Scientists pinpoint the exact moment in evolutionary time when mammals became warm-blooded

Scientists have pinpointed the point in time when our earliest ancestors evolved to be warm-blooded, and it happened much later and much faster than researchers expected.

The discovery, made by studying the tiny passageways of the inner ear, places the evolution of warm-blooded mammals around 233 million years ago, 19 million years later than scientists thought.

These semicircular canals are filled with a viscous fluid, called endolymph, which tickles the tiny hairs lining the canals as the fluid pours out. These hairs transmit messages to the brain, giving it instructions on how to keep the body balanced. Like some fluids, honey-like endolymph becomes more liquid the warmer it is, requiring the semicircular canals to change shape so the fluid can still do its job. In ectothermic or cold-blooded animals, this ear fluid is colder and therefore behaves more like honeydew and needs larger spaces to flow. But for endothermic or warm-blooded animals, the fluid is more watery and small spaces are enough.

This temperature-based property makes the small semicircular canals a perfect place to detect when the cold blood of ancient mammals warmed up, the researchers wrote in a paper published July 20 in the journal Nature.

“Until now, semicircular canals were generally used to predict the locomotion of fossil organisms,” study co-lead author Romain David, an evolutionary anthropologist at the Natural History Museum in London, said in a statement. “However, by looking closely at their biomechanics, we thought we could also use them to infer body temperature.

“This is because, like honey, the fluid contained within the semicircular canals becomes less viscous [syrupy] when the temperature increases, which affects function,” David explained. “Thus, during the transition to endothermy, morphological adaptations were required to maintain optimal performance, and we were able to trace them back to the ancestors of mammals.”

To discover the timing of this evolutionary change, the researchers measured three samples of the inner ear canal from 341 animals, 243 living species and 64 extinct species, spanning the entire animal kingdom. The analysis revealed that all 54 extinct mammals included in the study developed narrow internal ear canal structures suitable for warm-blooded animals 233 million years ago.

Before this study, scientists thought 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 of their first appearance. appearance 252 million ago. years ago. However, the new findings suggest that mammals diverged from their earliest ancestors more sharply than expected.

And this drastic change happened surprisingly quickly. Heat-friendly ear canals didn’t just show up later in the fossil record than scientists expected. It also happened much faster, appearing around the same time that the first mammals began to develop whiskers, fur, and specialized backbones.

“Contrary to current scientific thinking, our paper surprisingly demonstrates that the acquisition of endothermy seems to have happened very quickly in geological terms, in less than a million years,” said study co-author Ricardo Araújo, a geologist at the University of Lisbon. in Portugal, it said in the statement. “It wasn’t a slow, gradual process over tens of millions of years as previously thought, but it was perhaps accomplished quickly when triggered by new mammalian-like metabolic pathways and the origin of fur.”

Follow-up studies will need to confirm the findings by other means, but the researchers said they are excited their work will help answer one of the oldest questions about mammalian evolution.

“The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology,” study lead author Kenneth Angielczyk, the Field Museum’s MacArthur curator of paleomamalogy, said in the statement. “Many different approaches have been used to try to predict when it first evolved, but have often given vague or conflicting results. We think our method is really promising because it has been validated using a large number of modern species and suggests that endothermy it evolved at a time when many other features of the mammalian body plan were also falling into place.”

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