Mammals’ ancestors began to become warm-blooded about 20 million years later than previously thought, researchers suggested Wednesday after analyzing inner ear fossils in hopes of solving “one of paleontology’s great unsolved mysteries.”
Warm-bloodedness is one of the essential traits of mammals, along with fur, but at the very time they first evolved, the trait had long been the subject of debate.
Previous research has shown that the ancestors of mammals began evolving to be warm-blooded, or endothermic, about 252 million years ago — around the time of the Permian extinction event known as the “Great Dying Out.”
However, figuring out the schedule has proved difficult.
“The problem is that you can’t put thermometers in your fossils, so you can’t measure their body temperature,” said Ricardo Araujo of the University of Lisbon, one of the authors of a new study in the journal Nature.
He was part of an international team of researchers that found a new way to determine how body heat changed over time by studying the semicircular canals in the inner ears of 56 extinct species of mammalian ancestors.
Fluid flows through the tiny ear canals that help the animals balance.
The researchers found that as the body temperature increased, the fluid in the ear also warmed up.
Araujo gave the example of oil used to fry hot fries.
Before you warm up the oil, it’s “very viscous, very dense,” he told AFP.
“But when you heat it up, you see that the oil is a lot more liquid, it flows a lot easier.”
The more liquid ear fluid led animals to develop narrower canals – which can be measured in fossils and allows researchers to track body temperature over time.
Contrary to previous research on the subject, the team developed a model that works not only on extinct mammalian ancestors, but also on living mammals, including humans.
“It can look at your inner ear and tell you how warm-blooded you are – that’s how accurate the model is,” the study’s lead author, Romain David, of London’s Natural History Museum, told AFP.
Using the model, they traced the beginnings of warm-blooded animals to about 233 million years ago in the late Triassic period.
– ‘No step-by-step, slow process’ –
Michael Benton, a paleontologist at Britain’s University of Bristol who was not involved in the study, said the new metric “appears to work well for a wide range of modern vertebrates.”
“Not only does it provide a yes-no answer, it actually scales the ‘degree’ of endothermia in relation to actual typical body set point temperature,” he told AFP.
Benton, whose previous research had given the 252-million-year date, said the transition to warm-blooded animals likely occurred in stages, and “there were several significant prior steps before this semicircular canal change.”
Araujo said the new research suggested that warm-blooded animals arose easily and “very rapidly in geological terms, less than a million years.”
“It wasn’t a gradual, slow process over tens of millions of years, as was previously thought.”
David said it was unlikely that warm-blooded animals would start around the 252 million year ago extinction event because global temperatures were extremely hot at the time.
That would have been a disadvantage for warm-blooded animals — but they could have thrived as temperatures cooled over the millions of years that followed.
“Being an endotherm allows you to be more independent of the vagaries of climate, to run faster, run longer, explore different habitats, explore the night, explore polar regions, go on long hikes,” Araujo said.
“There was a lot of innovation back then that started to define what a mammal was – but ultimately what a human would be as well.”
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