Ear Fossil Reveals new Origins of our Warm-Blooded Ancestors

Warm-blooded animals, also called endotherms, can maintain a constant body temperature by employing a fast metabolic system of their body. In contrast, the cold-blooded animals or the ectotherms fail to maintain their body temperature owing to a slow rate of metabolism and rely on the environment to keep themselves warm.

Mammalian species are warm-blooded, but how they evolved to be and when this happened is a contested matter amongst the scientists manifested in the difference in their estimations. Some studies put the period of the emergence of the endotherms around as recently as 145 million to 66 million years ago. In comparison, others estimated it to be around 300 million to 250 million years ago. New research has brought new aspects to this field in this contested space. The interesting work was published in the journal Nature on July 20.

The new work is based on the findings of fossilised inner ear canals, which are made up of tiny bones. Earlier scientists used to study skeletal features like bone structure or fur and feathers that warm-blooded animals grow on their bodies. The warm-blooded animals usually have faster growth of bones and have fur and feathers on their body. Researchers studied these aspects to estimate when did mammalian species start their journey towards being warm-blooded or endotherms. The skeletal feature, like bone structure, also reflects metabolic rate. As warm-blooded animals have a higher metabolic rate compared to cold-blooded ones, scientists used these to estimate evolution points. The new research is a deviation from these early ways.

The new research led by Ricardo Araujo, a palaeontologist at the University of Lisbon, along with his colleagues, proposed that the shape and size of the bony canals of the inner ear can be a proxy of the body temperature. Another lead author of the study, Romain David, a palaeontologist at the natural history museum, London, did his doctoral studies on the structure of the bony canals of the inner ear.

During his research, he found that mammals have smaller semicircular canals than other vertebrates. David explained his clicking point for the study: "The whale shark is actually the animal with the largest semicircular canals on Earth, much larger than the canals in whales. I thought, maybe this has to do with body temperature.”

Along with it, David was also excited by the importance of the fluid inside the canals. The fluid, named endolymph, can change its viscosity depending on temperature. An example is oil in a hot pan, which becomes thinner when heated and thicker when getting cold. David had the idea that endolymph viscosity and the semicircular canal size are related, reflecting endothermy.

To test the hypothesis, David and his colleagues examined the ear canals of 277 living animal species, including alpacas, turkeys and lizards. The results they got indicated the confirmation of David's hypothesis. The warm-blooded animals have less viscous endolymph along with smaller and thinner canals. On the other hand, cold-blooded animals had thicker endolymph with more extensive and thicker canals.

With this, the researchers attempted to trace when animals with smaller and thinner canals appeared, which is a sign of the evolution of the endotherms. The team of researchers estimated this time as around 233 million years ago, during the late phase of the Triassic period. This is later than thought previously.

However, experts are not sure whether the new findings, although exciting, can settle the debate on the time of evolution of the endotherms. Nevertheless, they believe the findings have brought new aspects into the field of research.