Fish fossils unearthed in the Canadian Arctic are challenging the evolutionary paradigm, known as the “front-wheel drive” hypothesis, that hind limbs did not begin to form until creatures began living on land.
The 375-million-year-old fish Tiktaalik roseaewas first written into biology textbooks in 2006, when a team of three paleontologists discovered a fossil of the curious crocodile-like fish, showing it had front fins resembling limbs, with elbows and primitive wrists. The same team—which included a renowned Harvard paleontologist who has since died—announced Monday that Tiktaalik also had surprisingly large pelvic bones, suggesting the transitional creature was shifting toward “all-wheel drive” though it still lived in the sea.
The discovery provides a powerful insight into the pivotal episode in evolution when creatures emerged onto land: If the authors are right, we can trace our arms—and our legs—to fish fins.
“That wrist you use to write with, the neck you use to move your head around with, the lungs you’re using to breathe ... all derive from parts in the bodies of fish. Your hands and arms derive from parts of the fins,” said Neil Shubin, a professor of anatomy at the University of Chicago who was one of the leaders of the work. “What the fossil record tells us is how deeply we are connected to life on the rest of the planet. In this case, this tells us how closely we are related to fish.”
Researchers can’t tell whether Tiktaalik crawled out of the water on all fours, but think it may have been capable of using its hind fins to support its weight and manage a squirming walk in shallow water, similar to how a mudskipper moves. Its enlarged pelvic girdle is more primitive than those found in ancient land-dwelling tetrapods with four limbs, but is much larger than those found in fish.
The study, published in the Proceedings of the National Academy of Sciences, was bittersweet for the scientific team because Farish Jenkins, the adventurous Harvard palentologist who was an integral part of the work, did not live to see it published. Jenkins, whose career had more shades of Indiana Jones than a typical professor’s, passed away in 2012. But he was an integral part of the work, his coauthors said, from teaching them how to shoot to defend themselves against errant polar bears, to painstakingly searching for fossils with demolition hammers and by moving shovelfuls of rock.
The three outlined the paper at a meeting in Cambridge eight months before Jenkins’ death. He insisted, as always, that the first step to presenting their find to the wider scientific community was to assemble the visuals that would tell the story. Shubin said the published paper is almost exactly as they outlined it together.
The paper describes a pelvic girdle that is unusually large for a fish, with surfaces where large muscles could have attached. But Tiktaalik still had fins; it wouldn’t have been able to walk like a true tetrapod. It also has a hip joint that is intermediate, oriented in a way that is not quite like those of fish or limbed animals.
Paleontologists not involved in the work said that the find was not exactly a surprise—scientists knew that there must have been a transition from fins to limbs—but that the study fills in a gap in evolution and is both interesting and significant.
“It’s what we’ve all been waiting for,” said Jennifer Clack, curator of vertebrate palaeontology at the University of Cambridge’s Museum of Zoology in the United Kingdom. “Until this discovery, we weren’t able to see the changes by which the pelvic fins of the fish became much larger and more robust, and gradually turned into the tetrapod hind limb.”
Edward Daeschler, associate curator at the Academy of Natural Sciences at Drexel University and a leader of the work, said that it was at his laboratory in Philadelphia that the fossils were prepared. The key fossils came from a block of rock the research team had loaded onto a helicopter in the Arctic and had considered a low priority.
To their surprise, the rock revealed something quite unexpected.
“Something would take shape and appear and I’d take a picture and send it in an e-mail, and it was so fun for all of us to say, ‘This is so new; this is so exciting! What does this mean?’ ” Daeschler said. “It really makes you remember why you enjoy science.”
John Maisey, a curator in the division of paleontology at the American Museum of Natural History in New York, said that the discovery suggests there was likely an evolutionary advantage to being able to use limb-like fins for navigating shallow, muddy environs—perhaps enabling Tiktaalik to better evade predators.
“I look at these things in a slightly different way and say they’re highly adapted or on their way to becoming tetrapods, so they’re really lousy fish. This thing couldn’t get away from anything” in the water, Maisey said. "I suspect a lot of these adaptations were actually towards evading ending up as somebody’s breakfast—not so much conquest of the land as the escape from the water.”
Shubin and Daeschler plan to continue the work by exploring a different part of the Arctic, where they hope to probe the origins of fish by studying much older rocks. Last summer, however, they returned to the same spot in the Arctic where they found the Tiktaalik fossil, hoping that they might recover a complete hind fin. They didn’t find one, but they did turn up another pelvic bone.
Shubin said Jenkins’ spirit was with them: When Shubin jumped out of the helicopter, he realized he was standing in a ring of rocks—the ones that once held down Jenkins’ tent.