After Fish Developed Limbs, Some Might Have Returned to Swimming

Scientists think a recently discovered fossil is evidence that evolution is more like a branching tree than a ladder

Qikiqtania wakei (top) was more suited to swimming than its larger cousin Tiktaalik (bottom). Alex Boersma

Several hundred million years ago, fish began to grow limbs that enabled them to walk across the bottom of the water. Modern mammals, including humans, evolved from these fish. But on Wednesday, a group of scientists shared the discovery of a fossil that the scientists say indicates that shortly after the development of limbs, some of these creatures returned to full-time swimming.

“We think of evolution in directional terms,” Neil Shubin, a paleobiologist at the University of Chicago who co-discovered the fossil, tells the New York Times’ Carl Zimmer. “That’s not the case here. You have some species going to land and some actually returning to the water.”

The researchers actually found the fossil almost 20 years ago, during an expedition to the Canadian Arctic. Shubin noticed rock embedded with fish jaw parts and scales while eating lunch near camp one day, reports Defector’s Sabrina Imbler. Shubin tells Defector that he considered not even taking one brick-sized rock, thinking, “Do I want to take a whole block back, you know, just for a couple of scales?”

It would take another 15 years for the researchers to realize what that rock contained, in part because they were distracted by a different discovery made on that same expedition: the fossils of a fish they named Tiktaalik, which provided an important insight into the evolution of animals from sea creatures to land crawlers. According to the New York Times, Tiktaalik had four leg-like fins, and the two front fins had bones similar to the human humerus, ulna, radius and wrist. These bones made it possible for the fish to walk in shallow water, such as the bottoms of swamps.

When the scientists finally got around to scanning the ignored brick-sized stone in 2020, they found an intact pectoral fin and a boomerang-shaped humerus bone, writes Defector.

The researchers realized the fossil was from an animal of a previously undiscovered species, and named it Qikiqtania wakei. Qikiqtania (pronounced “kick-kiq-tani-ahh”) comes from the Inuktitut words for the region where the fossil was found, writes Stewart in a piece for The Conversation. The researchers published their findings this week in the journal Nature.

Qikiqtania “gives a richer picture of the diverse lifestyles and body plans that existed when the first vertebrates moved onto land hundreds of millions of years ago,” Richard Blob, an evolutionary biologist at Clemson University who did not contribute to the research, tells Defector.

Qikiqtania might be Tiktaalik’s closest known relative. After Qikiqtania branched off from Taktaalik, it seems to have shrunk. Taktaalik stretched to nine feet, while the researchers estimate Qikiqtania would have been only around 30 inches long, writes the New York Times.

The humerus the researchers unearthed didn’t have the knobs and ridges found on Tiktaalik that muscles would have been attached to, according to the New York Times. Nor did it have a Tiktaalik-like bendable elbow. “It’s not a flexible limb–it’s like a paddle,” Shubin tells the New York Times. The researchers theorize that these findings are evidence that Qikiqtania’s ancestors could walk, but Qikiqtania chose not to.

“The ancestors of Qikiqtania were already taking those steps [out of water], but this was a creature that said, ‘I’m not doing that, I like the water better,” Shubin tells the New Scientist’s Christa Lesté-Lasserre.

Some scientists who did not contribute to the study caution that more research needs to be done to confirm this hypothesis. Alice Clement, an evolutionary biologist and paleontologist at Flinders University in South Australia tells Defector that the idea that Qikiqtania returned to swimming “can only really be conclusively confirmed with the discovery of additional material.”

Stephanie Pierce, a paleobiologist at Harvard University, tells the New York Times she thought it wasn’t clear the fin stuck out as a paddle—she thinks more fossils are needed to confirm the Qikiqtania fin was for swimming. “It’s a great specimen, and it does open up a lot of questions that I would love to dig into,” she says.