A fish with legs? It sounds preposterous, but ancient fossils unearthed in the Canadian Arctic reveal a fish that had skeletal features similar to animals with legs, researchers said Monday.
The find challenges the widely held view of evolution that hind limbs did not begin to form until creatures left the oceans and began living on land.
And it provides a powerful insight into the pivotal episode 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.”
The 375-million-year-old fish Tiktaalik roseae was 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 — said in Monday’s announcement that Tiktaalik also had surprisingly large pelvic bones. That suggests the transitional creature did not just have what researchers often refer to as “front-wheel drive,” but was shifting to “all-wheel drive” although it still lived in the water.
Researchers cannot 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 paleontologist 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, died in 2012. But he was an integral part of the work, his coauthors said: from showing researchers on scientific expeditions how to shoot to defend themselves against errant polar bears, to painstakingly searching for fossils, one shovelful at a time.
Edward Daeschler, associate curator at the Academy of Natural Sciences at Drexel University, was the third team member.
The three drafted the paper at a meeting in Cambridge eight months before Jenkins’s 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 would not 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 significant.
“It’s what we’ve all been waiting for,” said Jennifer Clack, curator of vertebrate paleontology 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.”
Daeschler said 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 the discovery suggests there was probably an evolutionary advantage to being able to use limblike 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 will 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 did not find one, but they did turn up another pelvic bone.
Shubin said Jenkins’s spirit was never far from their thoughts: When Shubin jumped out of the helicopter, he realized he was standing in a ring of rocks — the ones that once held down Jenkins’s tent.Carolyn Y. Johnson can be reached at firstname.lastname@example.org. Follow her on Twitter @carolynyjohnson.