A graphic in an undergraduate geology textbook serendipitously led to the 2004 discovery of the missing link between fish and land animals far in the Canadian Arctic, one of the creature’s discoverers said during an April 16 lecture at Harvard.
Neil Shubin, a professor at the University of Chicago and leader of the expedition that discovered Tiktaalik roseae, dedicated his career to finding an intermediary between lobe-finned fishes, which existed some 380 million years ago and early land animals, the first of which is thought to have existed 365 million years ago.
After years of work fruitlessly seeking fossils of the right age — about 370 million years old — in outcroppings in Pennsylvania, Shubin realized the fossils he was finding were a bit too young. Rather than finding examples of the transition from fish to land animals, he was finding early land animals. He needed to find outcroppings that were a little older.
In the winter of 1998, Shubin was arguing a point with Ted Daschler, a graduate student who would accompany Shubin on the Tiktaalik expeditions, and pulled out an undergraduate geology textbook. As he flipped through the pages, he found a graphic that showed where major Devonian era rock outcroppings lie. Two were well-known to him — in Pennsylvania where he was currently working, and in east Greenland, which was well-explored. The third site was in the Canadian arctic and was largely unexplored.
Looking into what little had been written about the area, Shubin discovered a 1974 paper that compared the rock formations there with those he was familiar with. Shubin contacted his doctoral adviser, Farish Jenkins, the Agassiz Professor of Zoology at Harvard, who had previously worked in the Arctic. The following summer, Shubin, Jenkins, and colleagues were in the Arctic, hiking over bare rock formations under a sun that never set.
Shubin told the story of Tiktaalik’s discovery, which drew international attention when it was announced in 2006, before a packed audience in the Geological Lecture Hall. His talk, “Finding Your Inner Fish,” was the last in this year’s Evolution Matters lecture series, sponsored by the Harvard Museum of Natural History (HMNH). The event also marked the opening of a new permanent exhibit at HMNH on evolution, complete with a model of Tiktaalik.
It took several seasons of challenging fieldwork before Shubin’s team found what they were looking for. The amount of gear they could carry in and the amount of fossils they could carry out was limited by the small planes and helicopters they used to reach their research sites. They lived in tents on bare, wind-swept tundra, kept firearms nearby in case polar bears threatened them, and spent their days walking and looking for telltale trails of fossilized bones washing out of rock layers nearby.
They eventually found an area in southern Ellesmere Island where the washed-out bones were abundant. Though they had some difficulty tracking the bones back to the original rock outcropping, they eventually found it and, after extensive digging, unearthed three specimens ranging in length from 4 feet to 9 feet.
That was just the beginning of the discovery. The specimens were brought back from the Arctic still largely encased in rock. Preparers had to painstakingly remove the rock from the fossilized bone, bit by bit to avoid damaging the fossil, before Shubin and his team could see the creature.
The result was Tiktaalik — an Inuit word meaning “large fish” — a creature that had both fishlike and land animal features. It had fishlike scales, fins, and gills, but also had lungs and robust front fins with a wristlike bony structure similar to that which would be found in early land animals. Unlike a fish, its head was not connected to its torso, giving it a neck. It had a long, flat head with eyes on top, and ribs like the earliest tetrapods.
Shubin said it is unlikely Tiktaalik walked on its front fins, but the creature could support its weight in a push-up-like motion. It lived in shallow freshwater streams and could have lived on the bottom or in the shallow areas close to the surface.
Tiktaalik lived 12 million years before the first land animals but represents the missing intermediate step between them and fish. Those first land animals left an aquatic environment teeming with predators for a land environment that had been colonized by plants and invertebrates millions of years earlier.
Shubin said the first land animals would eventually evolve into dinosaurs, reptiles, birds, and mammals, including humans. Even so far into the distant past, traits we share today, such as a neck and articulated wrist, were present in these early creatures, that perhaps peered from the water out to a plant-covered landscape. And even earlier, in fish, bones and structures evolved that, modified over long stretches of time, are still part of us today. That’s why, he said, some of the most important discoveries about basic human anatomy in recent years result from work on model organisms such as worms, yeast, and sea slugs.
“I cannot imagine a more powerful connection to the rest of life on the planet than that,” Shubin said.