“Warm-blooded”, one of the defining characteristics of the organism of humans and other mammals, emerged 230 million years ago, at the beginning of the Age of Dinosaurs, says an international team of researchers.
According to them, this evolutionary innovation crucial to our lineage, which allows animals to maintain an adequate body temperature despite variations in heat and cold in the environment around them, is so old that it even predates the origin of mammals themselves. .
The data that support the hypothesis have just been published in the specialized journal Nature by a group led by the Portuguese Ricardo Araújo, from the Instituto Superior Técnico of the University of Lisbon. “Our estimate for the origin of endothermy [designação técnica do ‘sangue quente’] indicates that it occurred in the group of mammaliamorphs, between 35 million and 55 million years before the origin of mammals”, Araújo told Sheet.
Along with European, American and Argentinian colleagues, Araújo used an ingenious method to estimate what the body temperature of animals that lived in the Triassic period, the first subdivision of the Age of Dinosaurs, would be like. As it is obviously impossible to use a thermometer on fossils, the solution was to take into account the conformation of the inner ear, a structure whose details are linked to the heat of the organism.
It turns out that endothermic animals, in addition to not experiencing body temperature fluctuations due to climate, also tend to have a higher average temperature than vertebrates that do not have this ability, such as reptiles and amphibians. And that has an impact on the way the inner ear works.
This structure is formed by a series of ducts, inside which a fluid called endolymph circulates. The back-and-forth of the endolymph in these ducts acts as a motion sensor, activating cells that tell the brain about the position of the animal’s head and helping to refine its orientation in space.
It turns out that temperature influences precisely the way endolymph flows through the ducts of the inner ear. “Therefore, the ear had to adapt, in the case of mammalian ancestors, to these new conditions of higher temperatures. Think of a car’s speedometer: if one of its electronic components stopped working above 25 degrees Celsius, you would have to pay very expensive fines to the São Paulo police”, explains Araújo.
In this case, the ducts of the “warm-blooded” animals became narrower, for example, which helps to date the evolutionary moment in which endothermy appeared.
In fact, he points out, two important fossils from the Triassic found in Rio Grande do Sul — small animals and probably already with some hair, the Brasilodon it’s the Riograndense— precisely document how the transition took place.
“O Brasilodon is a mammaliamorph and one of the first endothermic animals, but on the other hand the probainognathus Riograndense, a basal animal, it was clearly ectothermic [de sangue frio]”, says the Portuguese researcher.
The process that triggered the emergence of this ability in mammals is still not entirely clear. One possibility is the presence of major environmental instabilities in the Triassic, a period that started out very hot, cooled down and then reached a phase of heavy rainfall and major climatic variations. This could have favored animals able to better control their body temperature.
“On the other hand, the mammaliamorphs already had characteristics that were markedly mammalian, with dental differentiation (incisors, canines and molars), regionalization of the vertebral column and increased brain capacity. Therefore, there was also an evolutionary context that conditioned the origin of endothermy”, concludes Araújo.
The other group of warm-blooded vertebrates today, the birds, probably achieved similar results in their physiology independently, from the characteristics of the subgroup of dinosaurs to which they belonged.
