Mammals were not the first to be warm-blooded
Karoo fossils provide “smoking gun” on clues to when warm-bloodedness evolved in pre-mammalian ancestors
Peer-Reviewed PublicationIMAGE: A WARM-BLOODED MAMMAL ANCESTOR BREATHING OUT HOT HAIR IN A FRIGID NIGHT. view more
CREDIT: LUZIA SOARES
Endothermy, or warm-bloodedness, is the ability of mammals and birds to produce their own body heat and control their body temperature.
This major difference with the cold-blooded reptiles underpins the ecological dominance of mammals in almost every ecosystem globally. Until now, it was not known exactly when endothermy originated in mammalian ancestry. A team of international scientists, including researchers from the University of the Witwatersrand (Wits University) in Johannesburg, South Africa, has found the smoking gun of this key evolutionary event in the inner ears of fossils from South Africa and around the globe.
A new study suggests that endothermy appeared in mammalian ancestors about 233 million years ago, well before the origin of mammals, which occurred about 200 million years ago. This study, titled Inner ear biomechanics reveals Late Triassic origin of mammalian endothermy is published in Nature.
“For the first time, we are able to trace through evolution the direct consequence of the origin of endothermy on the skeletal anatomy of our pre-mammalian ancestors,” says Dr Julien Benoit, Senior Researcher in Palaeontology at the Evolutionary Studies Institute at Wits University. “This is an exciting time for our field of study.”
The intuition
The inner ear is not only the organ of hearing, but also houses the organ of balance: the semicircular canals. The three semicircular canals of the inner ear are oriented in the three dimensions of space and are filled with a fluid that flows in the canals as the head moves and activates receptors to tell the brain the exact three-dimensional position of the head and body. The viscosity, or runniness, of this fluid (called the endolymph) is of paramount importance for the balance organ to efficiently detect head rotation and aid balance.
As for any other fluid, the viscosity of the endolymph changes with body temperature, just like a piece of butter turns from solid to liquid in a warm pan. Because of this physical property, the viscosity of the endolymph would be altered by the evolution of a higher body temperature. However, this change of viscosity cannot be left unchecked because the balance organ would stop working properly. The semicircular canals of the inner ear must adapt to the new viscosity imposed by higher body temperature: they have to change their geometry.
The key intuition of the two lead authors of the discovery, Dr Ricardo Araújo (University of Lisbon) and Dr Romain David (Natural History Museum of Paris), was to realise that this change in the semicircular canals shape would be easy to trace through geologic time using fossils. Pinpointing the species in which this change of geometry occurred would work as an accurate guide to when endothermy evolved: the smoking gun identifying when mammalian ancestors transitioned from cold-blooded to warm-blooded.
“Until now, semicircular canals were generally used to predict locomotion of fossil organisms. However, by carefully looking at their biomechanics, we figured that we could also use them to infer body temperatures. This is because, like honey, the fluid contained inside semicircular canals gets less viscous when temperature increases, impacting function. Hence, during the transition to endothermy, morphological adaptations were required to keep optimal performances, and we could track them in mammal ancestors,” says Dr Romain David, Post-Doctoral Researcher at the Natural History Museum and Lead Author on the paper.
The contribution of Karoo fossils
The team found that the inner ear canals geometry adapted to a relatively abrupt change in endolymph viscosity some 233 million years ago, indicating that the overall body temperature of mammal ancestors became warmer at this time.
Fossils from the South African Karoo played a key role in this discovery, in part because of the wealth of fossils of mammal ancestors that the Karoo-aged rocks have produced in more than a hundred years of study.
South African fossils offer an unbroken record of the evolution of life during an interval of almost 100 million years, documenting the transformation from reptilian-like animals (the therapsids) to mammals in exquisite detail. In addition, because the Karoo was situated closer to the South Pole at that time as a result of continental drift, 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 colder on average, the change in inner ear fluid viscosity can only have been caused by a generally warmer body temperature in mammalian ancestors,” says Benoit.
Using cutting edge CT-scanning techniques and 3D modelling, the researchers were able to reconstruct the inner ear of dozens of mammalian ancestors from the South African Karoo and elsewhere in the world, and managed to point out exactly which species had an inner ear anatomy consistent with a warmer body temperature, and which ones did not.
A change of paradigmn
Until now the general expectation was that endothermy arose very close to the Permo-Triassic boundary, about 252 million years ago, or perhaps closer to the origin of mammals 200 million years ago. The new results suggest that endothermy appeared in mammalian ancestors some 233 million years ago. This new date is consistent with the recent findings that many of the traits usually associated with “mammalness”, such as whiskers and fur, also evolved earlier than previously expected. More importantly, the results support that the evolutionary transition to warmbloodedness was unexpectedly fast.
“Contrary to current scientific thinking, our paper surprisingly demonstrates that the acquisition of endothermy seems to have occurred very quickly in geological terms, in less than a million years,” says Dr Ricardo Araújo, Junior Researcher at Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, University of Lisbon and Lead Author on the paper. “It was not a gradual, slow process over tens of millions of years as previously thought, but maybe was attained quickly when triggered by novel mammal-like metabolic pathways and origin of fur.”
