The origin of the warm blood of mammals is hidden deep in the ear

Stefan Glasauer didn’t think he knew much about the secrets of the inner ear. Professor at the Technological Institute and Medical School of the University of Brandenburg in Germany, doing the anatomy of hearing but above all the positional role of this strange maze. Visualize the angular movements of the head, record the acceleration of the body, adapt the vision, maintain balance … “I’ve been working on this vestibular apparatus for years. With today’s discovery that the size of the sensors can tell us the temperature of an extinct species, I admit it impresses me”trust.

An international team of scientists from seven European and American countries published Wednesday, July 20 in the journal temper naturean article that proposes a new chronology for a key stage in animal evolution: the emergence of so-called “warm-blooded” species. Until then, all the animals were out of heat. Their temperature was dependent on the temperature outside, Ricardo Araujo, a researcher at the University of Lisbon and first author of the study confirms. It is endothermic gain that has enabled mammals and birds to explore different climates, out in all seasons, day and night, to move faster and longer. All innovations attributed to our humanity also depend on this moment. »

“great idea”

To track this shift, scientists have, for several decades, doubled down on methods. Analysis of posture, diaphragm, nasal passages, teeth, chemical isotopes in bones… The results were often contradictory and unconvincing. says Romain David, a postdoctoral researcher at the Natural History Museum in London and co-first author of the research. But the main hypothesis was that this transformation occurred about 255 million years ago in a gradual manner. »

In an effort to verify their significance, in 2017 Araujo and David decided to look into the inner ear. Our reasoning was very simple, Recognize the French from London. The semicircular canals of the inner ear are sensors that lymph fluid traverses. When the temperature rises, as with honey, the viscosity of the liquid decreases, so the cilia that record the information pick it up less. We have known for more than fifty years that the ducts of mammals are relatively smaller than those of lizards or fish. We hypothesized that to compensate for the loss of efficiency, it evolved by reducing the size of the tubes. “Connecting endolymph viscosity, body temperature, and airway morphology in this way is really a great idea, comments Guillaume Billet, Lecturer in Paleontology at the National Museum of Natural History in Paris. They demonstrated this by integrating several scientific fields: fluid mechanics, physiology, morphology, and evolutionary biology. It all depends on a particularly large data set. It is very impressive. »

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