Harvard Researchers Link Arm Bones and Evolutionary Timelines

A team of researchers from Harvard and the University of Cambridge unveiled new research last month that uses novel techniques to examine how and when early tetrapods — four-limbed animals — transitioned from living in marine environments to terrestrial ones.

In the study published Nov. 24, Professor of Organismic and Evolutionary Biology Stephanie E. Pierce and graduate student researcher Blake V. Dickson used an analysis program that modeled and quantified changes in the humerus — a bone in the arm — over time. They then made predictions about how these evolutionary changes might have eased terrestrial movement.

“It's not surprising that the humerus changed in morphology. We were expecting that to happen, because you can see it in the fossil,” Pierce said. “The trick was trying to capture that in a quantitative fashion. And then once we have that, how do we actually predict what the function was?”

“We had this prediction that the longer the humerus gets, the better the animal would be at moving limbs back and forth, which is of course really important when you're walking on land,” she said.

Pierce added that these changes were gradual, though.

“These early tetrapods came up essentially with a solution to be just good enough to venture out onto land but maintain their ancestral ties to water, so that they can continue to feed, they could continue to breed — all the things that are associated with having that ancestral habitat.”

She noted that, in the study, she and the other researchers concluded that tetrapods underwent an intermediate period between living on water and living on land.

“[Tetrapods] were being sort of pushed up this terrestrial landscape but also being pulled back into their ancestral aquatic habitat,” she explained. “They had pressures that they still needed to feed in water; they still needed to reproduce in water. So, they have most of their biology still tied to water.”

Pierce said she believes the methods and findings of this study may help inform further research into evolutionary biology, as the process can be extrapolated to other bones besides the humerus.

“We still have much more to learn, because we've only looked at one bone. And the signal that you get from one bone might be different than the signal you get from another,” Pierce said.

University of Cambridge paleontology professor Jennifer A. Clack and paleontology researcher Timothy R. Smithson were co-authors on the paper.