Scientists pinpointed when our earliest ancestors evolved to become warm-blooded, and it happened much later and much faster than researchers predicted.
The discovery, made by studying the tiny tubes of the inner ear, places the evolution of warm-blooded mammals around 233 million years ago, 19 million years later than scientists previously thought.
These semicircular canals are filled with a viscous liquid, called endolymph, which tickles tiny hairs lining the canals as the liquid sloshes around. These hairs transmit messages to the brain, giving him instructions on how to maintain the balance of the body. Like some fluids, the honey-like endolymph becomes more liquid as it heats up, causing the semicircular canals to change shape so the fluid can continue to do its job. In ectothermic or cold-blooded animals, this auditory fluid is colder and therefore behaves more like molasses and needs larger spaces to drain. But for endothermic, or warm-blooded animals, the fluid is more watery and small spaces are sufficient.
Related: The ancient toothless ‘eel’ is your oldest known ancestor
This temperature-based property makes tiny semicircular canals a perfect place to spot when the cold blood of ancient mammals turned warm, the researchers wrote in a paper published July 20 in the journal Nature (opens in a new tab).
“Until now, semicircular canals have generally been used to predict the locomotion of fossil organisms,” said study co-lead author Romain David, an evolutionary anthropologist at the Natural History Museum in London. said in a press release (opens in a new tab). “However, by carefully examining their biomechanics, we thought we could also use them to infer body temperatures.
“This is because, like honey, the fluid in the semicircular canals becomes less viscous [syrupy] when the temperature increases, impacting function,” David explained. “Thus, during the transition to endothermy, morphological adaptations were necessary to maintain optimal performance, and we were able to track them in mammalian ancestors.
To uncover the timing of this evolutionary change, the researchers measured three samples of the inner auditory canal from 341 animals – 243 living and 64 extinct species – spanning the animal kingdom. The analysis revealed that the 54 extinct mammals included in the study evolved 233 million years ago the narrow structures of the inner auditory canal suitable for warm-blooded animals.
Prior to this study, scientists believed that mammals inherited warm blood from cynodonts – a group of scaly rat-like lizards that gave rise to all living mammals – which were thought to have developed warm blood at the time. from their first appearance 252 million years ago. However, the new findings suggest that mammals diverged from their earliest ancestors more markedly than expected.
And this radical change happened surprisingly quickly. Heat-friendly ear canals appeared no later in the fossil record than scientists had anticipated. It also happened much faster – appearing around the same time that early mammals began to develop specialized whiskers, body hair and spines.
“Contrary to current scientific thinking, our article surprisingly demonstrates that the acquisition of endothermy seems[s] happened very quickly in geological terms, in less than a million years,” Ricardo Araújo, co-lead author of the study, a geologist at the University of Lisbon in Portugal, said in the statement. “This was not a gradual, slow process over tens of millions of years as previously thought, but perhaps was achieved quickly when triggered by novel mammalian-like metabolic pathways and l origin of fur.”
Follow-up studies will need to confirm the findings in other ways, but the researchers said they’re glad their work helps answer one of the oldest questions about the evolution of mammals.
“The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology,” said study lead author Kenneth Angielczyk, MacArthur curator of paleomammalogy at the Field Museum, in the statement. . “Many different approaches have been used to try to predict when it first evolved, but they have often yielded vague or conflicting results. We believe our method holds great promise as it has been validated using a very large number of modern species, and it suggests this endothermy evolved at a time when many other features of the mammalian body plan were also falling into place.”
Originally posted on Live Science.