Mystery solved: Cutting-edge technology reveals when mammalian ancestors became warm-blooded

Mammals and birds generate their own body heat and control their body temperature.

Mammals and birds generate their own body heat and control their body temperature. This process is known as endothermic, or warm-blooded, and may be one reason why mammals tend to dominate nearly every global ecosystem.

Warm blooded animals are more active both day and night than cold blooded animals and they reproduce more quickly.

However, until now it is not known exactly when endotherms came from the ancestors of mammals. Our new study, just published in Nature, changes that.

A combination of scientific intuition, fossils from the Karoo region of South Africa and cutting-edge technology has provided the answer: endothermy developed in the ancestors of mammals about 233 million years ago during the Late Triassic period.

The origin of mammalian endothermy has been one of the great unsolved mysteries of paleontology.

Many different approaches have been used to try to determine the answer but they often give unclear or conflicting results. We think our method shows real promise as it has been validated using a large number of modern species.

This suggests that endothermy evolved at a time when many other features of the mammalian body plan also fell into place.

Warm-bloodedness was the key to what made mammals what they are today.

Endothermy is likely the starting point from which mammals evolved: the acquisition of insulating fur coats; the evolution of a larger brain, supplied with warmer blood; faster reproduction rate; and a more active life is a hallmark of mammals that evolved because they were warm-blooded.

Until now, most scientists had speculated that the transition to endothermic was a gradual and slow process over tens of millions of years that began near the Permo-Triassic boundary, although some suggest it occurred closer to a mammalian origin, around 200 million years ago. .

In contrast, our results suggest that it appeared in the ancestor of mammals about 33 million years before the origin of mammals.

The new date is consistent with recent findings that many traits normally associated with “mammals”, such as whiskers and fur, also evolved earlier than previously thought.

And according to our results, endothermia develops very rapidly in geological terms, in less than a million years. We suggest that the process may have been triggered by a new mammalian-like metabolic pathway and the origin of feathers.

Scientist’s intuition

Our research began with Dr Araújo and Dr David’s intuition about the inner ear. It is more than just the organ of hearing: it also houses the organ of balance, the semicircular canal.

The three semicircular canals of the inner ear are oriented in three spatial dimensions.

They are filled with fluid that flows in the canals as the head moves and activates receptors to tell the brain the precise three-dimensional position of the head and body.

The viscosity, or thickness, of this fluid (called endolymph) is critical to the ability of the balance organs to efficiently detect head rotation and aid balance.

In the same way that a piece of butter changes from solid to liquid in a warm saucepan, or honey becomes thicker when cold, the viscosity of endolymph changes with body temperature.

That means the viscosity of the endolymph will normally be altered by the evolution of higher body temperatures. But the body has to adapt because the change in viscosity will prevent the semicircular channel from working properly.

In mammals, the canals adapt to higher body temperatures by changing their geometry.

The researchers realized that the changes in the shape of this semicircular canal would be easy to trace through geological time using fossils.

Determining the species in which the geometry changes occurred, they argue, would provide accurate guidance on when endotherms evolved.

They needed fossils to test their hypothesis – and that’s where South Africa’s fossil wealth from the Karoo region comes in.

Reconstruction and study

The arid Karoo region holds a treasure trove of fossils, many of which belong to mammalian ancestors.

These fossils offer an unbroken record of the evolution of life over nearly 100 million years. They document the transformation from reptile-like animals (therapsids) to mammals in great detail.

Using state-of-the-art CT scanning techniques and 3D modeling, we were able to reconstruct the inner ears of dozens of mammal ancestors from the Karoo of South Africa and elsewhere in the world.

From there we were able to pinpoint which species had an inner ear anatomy consistent with warmer body temperatures, and which did not.

One thing we have to consider is the geographic position of the Karoo at the time these animals lived. It was located closer to the South Pole than it is today as a result of continental drift.

That means the warmer body temperature suggested by the inner ear geometry cannot be due to a warmer climate overall.

Because South Africa’s climate is on average cooler, changes in the viscosity of the inner ear fluid can only be due to the generally warmer body temperature of our mammalian ancestors.

Fun time

This is an exciting time for our field. Until recently, to reconstruct endothermic evolution, scientists had only access to skeletal features that were questionably correlated with warm-bloodedness.

Every attempt is a long shot to get an accurate result. The inner ear, as this study shows, changes it. We believe this may be the key to unlocking more knowledge about the ancestors of mammals in the future.

(Conversation)

By Julien Benoit, University of the Witwatersrand, Kenneth D. Angielczyk, University of Chicago, Ricardo Miguel Nóbrega Araújo, Universidade de Lisboa, and Romain David, Museum of Natural History

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