“The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology”, says Dr Kenneth D. Angielczyk, MacArthur Curator of Paleomammalogy at Field Museum of Natural History, USA and Senior Author on the paper.
“Many different approaches have been used to try to predict when it first evolved, but they have often given vague or conflicting results. We think our method shows real promise because it has been validated using a very large number of modern species, and it suggests that endothermy evolved at a time when many other features of the mammalian body plan were also falling into place.”
JOURNAL
Nature
ARTICLE TITLE
Inner ear biomechanics reveals Late Triassic origin of mammalian
ARTICLE PUBLICATION DATE
20-Jul-2022
The size of mammal ancestors’ ear canals reveal when warm-bloodedness evolved
Higher body temperatures went hand-in-hand with runnier “ear fluid” and narrower ear canals
Peer-Reviewed PublicationCAPTION
Size differences between inner ears (in grey) of warm-blooded mammaliamorphs (on the left) and cold-blooded, earlier synapsids (on the right). Inner ears are compared for animals of similar body sizes.
CREDIT
Credit: Romain David and Ricardo Araújo
One of the things that makes mammals, mammals is that we’re warm-blooded-- our bodies have high metabolisms that maintain our internal temperature independent of our surroundings, unlike cold-blooded animals like lizards that have to bask in the sun. Among modern animals, only mammals and birds are warm-blooded, and our ability to keep ourselves warm has enabled mammals to survive in icy weather and make long migrations. But it’s been a mystery exactly when mammals evolved their high metabolisms. In a new study in Nature, scientists point to an unlikely source for determining when ancient mammal ancestors became warm-blooded: the size of tiny structures in their inner ears.
It’s hard to tell whether a fossil animal was warm-blooded-- we can’t take the temperature of a creature that lived hundreds of millions of years ago, and we can only guess at whether its behavior matched an active, warm-blooded metabolism or a slower cold-blooded one. But a team of researchers led by London’s Natural History Museum, the University of Lisbon’s Instituto Superior Técnico, and the Field Museum in Chicago realized that animals’ ears provide an indirect clue about their body temperatures.
All vertebrate animals’ ears contain tiny canals filled with fluid that helps us balance. The viscosity, or runniness, of that fluid changes based on temperature, and our inner ears have evolved different sizes so that it can flow correctly. Cold-blooded animals’ ear fluid is cooler and thicker, so it needs wider spaces to travel through, while warm-blooded animals have runnier ear fluid, so our semicircular canals don’t need to be as big.
“Until now, semicircular canals were generally used to predict locomotion of fossil organisms. However, by carefully looking at their biomechanics, we figured that we could also use them to infer body temperatures,” says Romain David, a post-doctoral researcher at the Natural History Museum and one of the study’s lead authors. “This is because, like honey, the fluid contained inside semicircular canals gets less viscous when temperature increases, impacting function. Hence, during the transition to endothermy, morphological adaptations were required to keep optimal performances, and we could track them in mammal ancestors.”
To track these evolutionary changes, the researchers compared the sizes of the inner ear canals of 341 animals, including 243 living species and 64 extinct ones. They found that mammal ancestors didn’t develop the kinds of inner ear structures ideal for warm-blooded animals until 233 million years ago-- nearly 20 million years later than scientists had previously thought warm-bloodedness evolved. And, based on when those differently-sized semicircular canals showed up in the fossil record, it seems that when mammal ancestors did evolve warm-bloodedness, it happened much more quickly than scientists had thought, around the same time that proto-mammals started to evolve whiskers, fur, and specialized backbones. The evolution of fur and warm-bloodedness at about the same time especially make sense because fur traps the body heat generated by a higher metabolism, helping keep the body at the high temperature it needs to thrive.
“Contrary to current scientific thinking, our paper surprisingly demonstrates that the acquisition of endothermy seems to have occurred very quickly in geological terms, in less than a million years,” says Ricardo Araújo, junior researcher at Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, University of Lisbon and one of the paper’s lead authors. “It was not a gradual, slow process over tens of millions of years as previously thought, but maybe was attained quickly when triggered by novel mammal-like metabolic pathways and origin of fur.”
The study’s senior author, Ken Angielczyk, says he’s excited by how the study helps answer one of the longest-standing questions about the evolution of mammals.
“The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology. Many different approaches have been used to try to predict when it first evolved, but they have often given vague or conflicting results,” says Angielczyk, the Field Museum’s MacArthur Curator of Paleomammalogy. “We think our method shows real promise because it has been validated using a very large number of modern species, and it suggests that endothermy evolved at a time when many other features of the mammalian body plan were also falling into place.”
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JOURNAL
Nature
ARTICLE TITLE
Inner ear biomechanics reveals the Late Triassic origin of mammalian endothermy
ARTICLE PUBLICATION DATE
20-Jul-2022
