A team of researchers from the University of the Witwatersrand (Wits University) determined that the emergence of warm-bloodedness, a key trait in mammals, arose during a time of climatic instability around 233 million years ago. Just when mildly cold regions became extremely hot.
Hot blood or not?
Using a novel method that analyzes the size and shape of inner ear canals, scientists suggest in new research published in the journal Nature that mammalian ancestors suddenly became warm-blooded about 233 million years ago.
Although endothermy (warm blood) is not exclusive to mammals, since birds, the only living dinosaurs, are also warm-blooded, it is one of the key characteristics of mammals, since it allows them to regulate temperature inside your body by controlling your metabolic rates. It is what allowed mammals to occupy environmental niches from the pole to the equator and weather the instability of past climates.
“For the first time, we can trace through evolution the direct consequence of the origin of endothermy in the skeletal anatomy of our premammalian ancestors ,” the authors explain.
Historical moment: about 230 million years ago
The inner ear structures of mammals and their ancestors hold the key to solving that mystery.
The researchers compared semicircular canals in 341 animals: most of the fossils in this study were found in South Africa. There is a fluid that responds to the movements of the head, rubbing against the tiny hair cells of the ear and helping to maintain the sense of balance. That liquid can become thicker or thinner depending on body temperature.
When temperatures rose, the viscosity of the inner ear fluid became runny and in a large inner ear it wouldn’t have the ability to tell the brain what to do, so that ‘s when the size of the inner ear shrank.
It’s a novel approach, since until this study, the inner ear was only used to study the movement of fossil organisms.
The researchers created a tool called the “thermomotility index” to link warm blood to those inner ear dimensions in 341 different vertebrates. Taking into account the differences in size, the value of this index turned out to closely follow the body temperature of an animal, from fish to reptiles and mammals. The reptiles had low index values; mammals were tall. The team then applied this index to the fossilized ear canals of 56 species of extinct mammalian ancestors. To their surprise, the data showed an abrupt change in the morphology of the inner ear about 233 million years ago. That would correspond to an increase in body temperature of between 5 and 9 degrees Celsius, suggesting that the endothermy evolved abruptly at that time, the experts conclude.
The time of the supposed change, about 233 million years ago, corresponds to a geologically brief interlude of highly unstable climate known as the Carnian pluvial event or Carnian pluvial event (of the late Triassic period).
This finding highlights that the Triassic was a slide in history, with a tremendously hot start after the “Great Dying”; then the Carnian pluvial episode and, finally, the dawn of the mammals and dinosaurs, who managed to survive. A pivotal moment in the history of life on Earth.
Referencia: R. Araújo et al. Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy. Nature. Published online July 20, 2022. doi: 10.1038/s41586-022-04963-z.
