Decoding the mystery: Since when did our ancestors become warm-blooded animals?

The origin of endothermia in mammals is one of the great unsolved mysteries of paleontology.

In the animal kingdom, only mammals and birds have the ability to generate their own body temperature and control it. This process is called endothermic, or warm-blooded, process. This is one of the reasons why mammals dominate almost every global ecosystem. Warm-blooded animals are more active both during the day and at night and reproduce faster than cold-blooded animals.

Reconstructed image of Tritylodon, an extinct archaean vertebrate, a warm-blooded nocturnal species, found to be characterized by water vapor escaping from the lungs. (Illustration: Luzia Soares)

But until now, it was not known exactly when the ancestors of mammals became warm-blooded animals. Now, scientists have used cutting edge technology to study fossils from the Karoo region of South Africa to find the answer: thermogenesis evolved in the ancestors of mammals about 233 million years old. in the Late Triassic (252 to 201 million years ago).

The origin of endothermia in mammals is one of the great unsolved mysteries of paleontology. Scientists have tried many explanations, but often come to an unsatisfactory answer. This method of study is very reliable because it has been validated in many specimens of modern animals, and shows that endothermogenesis evolved at a time when mammalian bodies appeared many characteristics. another important point.

Warm blood is the key to making mammals what they are today. Endothermic generation is like the starting point in mammalian evolution: an insulated coat, a larger brain, warmer blood circulation, a faster reproductive rate, and a more active life. All of these traits were born of warm blood.

Until now, most scientists have assessed the transition to autothermia as a slow process over tens of millions of years that began near the extinction event in the Triassic period. Meanwhile, some other ideas suggest that it happened closer to the time of the appearance of mammals, about 200 million years ago.

However, this new study suggests that endothermogenesis appeared in mammalian ancestors about 33 million years before mammals appeared. This date is consistent with recent findings that many mammalian features, such as antennae and fur, also appeared earlier than previously thought.

This research comes from the research of scientists about the inner ear. The inner ear is not only the organ of hearing, but also contains the organ that controls balance, the semicircular canals. These three canals are oriented in three spatial dimensions. They are filled with fluid that moves in the canals each time the head moves and activates receptors to tell the brain the exact three-dimensional position of the head and body. The viscosity of this fluid (intra-lymphatic) is important so that the balance organ can accurately detect the rotation of the head and aid in balance.

Just as a piece of butter comes out of the fridge and melts when placed in a hot pan or honey becomes thicker in cold weather, the viscosity of the lymphatic organs changes with body temperature. This means that as body temperature is higher, intralymphatic viscosity also changes. Before this change, the body must also make adjustments to adapt. In mammals, semicircular canals are adapted by changing their shape.

The researchers noticed a change in the shape of these canals in fossil specimens. They think that if we find species with such changes, it is possible to pinpoint the exact time of evolution of endothermic generation. And it is from the abundant fossils in Karoo, South Africa, that they have come to a convincing conclusion.

The arid Karoo region offers very good conservation conditions for a whole treasure of fossils, including fossils of mammalian ancestors. These fossils serve as an unbroken continuous record of the evolution of life over a period of about 100 million years. They document the transition from reptile to mammal in great detail.

Using 3D CT scanning, the researchers reconstructed the inner ears of dozens of mammal ancestors in Karoo and other parts of the world. From there, they discovered exactly which species have inner ear anatomy suited to warmer body temperatures and which don't.

Brain (pink) and inner ear (blue) of a primate - today's mammal, reconstruction illustrated by Julien Benoit.

One point to note is the geographical location of the Karoo region at the time these animals lived. At that time, this land was located closer to Antarctica than it is now due to continental drift. This means that warmer body temperature (from evidence of altered inner ear shape) is not due to a warmer climate. Since the average climate in South Africa was then colder, the variation in intra-lymphatic viscosity in the inner ear could only be attributed to the body temperature of warmer, evolved mammalian ancestors.

Until now, scientists could only rely on the characteristics of the skeleton to reconstruct the evolution of endothermic generation, making the research results inaccurate. With the findings of the inner ear research, they believe this could be the key to unlocking a treasure trove of knowledge about mammalian ancestors.