These Fins Were Made for Walking … and Then Swimming

One of the biggest myths of evolution is that it is a relentless march of progress. In fact, evolution is not a linear track, but a branching tree. New species do not arise as part of some long-term goal; they adapt to new opportunities in their surroundings.

On Wednesday, paleontologists unveiled a fossil that proved a potent antidote for the march-of-progress myth. It was a fish that lived about 375 million years ago, when our ancestors were scaly creatures vaguely resembling giant eels, walking across mud flats with four limbs complete with elbows, knees, wrists and ankles. The newly discovered fossil, called Qikiqtania wakei, belonged to this lineage.

But its anatomy suggests that its ancestors, unlike ours, did not continue the move to land. Instead, they gave up walking to swim again.

“We think of evolution in directional terms,” said Neil Shubin, a paleobiologist at the University of Chicago. “That’s not the case here. You have some species going to land and some actually returning to the water.”

In 2004, Dr. Shubin and his colleagues made a headline-grabbing discovery while searching for fossils in Nunavut, an Arctic territory of Canada. They discovered a large, 375-million-year-old fish closely related to land vertebrates. Its most striking similarity was in its four leg-like fins.

The creature’s two front fins had bones corresponding to our humerus, radius, ulna and wrist bones. The combination allowed the fish, which they named Tiktaalik, to walk on mud flats and the bottom of swamps.

Tiktaalik’s importance came into sharp focus when scientists put it on an evolutionary tree along with land vertebrates — known as tetrapods — and other tetrapod-like fish. By looking at these branches, scientists could see how the tetrapod body evolved, step by step. Fish first evolved the long bones in their legs, later adding wrists and ankles. Later still, fingers and toes arose.

Now, Dr. Shubin and his colleagues have added yet another branch to our evolutionary tree with a fossil that they unknowingly discovered in Nunavut, even before finding Tiktaalik.

The team first went to Nunavut in 1998, attracted to rocks that looked as if they might contain fossils from the age of the earliest tetrapods. But one field season after another ended in disappointment.

When the researchers returned in 2004, they found something promising on a small hill next to their tents. “One day, I was having lunch at this spot, and I looked down, and I saw a field of white scales on dark rock,” Dr. Shubin said.

The scales had a distinctive diamond-like pattern only found on fish that are closely related to tetrapods. Near the dark rock, Dr. Shubin noticed a fish jaw fossil. And near that was a rock the size of a Frisbee, with bone-like specks on its surface.

Dr. Shubin socked away everything in a bag to take back to his laboratory, but four days later, the researchers discovered the first fossils of Tiktaalik at another site a mile away from camp. They immediately recognized it as revolutionary, and by the time they got back to Chicago, Dr. Shubin’s lunchtime find had sunk into oblivion.

“This essentially sat in a drawer because we were focused like a laser beam on Tiktaalik,” Dr. Shubin said.

In subsequent field seasons, the researchers found at least 10 specimens of Tiktaalik, some young and some adult. They were able to chart the growth of the animal over its lifetime into a nine-foot-long beast.

The fossils allowed the scientists to reconstruct the walking style of Tiktaalik, a fish version of four-wheel-drive. They discovered that the animals hunted fish by biting down with their long fangs and sucking it down their throats.

In 2019, the researchers turned their attention back to the Frisbee rock. The University of Chicago had purchased a CT scanner designed specifically to produce high-resolution images of fossils, even when they are still in rocks. After scanning the jaw and the scales, Thomas Stewart, a postdoctoral researcher in Dr. Shubin’s lab, finally got around to scanning the rock.

To his astonishment, it contained a fairly complete fin. Although it was similar to Tiktaalik’s fin, it had some crucial differences that marked a second species of tetrapod-like fish in Nunavut.

“You could have knocked me over with a feather,” Dr. Shubin said.

In normal times, the researchers would have frantically converged on their lab to make sense of their discovery. But Dr. Stewart discovered the hidden fin on March 13, 2020. Within days, the scientists were shut out of their lab as the pandemic closed the university.

It was not until June 2020 that they were able to get back in, and then only one at a time. They managed to trim some of the rock away, so that they could take a better scan of the bones inside. The researchers then pored over the images for months.

“This became our pandemic lockdown project,” Dr. Shubin said. “It kept us sane, when the world was not.”

The scientists dubbed the fossil Qikiqtania (pronounced kick-kick-TAN-ee-ya) after the Inuktitut names for the region where it was found, Qikiqtaaluk and Qikiqtani. The second part of its name, wakei, honored David Wake, an evolutionary biologist at the University of California, Berkeley who was a mentor to Dr. Shubin and died last year.

A careful comparison of its anatomy confirmed that Qikiqtania was closely related to tetrapods and might be the closest known relative to Tiktaalik. But after Qikiqtania branched off from Tiktaalik, its evolution took a strikingly different path. For one thing, it got much smaller, likely measuring only about 30 inches long.

An even more dramatic change happened to Qikiqtania’s fins.

On Tiktaalik and other tetrapod-like fish, the humerus had knobs and ridges where powerful walking muscles were anchored. But Qikiqtania had a smooth humerus that offered little support for muscles.

The researchers found another striking difference in the elbow. Tiktaalik relied on its elbow to walk, bending its limb at a 90-degree angle into a push-up position. Qikiqtania’s elbow was locked, with its fin extended out in a straight line.

“It’s not a flexible limb — it’s like a paddle,” Dr. Shubin said.

Qikiqtania also had a bigger fan of rays at the end of its fin, which may have helped it to swim, Dr. Shubin said. It would not have offered any help for walking.

Dr. Shubin suspected that Qikiqtania abandoned the walking habit that its ancestors had recently evolved, opting instead to swim in the open water something like a modern paddlefish.

To understand Qikiqtania’s striking evolutionary shift, Dr. Shubin pointed to tetrapods that returned to the water millions of years later. About 50 million years ago, for example, land mammals adapted into aquatic animals that would eventually become whales and dolphins. The discovery of Qikiqtania suggested that some of our ancient relatives gave up walking almost as soon as walking evolved.

But Qikiqtania did not return to the water simply by reverting to the bodies of its swimming ancestors. It probably used the new bite-and-suck attack that tetrapod-like fish evolved. “Not only are they returning to the water, but they’re doing it in new ways,” Dr. Shubin said.

“It’s great to see us fill out the tree of life here,” said Stephanie Pierce, a paleobiologist at Harvard University who was not involved in the new study. “Any new fossil that can help us understand what’s happening during the early stages of the evolution of the tetrapod body plan is incredibly important because we have such few fossils that document this period.”

Still, Dr. Pierce said that more fossils of Qikiqtania would be required to test Dr. Shubin’s hypothesis. It was not clear to her whether the tetrapod’s fin stuck straight out as a rigid paddle, for example.

“It’s a great specimen, and it does open up a lot of questions that I would love to dig into,” she said.

Dr. Shubin and his colleagues are taking a fresh look at some their Tiktaalik fossils to see if they actually come from Qikiqtania. They also wonder if a mysterious tetrapod-like fossil discovered in Scotland in the 1990s could belong to Qikiqtania’s lineage, too.

Next year, Dr. Shubin and his colleagues are planning an expedition back to Nunavut for the first time in nine years. They intend to return to Dr. Shubin’s 2014 lunchtime spot and to dig for more fossils. It is possible that they will find more tetrapod-like fish that evolved strange adaptations of their own.

“I feel like a kid in a candy store waiting to get back in the store,” Dr. Shubin said.

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