A flounder looks like a hallucination of a fish. His body is flat as a pancake, his head is permanently tilted to one side, and instead of having one eye on each side of his head, both eyes are tightly packed on one side.
This anatomy, strange as it may be, is one of evolution’s remarkable success stories. Flounder, like more than 800 other species of flatfish, lie flat on the seabed, with their two eyes staring up at the water above. When a smaller fish swims by, a flatfish shoots up and strikes. One species, the Pacific halibut, can grow to the size of a barn door.
The bizarre bodies of flatfish have long puzzled biologists. In fact, Charles Darwin’s critics used it as evidence against his theory of evolution.
In ‘The Origin of Species’, Darwin argued that natural selection favors small variations. Every small step benefited an organism. Over countless generations, he said, these steps gradually led to large-scale transformations.
Darwin’s critics rejected the idea that such changes could actually occur. St. George Jackson Mivart, a British biologist, used flatfish as Exhibit A: It seemed impossible to him that the slow migration of the eye around the head of the fish would be beneficial at any stage of the journey.
“How such a displacement of one eye, a small part of the journey to the other side of the head, could benefit the individual is indeed far from clear,” Mivart wrote in 1871. The idea that natural selection is gradually a flatfish could create, he added, “seems to contradict not imagination, but reason.”
Darwin’s theory survived Mivart’s attacks, but the flatfish riddle remained. 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 cousins.
The mystery began to unravel in the early 2000s, when biologists discovered that the closest genetic relatives of flatfish looked nothing like them. Their cousins included large, fast swimmers that spent their lives in the open ocean, including tuna, barracuda and marlin.
“That’s quite shocking,” says Ricardo Betancur-R., a marine biologist at Scripps Institution of Oceanography, who discovered the connection in 2013.
Flatfish DNA offered some clues as to how the transformation occurred. By adding up the mutations collected by flatfish and their relatives, researchers were able to estimate when their evolutionary branches split. It turns out that flatfish and their fast-swimming cousins split up not long after Earth suffered a massive catastrophe 66 million years ago.
At that moment, a six-mile-wide asteroid hit the planet and turned the sky black, wiping out more than half of the species both on land and in the ocean. The mass extinction opened ecological opportunities for the survivors. One remaining lineage split, with some individuals finding opportunity in the open ocean and others settling on the seafloor.
The DNA of flatfish looks as you would expect if they evolved as Darwin imagined. Natural selection favored a series of mutations that gradually changed the body of an ordinary-looking ancestor, creating the anatomy of flatfish.
Fossils also offer a glimpse of this transformation. In 2008, Matt Friedman, now director of the Museum of Paleontology at the University of Michigan, 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 skull. The fossils documented exactly the kind of transitional form that Darwin predicted – and that Mivart claimed was impossible.
To understand how flatfish eyes changed so dramatically, some biologists are looking at how flatfish eggs hatch and develop into adult fish. The larvae initially look like normal fish. Only when they metamorphose into adults does one eye migrate to the opposite side of the head. The fish then settle on the seabed and wait for prey.
Hormones from the thyroid gland ensure that larvae undergo this metamorphosis. The hormones switch on genes in the flatfish skull, causing it to change shape, pushing one eye into a new position. Within the eye itself, other genes stimulate the growth of neurons so they can stay connected to the brain as it travels to a new place.
The new discoveries of flatfish have provided some answers to questions about their evolution – and sparked new debates. In 2021, a team of researchers in China created a new evolutionary tree of flatfish by looking at 1,700 DNA segments in 13 species of flatfish and their relatives. The researchers concluded that the body of flatfish has evolved twice. One of these transitions gave rise to a group of flatfish species living in tropical oceans known as 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 would grow twice the bizarre flatfish body. “I’m pretty sure that innovation evolved at some point,” he said.
Because flatfish evolved so quickly after the asteroid hit 66 million years ago, their evolutionary tree was difficult to reconstruct. Dr. Friedman tried this together with Dr. Betancur-R to do. and Emanuell Duarte-Ribeiro, an evolutionary biologist at the University of Basel in Switzerland. They focused on certain DNA segments that they predicted would 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 only one origin of flatfish. “We’re happy to find the sole origin because it’s the simpler explanation,” said Dr. Duarte Ribeiro. “There are so many genes that may be involved in this transition that it is quite unlikely that this happened twice.”
The Chinese researchers published a response supporting their results. Three members of the team did not respond to emails seeking comment.
Dr. Betancur-R. and his colleagues are now collecting more data from flatfish DNA to see if their finding of a single origin holds up. “That’s what I think would happen,” he said. “But that’s difficult to say, because these are difficult problems.”