Scientists have pinpointed the exact moment our earliest ancestors evolved as warm-blooded, and it happened later and earlier than researchers expected.
The discovery, made by studying the tiny tubes of the inner ear, dates the evolution of warm-blooded mammals to around 233 million years ago – 19 million years later than scientists previously thought.
These semicircular canals are filled with a viscous fluid called endolymph, and as the fluid splashes around it tickles the tiny hairs that line the canals. These hairs transmit messages the brain, instructing him to keep the body in balance. Like some fluids, the pupal endolymph becomes fluid as it heats up, requiring the semicircular canals to change their shape so that the fluid can still do its job. In ectothermic or cold-blooded animals, this ear fluid is colder and therefore behaves more like molasses and needs larger spaces to flow through. However, for endothermic or warm-blooded animals, the fluid is more watery and smaller cavities are sufficient.
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This temperature-driven feature makes the small, semicircular canals a perfect place to see the moment the ancient mammals’ cold blood warmed up, the researchers wrote in a July 20 journal article. Nature (opens in new tab).
“Until now, semicircular canals have generally been used to predict the movement of fossil organisms,” said Romain David, an evolutionary anthropologist at the Natural History Museum in London. said in the statement (opens in new tab). “However, by looking closely at their biomechanics, we realized that we could also use them to determine body temperature.
“This is because the liquid inside the semicircular canals, like honey, is less viscous [syrupy] When the temperature increases, it affects the function, “David explained.” Thus, morphological adaptations were required to maintain optimal performance during the transition to endothermy, and we were able to track them in mammalian ancestors.
To find the timing of this evolutionary change, the researchers measured three samples of the inner ear canal from 341 animals across the animal kingdom – 243 living species and 64 extinct species. The analysis revealed that the 54 extinct mammals included in the study developed narrow inner ear canal structures suitable for warm-blooded animals by 233 million years ago.
Before this study, scientists thought that mammals inherited warm-bloodedness from cynodonts—the scaly, rat-like lizards that gave rise to all living mammals. years ago. However, the new findings suggest that mammals diverged from their early ancestors more markedly than expected.
And this drastic change happened with surprising speed. Heat-friendly ear canals didn’t appear in the fossil record until later than scientists expected. This happened much faster – at the same time the first mammals began to develop whiskers, fur and specialized backbones.
“Contrary to current scientific thinking, our paper surprisingly shows that endothermy does appear to be achieved[s] “It happened geologically very quickly, in less than a million years,” the study’s lead author, Ricardo Araujo, a geologist at the University of Lisbon in Portugal, said in a statement. It was not a gradual, slow process. tens of millions of years, as previously thought, but may have been acquired quickly, triggered by novel mammalian-like metabolic pathways and the origin of fur.
Further studies will need to confirm the findings with other means, but the researchers said they are excited that their work will help answer one of the longest-standing questions. evolution of mammals.
“The origin of mammalian endothermy is one of paleontology’s greatest unsolved mysteries,” said lead study author Kenneth Angielczyk, curator of paleomammology at the Field Museum, MacArthur. “Many different approaches have been used to try to predict when it first evolved, but they have often produced ambiguous or conflicting results. We think our method holds real promise because it has been validated using a large number of modern species and he suggests that endothermy evolved at a time when many other features of the mammalian body plan were also in place.
Originally published in Live Science.
