Johannesburg - From clues discovered deep inside Karoo fossils, palaeontologists believe they have cracked a nearly quarter of a billion-year-old mystery.
Scientists have long been trying to work out when endothermy – warm bloodedness – evolved, and now they say they have the smoking gun.
They found it by examining some very old ears.
The research involved a team of international scientists. This week, they published their findings in the journal Nature.
“It's very difficult to find something like that preserved in the bone that is directly correlated to warm bloodedness. But we have found something that depends directly on body temperature.” says Dr Julien Benoit, senior researcher in palaeontology at the Evolutionary Studies Institute at Wits University.
The scientists found the clues after scanning the inner ears of not only prehistoric animals, but also modern birds and mammals.
Through CT scanning techniques and 3D modelling, the researchers were able to reconstruct the inner ear.
What they were looking for were changes in the semicircular canals that are responsible for balance. These three canals are filled with fluid, which when the head moves activates receptors that tell the brain the three dimensional position of the body and head. This fluid is called endolymph and for it to work efficiently it needs to be at a certain viscosity or runniness. The viscosity, however, changes with heat. And the scientists realised that the semicircular canals would have to evolve to adapt to a new viscosity if an animal was warm blooded and had a higher body temperature.
Their findings suggested that endothermy appeared in mammalian ancestors, in what is now the Karoo, 233 million years ago. The speed of this change surprised the researchers.
“We were expecting a gradual transition, but this all changed in a snap,” says Benoit.
This evolutionary transition from cold to warm blooded was extraordinarily fast.
“Contrary to current scientific thinking, our paper surprisingly demonstrates that the acquisition of endothermy seems to have occurred very quickly in geological terms, in less than a million years,” says Dr Ricardo Araújo, junior researcher at Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, University of Lisbon and lead author on the paper.
At the time this happened, what was to become South Africa was a colder place. It was closer to the South Pole and drier than today.
“This would have been a difficult environment to survive in,” explains Benoit.
Something else was happening. There was an extinction event that might have wiped out 70% of animals on the land.
During this time, there is evidence that some of the early mammalian species evolved fur.
“A lot of things were happening; mammals were getting smaller, they were becoming nocturnal and developing bigger brains, which all could be a source for warm bloodedness,” Benoit explains.
Now the scientists need to understand what drove this evolutionary adaptation, and why it happened so fast. It means collecting more samples and getting high-resolution images.
“We need to go into the field, we need to go into the museums and find those key species,” says Benoit.