Birds and mammals got an evolutionary edge by getting their legs under them

New research suggests that the ancestors of mammals and birds evolved warm-bloodedness and the ability to walk upright at the same time — and it was triggered by the greatest extinction event in Earth's history.

About 250 million years ago, Earth was going through the Permian-Triassic mass extinction event. Massive volcanic eruptions led to catastrophic global warming and ocean acidification, wiping out most of the life on our planet.

"The scale of this extinction was huge. It left only about one in 20 of the existing species and they were mostly smaller animals," paleontologist Mike Benton told Quirks & Quarks host Bob McDonald. "The survivors were quite few and far between."  

Among those survivors were the ancestors of modern-day birds and mammals. By looking at hundreds of fossilized footprints from before and after this tumultuous time in history, Benton found evidence these animals quickly evolved to become warm blooded and adopt upright postures. In the aftermath of the extinction, this allowed them to become the dominant animals we know today.

Warm-bloodedness key to success

Before the Permian-Triassic mass extinction event, life on land looked different. The largest land animals were cold-blooded and had a sprawling posture, with arms and legs that stuck out at their sides like modern-day lizards and salamanders.

"Life was going on well, but to our eyes, we might have thought this was a bit slow. They're kind of plodding around at a slow pace," said Benton, a professor of paleontology at the University of Bristol.

After the extinction, the fossil record has revealed an incredible explosion in diversity of animals — including the rise of the earliest proto-dinosaurs and the ancestral lineage that led to mammals. Both of these lineages are now believed to have quickly moved toward warm-bloodedness at that time. But a simultaneous and linked adaptation in their posture also seems to have occurred.

By analyzing fossilized trackways, Benton noticed that, before the extinction event, animals showed a sprawling, lizard-like posture. But afterwards, the pattern of footprints changed. The ancestors of both birds and mammals walked with their legs directly beneath them — a more upright posture, just as how horses, dogs, cats, and even humans walk today. 

In biology, walking upright is tied to warm bloodedness because both facilitate efficient movement and endurance.  Because the posture is biomechanically efficient it allows animals to run better. But to take advantage of it fully requires an active metabolism.

"There's a link between upright posture and fast movement. You need the warm bloodedness because you need a lot of food. You need a lot of oxygen to fuel the internal engine. And this seems to be the case also for birds and dinosaurs and their ancestors, because they go all the way back to this point in time," said Benton.

Being upright, being warm blooded, they wiped out the others that were not able to keep up.- Professor Mike Benton, University of Bristol

Previously, scientists had thought that this change had happened somewhat later in the bird/dinosaur lineage. So it was a surprise that the evidence shows this happening in the ancestors of both birds and mammals at the same time. 

"The ones that chanced to have an upright posture, they were able to run a bit further and a bit faster," he said. "And I suspect that once a few of them began this being upright, being warm blooded, they wiped out the others that were not able to keep up."

Extinction events offer opportunities for evolution

Big evolutionary changes like these often happen after massive extinctions, said Benton, as it gives "a chance for the whole of life to reset itself."

Because of intense competition in the brutal world left after the extinction event, animals needed to evolve quickly to survive.

"When we think about the end of the dinosaurs, it was the same thing," said Benton. "I think extinction events are very interesting. We obviously see them in a negative way, but the Earth eventually stabilizes and offers opportunities."

The research was published in the journal Gondwana Research.

Produced and written by Amanda Buckiewicz.