A flounder looks like a fish hallucination. Its body is flat as a pancake, its head permanently tilted to one side, and instead of having one eye on each side of its head, its two eyes are gathered together on one side.
This anatomy, strange as it is, is one of the remarkable achievements of evolution. Flounder, like more than 800 other species of flatfish, lie flat on the seafloor with both eyes looking at the water above its head. When a smaller fish swims, a flatfish springs up and strikes. One species, the Pacific halibut, can grow to the size of a barn door.
The bizarre bodies of flatfish have long intrigued biologists. In fact, critics of Charles Darwin used it as evidence against his theory of evolution.
In “The Origin of Species,” Darwin argued that natural selection favors tiny variations. Every small increment benefited an organization. Over countless generations, he said, these advances gradually added up to large-scale transformations.
Darwin’s critics rejected the idea that such changes could actually occur. St. George Jackson Mivart, a British biologist, used the flatfish as evidence A: it seemed impossible to him that the slow migration of the eye around the fish’s head would be advantageous every step of the way.
“How such a transit of one eye, a tiny fraction of the journey to the other side of the head, could benefit the individual is indeed far from clear,” wrote Mivart in 1871. The idea that natural selection could gradually create a flatfish, he added: “seems to contradict not imagination, but reason. »
Darwin’s theory survived Mivart’s assaults, but the enigma of the flatfish remains. For decades, scientists have wondered what kind of ancestor they evolved from. Flounder, halibut, sole, and all other flatfish had very similar bodies, indicating that they were closely related to each other. But they were all so strange that it was impossible to identify their closest living cousins.
The mystery began to unravel in the early 2000s, when biologists discovered that the flatfish’s closest genetic relatives looked nothing like them. Their cousins included large, fast swimmers who spent their lives in the open ocean, including tuna, barracuda, and marlin.
“It’s pretty shocking,” said Ricardo Betancur-R., a marine biologist at the Scripps Institution of Oceanography, who discovered the link in 2013.
Flatfish DNA provided some clues as to how the transformation occurred. By counting the mutations accumulated by flatfish and their relatives, the researchers were able to estimate when their evolutionary branches split. It turned out that flatfish and their fast-swimming cousins went their separate ways shortly after Earth suffered a massive catastrophe 66 million years ago.
Around this time, a six-mile-wide asteroid crashed into the planet and turned the sky black, wiping out more than half of the land and ocean species. The mass extinction opened up ecological opportunities for survivors. A surviving lineage split, with some individuals finding opportunities in the open sea and others settling on the seafloor.
The DNA of flatfish looks like what you would expect if they evolved as Darwin envisioned. Natural selection favored a series of mutations that gradually altered the body of an ordinary-looking ancestor, creating the anatomy of the flatfish.
Fossils also provide insight into this transformation. In 2008, Matt Friedman, now director of the University of Michigan Museum of Paleontology, discovered that fossils of two early species of flatfish had eyes on both sides of their heads. But one of the eyes was close to the top of the head. The fossils documented exactly the type of transitional form that Darwin had predicted – and that Mivart claimed was impossible.
To understand how flatfish eyes changed so dramatically, some biologists watch flatfish eggs hatch and develop into adult fish. The larvae start out looking like normal fish. It is only when they metamorphose into adults that an eye migrates to the other side of the head. The fish then settle on the seabed to wait for their prey.
Hormones from the thyroid gland trigger this metamorphosis in the larvae. The hormones activate genes in the flatfish’s skull that cause it to change shape, helping push an eye into a new position. Inside the eye itself, other genes stimulate the growth of neurons so that they can stay connected to the brain as they move to a new location.
New flatfish discoveries have answered questions about their evolution and led to new debates. In 2021, a team of Chinese researchers created a new flatfish evolutionary tree by examining 1,700 DNA segments in 13 species of flatfish and their relatives. The researchers concluded that the flatfish’s body evolved twice. One of these transitions gave rise to a group of flatfish species living in tropical oceans called spiny turbots, and the other transition gave rise to all other flatfish, including flounder.
Dr. Friedman was skeptical. All flatfish share many of the same extreme changes in their anatomy, made possible by dramatic changes in their larvae. Dr. Friedman found it hard to believe that ordinary fish could acquire twice the bizarre body of a flatfish. “I’m pretty sure this innovation evolved once,” he said.
Because flatfish evolved so quickly after the asteroid impact 66 million years ago, their evolutionary tree was difficult to reconstruct. Dr. Friedman tried to do this with Dr. Betancur-R. and Emanuell Duarte-Ribeiro, an evolutionary biologist at the University of Basel in Switzerland. They focused on certain segments of DNA that they believed would be more likely to provide a more accurate picture. They also expanded their analysis to compare more than 400 species of flatfish and their relatives.
Last month, the researchers reported that their new analysis pointed to a single origin of flatfish. “We are happy to find the single origin, because it is the simplest explanation,” said Dr. Duarte-Ribeiro. “There are so many genes potentially involved in this transition that it is unlikely to happen twice.”
The Chinese researchers published a response in which they maintain their results. Three team members did not respond to emails seeking comment.
Dr. Betancur-R. and colleagues are now gathering more data on flatfish DNA to see if their discovery of a single origin holds water. “That’s what I think would happen,” he said. “But it’s hard to say, because these are difficult issues.”