The evolution of warm-bloodedness, technically known as endothermy, is considered key to mammals' survival of the end-Cretaceous extinction and our subsequent world domination. This makes the question of when it evolved an important one for paleontology. New evidence suggests it appeared about the time our ancestors became recognizably mammalian.

Humans consider temperature regulation so central to our identity we use “cold-blooded” as an insult, suggesting reptiles and fish are incapable of empathy. Arguably this has contributed to the heat of the debate about dinosaur body temperature, a particularly long-lasting and heated paleontological battle.

Less attention has been paid to the question of when our own ancestors developed this capacity, but the Natural History Museum's Dr Romain David and co-authors have addressed it in a novel way in Nature. 

The bones of a creature capable of generating its own body heat look the same as one that needs sunlight to get moving. Nevertheless, the authors realized there is one part of the body that could preserve evidence on this matter.

Vertebrates balance and navigate using fluid in our semicircular ear canals, and like most fluids this gets runnier (less viscous) at higher temperatures. Warm-blooded creatures have taken advantage of this fact by evolving smaller ear canals – reptiles need to use up a little extra space to allow their viscous ear-fluids to flow. Being more active and fast-moving, mammals require more efficiently shaped systems.

“Until now, semicircular canals were generally used to predict locomotion of fossil organisms. However, by carefully looking at their biomechanics, we figured that we could also use them to infer body temperatures,” David said in a statement. 

The difference may be small, but David and co-authors were still able to track it across 243 living species and 64 extinct counterparts. They found narrow inner-ears first appeared in the mammalian ancestry 233 million years ago.

Ancient as that may seem, it's actually 20 million years later than warm-bloodedness was previously thought to appear. It's unlikely there was a gap anything like that long between endothermy evolving and the ear canals catching up.

Once proto-mammals gave warm-bloodedness a try, however, they seem to have taken to it with enthusiasm. “Contrary to current scientific thinking, our paper surprisingly demonstrates that the acquisition of endothermy seems to have occurred very quickly in geological terms, in less than a million years,” Dr Ricardo Araújo of the University of Lisbon said. “It was not a gradual, slow process over tens of millions of years as previously thought.” Average body temperatures jumped 5-9°C (9-16°F) during this time, and would also have become much more stable.

Endothermy appears to have coincided with other distinctive mammalian traits such as fur, whiskers, and changes to the backbone. Aerobic and anaerobic capacities increased to provide the extra heat, accompanied by additional food requirements. The association in time with fur makes sense, since warm-blooded creatures, particularly small ones, need some way to prevent the warmth they are generating from escaping.

Canal size is not a perfect guide – a few living species would be placed in the wrong category if we tried to assess their body temperatures based on ear canals alone, but it works in the overwhelming majority of cases.

The findings could be seen as a blow to mammalian prestige in the context of a recent paper providing evidence early dinosaurs and pterosaurs were warmblooded about the same time or even a little earlier. On the plus side, the acquisition of endothermy appears to be associated with the acquisition of larger brains, something dinosaurs famously missed.