PARIS: The ancestors of mammals started to become warm-blooded around 20 million years later than previously thought, researchers suggested on Wednesday, after analysing inner-ear fossils hoping to solve “one of the great unsolved mysteries of palaeontology”.

Warm-bloodedness is one of the quintessential characteristics of mammals, along with fur, but exactly when they first evolved the feature has long been a subject of debate.

Previous research has indicated that the ancestors of mammals began evolving warm-bloodedness, or endothermy, around 252 million years ago — around the time of the Permian extinction, known as the “Great Dying”. However figuring out the timeline has proved difficult.

“The problem is that you cannot stick thermometers in your fossils, so you cannot measure their body temperature,” said Ricardo Araujo of the University of Lisbon, one of the authors of a new study in the journal Nature. He was part of an international team of researchers that found a new way to determine how body heat changed throughout time, by examining the semicircular canals in the inner ears of 56 extinct species of mammal ancestors. Fluid runs through the tiny ear canals, which help animals keep their balance.

The researchers realised that as body temperatures warmed up, so did the ear fluid. Araujo gave the example of oil used to fry hot chips. Before you warm the oil up, it is “very viscous, very dense,” he said.

“But then when you heat it up, you’ll see that the oil is much runnier, it flows much more easily.” The runnier ear fluid led to animals evolving narrower canals — which can be measured in fossils, allowing the researchers to track body temperature over time.

Unlike previous research on this subject, the team developed a model that not only works on extinct mammal ancestors, but also living mammals, including humans.

Published in Dawn, July 21st, 2022