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

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

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

Mammals and birds produce their own body heat and control their body temperature. This process is known as endothermy, or warm-bloodedness, and it may be one of the reasons why mammals tend to dominate almost all global ecosystems.

Warm-blooded animals are more active during both days and nights than their cold-blooded counterparts and they reproduce faster.

But until now, it has not been known exactly when endothermy originated in the lineage of mammals. Our new study, just published in Nature, changes that.

A combination of scientists’ intuition, fossils from South Africa’s Karoo region and cutting-edge technology has provided the answer: endothermy evolved in mammalian ancestors about 233 million years ago during the late Triassic period.

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

Many different approaches have been used to try to find the answer but they have often produced vague or contradictory results. We find our method very promising because it has been validated with a very large number of modern species.

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

Warm-bloodedness is the key to what makes mammals what they are today.

Endothermy was likely the starting point at which the mammal evolved: the acquisition of an insulating coat; the development of a larger brain, supplied with warmer blood; a faster reproduction rate; and a more active life define all mammalian traits evolved due to warm-bloodedness.

Until now, most scientists have speculated that the transition to endothermy was a gradual, slow process over tens of millions of years that began near the Permo-Triassic boundary, although some suggested that it happened closer to the origin of mammals, about 200 million years ago.

In contrast, our results suggest that it was present in mammalian ancestors about 33 million years before the origin of mammals.

The new date is consistent with recent findings that many of the features typically associated with “mammals,” such as whiskers and fur, also evolved earlier than previously expected.

And according to our results, endothermy evolved very quickly in geological terms, in less than a million years. We suggest that the process may have been triggered by novel mammalian-like metabolic pathways and the origin of fur.

Scientists’ intuition

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

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

They are filled with a fluid that flows in the channels when the head moves and activates receptors to tell the brain the exact three-dimensional position of the head and body.

The viscosity, or fluidity, of this fluid (called endolymph) is critical to the balance organ’s ability to effectively detect head rotation and facilitate balance.

In the same way that a piece of butter turns from solid to liquid in a warm panor honey thickens when it is cold, endolymph viscosity changes with body temperature.

This means that the viscosity of the endolymph would normally change through the development of a higher body temperature. But the body has to adapt because changing viscosity would prevent the semicircular canals from working properly.

In mammals, the ducts adapt to higher body temperature by changing their geometry.

The researchers realized that this change in the shape of the semicircular canals would be easy to trace through geologic time using fossils.

Pinpointing the species where the change in geometry occurred would, they reasoned, provide an accurate guide to when endothermy evolved.

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

Reconstruction and studies

The arid Karoo region preserves a treasure trove of fossils, many of them belonging to mammalian ancestors.

These fossils offer an unbroken record of the development of life over a period of almost 100 million years. They document the transformation from reptilian animals (therapists) to mammals in exquisite detail.

Using cutting-edge CT scanning techniques and 3D modeling, we were able to reconstruct the inner ear of dozens of mammalian ancestors from South Africa’s Karoo and elsewhere in the world.

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

One what we had to take into account was the geographical location of the Karoo at the time these animals lived. It was located closer to the South Pole than it is now due to continental drift.

This means that the warmer body temperature suggested by the geometry of the inner ear cannot be due to an overall warmer climate.

As the South African climate was on average colder, the change in the viscosity of the fluid in the inner ear can only have been caused by a generally warmer body temperature in mammalian ancestors.

An exciting time

This is an exciting time for our area. Until now, to reconstruct the evolution of endothermy, researchers only had access to skeletal traits that questionably correlated with warm-bloodedness.

Each attempt was a long shot to get accurate results. The inner ear, as this research shows, changes this. We believe it could be the key to unlocking more knowledge about mammalian ancestors in the future.

(The conversation)

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

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