Stefan Glasauer didn’t think he knew much about the secrets of the inner ear. Professor at the Technological Institute and at the School of Medicine of the University of Brandenburg, in Germany, he dissects the auditory but above all positional role of this curious labyrinth. Perceive the angular movements of the head, record the accelerations of the body, adapt the vision, maintain the balance… “I have been working on this vestibular system for years. Discovering today that the size of the sensors could tell us about the temperature of extinct species, I admit that it impresses me”he confides.
An international team bringing together scientists from seven European and American countries published Wednesday, July 20, in the journal Naturean article proposing a new chronology of an essential stage in animal evolution: the appearance of so-called “warm-blooded” species. “Until then, the animals were all ectothermic. Their temperature depended on that of the outside, underlines Ricardo Araujo, researcher at the University of Lisbon and first author of the study. It is the acquisition of endothermy that has enabled mammals and birds to explore different climates, to go out in all seasons, day and night, to move faster and longer. All the innovations attributed to our humanity also depend on this moment. »
“A Brilliant Idea”
To track this shift, scientists have, for several decades, multiplied approaches. Analyze the posture, the diaphragm, the nasal passages, the dentition, the chemical isotopes in the bones… “The results were often contradictory and unconvincing, says Romain David, postdoctoral researcher at the Natural History Museum in London and co-first author of the research. But the main hypothesis was that this transition took place about 255 million years ago in a gradual way. »
It was in an attempt to verify its relevance that in 2017 Araujo and David decided to look into the inner ear. “Our reasoning was quite simple, confides the Frenchman from London. The semicircular canals of the inner ear are sensors traversed by a fluid, the lymph. When the temperature increases, it’s like with honey, the viscosity of the fluid decreases, so the cilia that record the information pick it up less well. We have known for more than fifty years that the canals of mammals are proportionally smaller than those of lizards or fish. We hypothesized that, to compensate for the loss of efficiency, they had evolved by decreasing the size of the tubes. “Linking endolymph viscosity, body temperature and duct morphology in this way is really a brilliant idea, comments Guillaume Billet, lecturer in paleontology at the National Museum of Natural History in Paris. And they demonstrated it by integrating several scientific fields: fluid mechanics, physiology, morphology and evolutionary biology. All based on a particularly large data set. It is very impressive. »
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