by Researchers have pointed to the moment our earliest ancestors evolved to be warm-blooded, and it happened much later and far faster than scientists expected.
The discovery, made by studying the small tubes in the inner ear, places the evolution of mammalian warm-bloodedness around 233 million years ago – 19 million years later than scientists previously thought.
These semicircular channels are filled with a viscous fluid, called the endolymph, which wedges small hairs along the channels as the fluid swirls around. These hairs transmit messages to brain, and gives it instructions on how to keep your body balanced. Like some liquids, the honey-like endolymph becomes runner the hotter it is, and requires the semicircular channels to change shape so that the liquid can still do its job. In ectothermic or cold-blooded animals, this ear fluid is colder and thus behaves more like molasses and needs wider room to flow in. But for endothermic or warm-blooded animals, the fluid is more watery and small spaces are enough.
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This temperature-based property makes small, semicircular canals a perfect place to discover the moment when the cold blood of ancient mammals became hot, researchers wrote in an article published July 20 in the journal Nature (opens in new tab).
“Until now, semicircular channels were generally used to predict the movement of fossil organisms,” study co-author Romain David, an evolutionary anthropologist at the Natural History Museum in London, said in a statement (opens in new tab). “But by looking closely at their biomechanics, we found that we could also use them to derive body temperatures.
“This is because, like honey, the liquid inside semicircular channels becomes less viscous [syrupy] when the temperature rises, it affects the function, “David explained.” Therefore, during the transition to endothermia, morphological adaptations were necessary to maintain optimal performance, and we could trace them in mammalian ancestors. “
To detect the time of this evolutionary change, researchers measured three samples from the inner ear canal from 341 animals – 243 living species and 64 extinct species – spanning the animal kingdom. The analysis revealed that the 54 extinct mammals included in the study developed the narrow inner ear canal structures suitable for warm-blooded animals 233 million years ago.
Prior to this study, researchers thought mammals inherited warm-bloodedness from the cynodons – a group of scaly, rat-like lizards that gave rise to all living mammals – that were thought to have developed warm-bloodedness around the time of their first appearance 252 million years ago. However, the new findings suggest that mammals diverged from their early ancestors more markedly than expected.
And this drastic change happened surprisingly quickly. Heat-friendly ear canals not only appeared later in the fossil record than researchers expected. It happened much faster too – appeared around the same time as the earliest mammals began to develop whiskers, fur and specialized spines.
“Contrary to current scientific thinking, our article surprisingly shows that the acquisition of endothermia works[s] “It was not a gradual, slow process over tens of millions of years as previously thought,” he said. , but was perhaps achieved quickly when it was triggered by new mammalian-like metabolic pathways and the origin of fur. “
Follow-up studies will have to confirm the findings in other ways, but the researchers said they are happy that their work will help answer one of the longest questions about development of mammals.
“The origins of mammalian endothermia are one of the great unsolved mysteries of paleontology,” said senior author Kenneth Angielczyk, Field Museum’s MacArthur curator of paleo-mammalogy, in the statement. “Many different approaches have been used to try to predict when it first evolved, but they have often yielded vague or contradictory results. We think our method shows real promise because it has been validated using a very large number of modern species,” and it suggests that endothermia developed at a time when many other features of the mammalian body plane also fell into place. “
Originally published on Live Science.
