When did warm blood appear? The answer may lie in the ears. |

The temperature of our blood sets us apart from each other in the animal world. Although the popular saying is that more than one person is cold-blooded, the truth is that people work because their blood is warm. Also known as endothermy, but it is not exclusive to mammals: birds, among others, are similar to us for this exact reason.

In the case of mammals, this allows them to control their internal body temperature by controlling their metabolic rate. It is precisely this characteristic that, millions of years ago, allowed them to occupy environmental niches from the pole to the equator, resisting the instability of ancient climates. But is this always the case?

At the beginning of its history, blood only cooled in the bodies of the creatures that lived on the planet. Little by little, life has changed, and now a group of researchers has found a way to tell if these animals that are impossible to find today are cold-blooded or warm-blooded. Just look inside their ears, even if they are fossils.

millions of years ago

Thanks to a new method of analyzing the size and shape of the inner ear canals, researchers led by Ricardo Araújo, a vertebrate paleontologist from the University of Lisbon, suggest that the ancestors in mammals suddenly becomes whole blood. hot about 233 million years ago.

The data, published in the journal Nature in July, do not shed light on when endothermy evolved, which remains an evolutionary mystery, but it may not have been long. However, based on fossil analyzes of growth rates and oxygen isotopes found in the skulls, they suggested dates for the emergence of warm blood that spanned 300 million years.

The key to determining the dates may lie in the inner ear structures of mammals and their ancestors. Consisting of a maze of semicircular canals, it contains a fluid that reacts to the movements of the head of all mammals and their ancestors, brushing the tiny hair cells of the ear and helping to maintain in balance. Well, it turns out that this fluid can be thick or thin depending on the body temperature.

The search for fossils

According to Ricardo Araújo, mammals have inner ears. very particular compared to cold-blooded vertebrates of the same size. The dimensions of their semicircular canals, such as thickness, length and radius of curvature, are very small: ” Their canals are very thin and tend to be very round compared to other animals. »

This led them to ask the key question of their study: what if, according to this hypothesis, the size and shape of the ear canals is related to the animal’s body temperature? In warm-blooded animals, the fluid may be less viscous, and the ducts may contract to recover. If this is true, it may be possible to trace in the fossils how the inner ear canals changed shape, and thus determine when warm blood first appeared in the mammal line.

The researchers hypothesized this using a tool they called ” the thermo-motility index“. They were able to link the warm blood to these measurements in the inner ear of up to 341 different vertebrates. Considering the differences in size, the value of this index was found to closely follow the animal’s body temperature, from fish to reptiles to mammals.

Therefore, while reptiles have low values ​​for the index, mammals have high values. All this adds up, and the team continues to point the finger: they apply that clue to the fossilized ear canals of 56 species of extinct mammalian ancestors.

Then came the surprise: the data revealed a sudden change in the morphology of the inner ear about 233 million years ago. This corresponds to an increase in body temperature of 5 to 9 degrees Celsius. Although the research still needs to be refined, this discovery represents a significant advance in the knowledge of evolutionary science.

To show hide the table of contents

Leave a Reply

Your email address will not be published. Required fields are marked *