Scientists pinpointed the point in time when our earliest ancestors became warm-blooded, and it happened much later and much faster than researchers expected.
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 thought.
These semicircular canals are filled with a viscous fluid called endolymph, which tickles the tiny hairs that line the canals as the fluid sloshes around. These hairs carry messages braingiving him instructions on how to maintain the balance of the body. Like some fluids, the honey-like endolymph becomes more fluid the hotter it gets, requiring the semicircular canals to reshape in order for the fluid to do its job. In ectothermic or cold-blooded animals, this ear fluid is colder and thus behaves more like molasses and requires wider spaces to flow. But in endothermic, or warm-blooded, animals, the fluid is more watery and small spaces are enough.
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This temperature-based property makes the tiny semicircular channels an ideal place to determine when the cold blood of ancient mammals became hot, the researchers write in a paper published July 20 in the journal. Nature (will open in a new tab).
“Until now, semicircular canals have been commonly used to predict the movement of fossil organisms,” says study co-author Romain David, an evolutionary anthropologist at the Natural History Museum in London. said in a statement (will open in a new tab). “However, after carefully studying their biomechanics, we realized that we could also use them to determine body temperature.
“This is because, like honey, the fluid contained within the semicircular canals becomes less viscous. [syrupy] when the temperature rises, it affects function,” David explained. “Therefore, during the transition to endothermy, morphological adaptations were required to maintain optimal performance, and we could track these in mammalian ancestors.”
To determine the timing of this evolutionary change, the researchers measured three samples of the internal auditory meatus in 341 animals — 243 living species and 64 extinct species spanning the animal kingdom. The analysis showed that the 54 extinct mammals included in the study evolved narrow structures of the internal auditory meatus 233 million years ago, suitable for warm-blooded animals.
Prior to this study, scientists believed that mammals inherited warm-bloodedness from the cynodonts — the group of scaly, rat-like lizards that gave rise to all modern mammals — who were thought to have developed warm-bloodedness around the time of their first appearance. many years ago. However, new evidence suggests that mammals differed more markedly from their early ancestors than expected.
And this dramatic change happened surprisingly quickly. Thermal ear canals appeared in the fossil record not only later than scientists expected. It happened much faster—they appeared around the same time that whiskers, fur, and a specialized spine began to develop in the earliest mammals.
“Contrary to current scientific thinking, our paper unexpectedly demonstrates that acquiring endothermy seems[s] geologically speaking, happened very quickly, in less than a million years,” study co-author Ricardo Araujo, a geologist at the University of Lisbon in Portugal, said in a statement. tens of millions of years, as previously thought, but this may have been achieved quickly when prompted by new mammalian-like metabolic pathways and the origin of fur.”
Future studies will need to confirm the results through other means, but the researchers said they are excited that their work will help answer one of the longest-standing questions about evolution mammals.
“The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology,” study senior author Kenneth Angilchik, curator of paleomammology at the Field MacArthur Museum, said in a statement. “Many different approaches have been used to try to predict when it first developed, but they often gave vague or conflicting results. that endothermy evolved at a time when many other mammalian bodily features also fell into place.”
Originally published on Live Science.
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