Scientists have identified the time when our first ancestors evolved to be warm-blooded, and it happened much later and much faster than researchers expected.
The discovery, made by studying the tiny tubes in the inner ear, places the evolution of warm mammalian blood about 233 million years ago, 19 million years later than scientists previously thought.
These semicircular canals fill with a viscous liquid, called an endolymph, that tickles the small hairs that line the canals as the liquid flows. These hairs transmit messages to the brain, giving you instructions on how to keep your body balanced. Like some fluids, the honey-like endolymph becomes more liquid as it gets hotter, and requires the semicircular channels to change 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 wider spaces to flow. 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 detect when the cold blood of ancient mammals became warm, the researchers wrote in an article published July 20 in the journal. Nature (opens in a new tab).
“Until now, semicircular canals were generally used to predict the locomotion of fossil organisms,” study lead author Romain David, an evolutionary anthropologist at the Museum of Natural History in London. he said in a statement (opens in a new tab). “However, looking closely at their biomechanics, we thought we could also use them to infer body temperature.
“This is because, like honey, the liquid contained within the semicircular canals becomes less viscous [syrupy] “When the temperature rises, it affects function,” David explained. “Therefore, during the transition to endotherm, morphological adaptations were required to maintain optimal yields and we could follow them in mammalian ancestors.”
To discover the timing of this evolutionary change, the researchers measured three samples of internal auditory behaviors 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 structures of the internal auditory canal suitable for warm-blooded animals 233 million years ago.
Prior to 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 evolved from warm blood around the world. moment of its first appearance 252 million. years ago. However, new findings suggest that mammals diverged from their early ancestors more markedly than expected.
And that drastic change came surprisingly quickly. Heat-favorable auditory channels not only appeared later in the fossil record than scientists expected. It also happened much faster: they appeared at the same time as the first mammals began to evolve mustaches, skins, and specialized spine.
“Contrary to current scientific thinking, our article surprisingly demonstrates that endothermic acquisition seems[s] that it has occurred very rapidly in geological terms, in less than a million years, “said co-author of the study Ricardo Araújo, a geologist at the University of Lisbon in Portugal.” It was not a gradual and slow process. tens of millions of years as previously thought, but perhaps it was quickly achieved when it was triggered by new mammalian-like metabolic pathways and the origin of the skin. “
Follow-up studies will have to confirm the findings by other means, but the researchers said they are excited that their work will help answer one of the longest questions about the evolution of mammals.
“The origin of mammalian endothermia is one of the great unsolved mysteries of paleontology,” study lead author Kenneth Angielczyk, a paleomalogy curator at Field Museum MacArthur, said 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 believe our method is promising because it has been validated using a very large number of modern species and suggests this endotherm evolved at a time when many other features of the mammalian body plane were also coming into place. “
Originally published in Live Science.