The ability to produce and regulate our body temperature, known as endothermy or "warm-blooded", is one of the reasons that mammals have been able to survive in the freezing poles and the steaming rainforests. But exactly when this ability evolved has been difficult to answer. Now, new research published in eLife claims that in the lineage that gave rise to mammals, it might not have evolved just once, but twice.
Most endothermic animals share a variety of traits, such as feathers or fur, and a secondary palate that allows for continuous breathing even while eating. In the lineage of ancient animals called therapsids, also known as “mammal-like reptiles” as they are seen in effect as a transition between reptiles and mammals, these traits are thought to have emerged during the Permian.
But putting an exact timing on this is something of a problem. So rather than looking at the physical features of the fossil therapsids alive at this time, the researchers from the University of Witwatersrand, Johannesburg, instead turned to chemistry. They focused on the ratio of two different oxygen isotopes locked within the fossil bone and teeth, namely oxygen-16 and oxygen-18. The ratio of these two isotopes changes depending on whether or not an animal is able to maintain a stable internal body temperature.
After comparing different therapsid fossils, they found that two different lineages had significantly different ratios of the oxygen isotopes, suggesting that their body temperatures were therefore different to the rest of the animals alive at the time, and drawing the conclusion that this might mean they were endothermic.
One of these lineages was that of Cynodontia, which eventually gave rise to all mammals, but the other was that of a closely-related group known as Dicynodontia. The study shows that endothermy seems to have evolved between 259 and 252 million years ago, before the rise of the dinosaurs, and crucially just before the massive end Permian extinction event, which wiped out much of life on Earth.
As the last common ancestor of the two groups was ectothermic (cold-blooded), it suggests that both groups, therefore, became endothermic independently. They postulate that the evolution of endothermy just before the extinction event may have helped the early mammal-like reptiles survive the changing climate and environmental conditions.
We already suspect that endothermy evolved independently at least twice in the evolution of animals, as both mammals and birds produce their own body heat, but this latest evidence suggests that even within the lineage that gave rise to mammals, it could well have sprung up multiple times. This implies that it is not that difficult a trait to evolve, and may simply be down to whether or not the right selection pressure is present on the animals in question.