The flounder looks like a hallucination of a fish. His body is flat as a pancake, his head is constantly tilted to one side, and instead of having one eye on each side of his head, he has both eyes crowded on one side.
This anatomy, strange as it may be, is one of the remarkable achievements of evolution. The flounder, like more than 800 other species of flounder, lies on the bottom of the sea with its two eyes staring into the water overhead. When a smaller fish swims by, the flounder shoots and hits. One species, the Pacific halibut, can grow up to the size of a barn door.
The bizarre bodies of flounder have long puzzled biologists. Charles Darwin’s critics actually used this as evidence against his theory of evolution.
In “The Origin of Species,” Darwin argued that natural selection favors small variations. Every small increment benefited the organism. Over countless generations, he said, these additions gradually added up to vast transformations.
Darwin’s critics rejected the idea that such changes could actually occur. St. George Jackson Mivart, a British biologist, used the flounder as Exhibit A: It seemed impossible to him that the slow migration of the eye around the fish’s head would be advantageous at every stage of the journey.
“How such a transition of one eye a minute fraction of the way to the other side of the head can benefit the individual is not really clear,” Mivart wrote in 1871. The idea that natural selection could gradually create the flounder, he added, “It seems it defies not imagination but reason.”
Darwin’s theory survived Mivart’s attacks, but the flounder conundrum remained. For decades, scientists have puzzled over which ancestors they evolved from. Flounder, plaice, sole, and all other flounder 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 21st century, when biologists discovered that the flounder’s closest genetic relatives were nothing like them. Their cousins included large, fast swimmers that spent their lives in the open ocean, including tuna, barracuda, and marlin.
“It’s kind of shocking,” said Ricardo Betancur-R., a marine biologist at the Scripps Institution of Oceanography who discovered the link in 2013.
Flounder DNA provided several clues as to how the transformation occurred. By counting the mutations that flounder and their relatives had accumulated, scientists could estimate when their evolutionary branches diverged. It turns out that the flounder and their fast-swimming relatives diverged not long after the Earth went through a huge cataclysm 66 million years ago.
That’s when a six-mile-wide asteroid slammed into the planet, blackening the sky and wiping out more than half the species on land and in the ocean. Mass extinctions have opened up ecological opportunities for survivors. One surviving lineage diverged, with some individuals finding opportunities in the open ocean and others settling on the sea floor.
The DNA of flounder looks like what you would expect if they had evolved the way Darwin imagined. Natural selection favored a series of mutations that gradually changed the body of an ordinary-looking ancestor to create the anatomy of the flounder.
Fossils also provide insights into 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 flounder had eyes on both sides of their heads. But one eye was near 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 flounder eyes have changed so drastically, some biologists are watching flounder eggs hatch and develop into adult fish. Larvae begin to look like normal fish. Only when they metamorphose into adults does one eye migrate to the other side of the head. The fish then settle on the sea floor and lie in wait for their prey.
Hormones from the thyroid gland trigger the larvae to go through this metamorphosis. The hormones turn on genes in the flounder’s skull that cause it to change shape, helping to push one eye into a new position. In the eye itself, other genes stimulate the growth of neurons so it can stay attached to the brain as it travels to a new location.
New discoveries of flounder have provided some answers to questions about their evolution – and led to new debates. In 2021, a team of researchers in China created a new flounder evolutionary tree by looking at 1,700 DNA segments in 13 species of flounder and their relatives. Scientists have concluded that the body of the flounder evolved twice. One of these crossings gave rise to a group of flounder species that live in tropical oceans known as spiny flounder, and the other crossing gave rise to all other flounder, including the flounder.
Dr. Friedman was skeptical. All flounder share many of the same extreme changes in their anatomy that allow for dramatic changes in their larvae. Dr. Friedman found it hard to believe that an ordinary fish would acquire the bizarre body of a flounder twice. “I’m pretty sure this innovation evolved once,” he said.
Because flounder evolved so quickly after an asteroid impact 66 million years ago, it has been difficult to reconstruct their evolutionary tree. Dr. Friedman attempted 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 predicted would be more likely to provide a more accurate picture. They also expanded their analysis by comparing more than 400 species of flounder and their relatives.
Last month, scientists announced that their new analysis pointed to only one origin for flounder. “We are happy to have found a single origin because it is a simpler explanation,” said Dr. Duarte-Ribeiro. “There are so many genes potentially involved in this transition that it’s quite unlikely that it happens twice.”
Chinese scientists have published a response in which they stand behind their results. Three team members did not respond to emails seeking comment.
Dr. Betancur-R. and his colleagues are now gathering more data from the DNA of flounder to see if their finding of a single origin holds up. “That’s what I think would happen,” he said. “But it’s hard to say because these are hard problems.