Mammals and birds produce their own body heat and regulate their body temperature. This process is known as endothermy, or warm-bloodedness, and may be one of the reasons why mammals tend to dominate almost every global ecosystem. Warm-blooded animals are more active during the day and night than their cold-blooded counterparts and reproduce faster.
But until now, it was not known exactly when endothermy arose in mammalian ancestors. Our new study, just published in Nature, changes that. A combination of scientists' intuition, fossils from South Africa's Karoo region, and state-of-the-art technology provided the answer: endothermy evolved in mammalian ancestors about 233 million years ago during the Late Triassic period.
The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology. Many different approaches have been used to try to determine the answer, but often provide vague or conflicting results. We think our method is really promising because 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 were also fitting together.
Warm-bloodedness is the key to what makes mammals what they are today. Endothermy was probably the starting point where mammals evolved: acquiring an insulating fur coat; the evolution of a larger brain, supplied with warmer blood; faster reproduction; and a more active life are all defining characteristics of mammals that have evolved through warm-bloodedness.
Until now, most scientists assumed that the transition to endothermy was a gradual, slow process lasting tens of millions of years, starting near the Permo-Triassic boundary, although some suggest that it occurred closer to the origin of mammals, about 200 million years ago. .
In contrast, our results suggest that it appeared in mammalian ancestors about 33 million years before the origin of mammals. The new date is consistent with recent findings that many features typically associated with "mammals," such as beards and fur, also evolved earlier than previously expected. And according to our results, endothermy evolved very quickly from a geological point of view, in less than a million years. We propose that this process may have been triggered by novel mammalian-like metabolic pathways and the origin of fur.
Intuition of scientists
Our research began with Dr. Araúj 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 of the inner ear are oriented in three dimensions of space. They are filled with fluid that flows in the channels when the head moves and activates receptors that 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 aid balance. Brain (pink) and inner ear (green) of a modern mammal, a primate, reconstructed in 3D. Julien Benoit
Just as a piece of butter changes from solid to liquid in a warm pan, or honey thickens when cold, the viscosity of endolymph changes with body temperature. This means that endolymph viscosity would normally change with the development of a higher body temperature. But the body has to adapt because the change in viscosity would prevent the semicircular canals from working properly. In mammals, the ducts adapt to higher body temperature by changing their geometry.
Scientists realized that this change in the shape of the semicircular canals would be easy to trace through geologic time using fossils. Pinpointing the exact species in which the geometry change occurred would provide a precise clue as to when endothermy evolved, they say.
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 study
The arid Karoo region holds a treasure trove of fossils, many of which belong to the ancestors of mammals. These fossils offer an intact record of the evolution of life over nearly 100 million years. They document the transformation of reptile-like animals (therapsids) into mammals in exquisite detail.
Using state-of-the-art CT scanning and 3D modeling techniques, we have been able to reconstruct the inner ear of dozens of mammalian ancestors from the South African Karoo and elsewhere in the world. From there, we were able to pinpoint which species had inner ear anatomy consistent with a higher body temperature and which did not.
Read more: What fossils reveal about the hairy history of mammalian ancestors
One thing we had to take into account was the geographical location of the Karoo at the time these animals lived. It was placed closer to the South Pole than it is now due to continental drift. This means that the higher body temperature suggested by the geometry of the inner ear cannot be caused by an overall warmer climate. As the climate of South Africa was on average cooler, the change in inner ear fluid viscosity could only be due to a generally higher body temperature in mammalian ancestors.
An exciting time
This is an exciting time for our industry. Until now, to reconstruct the evolution of endothermy, scientists only had access to skeletal traits that were questionably correlated with warm-bloodedness. Each attempt was a long shot to achieve accurate results. The inner ear, this research shows, changes that. We believe this may be the key to unlocking more knowledge about mammalian ancestors in the future.
Julien Benoit receives funding from the Paleontological Science Foundation (PAST) and its dispersal projects; NRF; and the DST-NRF Center of Excellence in Palaeosciences (GENUS, CoE in Palaeosciences).
Kenneth D. Angielczyk receives funding from the US National Science Foundation and the Field Museum of Natural History.
Ricardo Miguel Nóbrega Araújo is funded by Fundação para a Ciência e a Tecnologia postdoctoral fellowship SFRH/BPD/96205/2013, FCT–AGA KHAN Development Network grant number 333206718, member of the National Geographic Society, MRI platform national grant number CP-109R- infrastructure France -BioImaging supported by the French National Research Agency (ANR-10-INBS-04, «Investment for the future»), labex CEMEB (ANR-10-LABX-0004) and NUMEV (ANR-10-LABX-0020 ). IPFN activities received financial support through projects UIDB/50010/2020 and UIDP/50010/2020
Romain David receives funding from the Calleva Foundation.
Julien Benoit, Senior Research Fellow in Vertebrate Paleontology, University of Witwatersrand And
Kenneth D. Angielczyk, Lecturer, University of Chicago And
Ricardo Miguel Nóbrega Araújo, Junior Researcher, Universidade de Lisb
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