshaping to eye migration to the genes that keep the moving eye connected to the brain — are so complex and interconnected that it seems very unlikely nature would stumble upon them twice. Their own analysis, published recently and covering more than 400 species, points to a single origin. The Chinese team stands by their results, and the scientific back-and-forth continues.
That ongoing debate is actually a sign of science working exactly as it should. Researchers on both sides keep pulling in more data, testing their ideas, and pushing each other to find stronger evidence. The flatfish, which once seemed like proof that evolution could not work, has become one of the clearest examples of how evolution really does work — slowly, creatively, and sometimes in ways that look completely bizarre until you understand the full story.
Source: The New York Times