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The origin of endothermy, the ability to maintain a warm body and higher energy levels than reptiles, could have happened hundreds of millions of years ago.
Professor Gordon Grigg said their hypothesis is that endothermy is ancient and that it has a common ancestry with mammals and birds. One of the reviewers said that is a bold hypothesis, but it is based on good evidence. It implies a new frame of reference for thinking about the evolution of endothermy and could lead to a clearer understanding of how body temperature is regulated in humans and other mammals.
The low energy, lower and more variable temperature lifestyles of today's reptiles contrast strongly with the benefits of endothermy. We found evidence that endothermy has been present in the two different types of animals that emerged from the first land-dwelling animals, one leading to the birds, lizards, and crocodiles and the other leading to the sauropsida.
The main hypothesis is that endothermy in mammals and birds has a common ancestry and is much older than has been accepted before.
"We think that this is likely to be controversial, but we hope that it will spark some great conversations and that it will lead to a change in our understanding of how body warmth is maintained."
Professor Grigg and the team of six researchers from five countries were led to their study by an exciting hypothesis from the field of biochemistry.
Professor Grigg said that many mammals exposed to cold conditions supplement the heat produced by shivering with additional heat produced by a special tissue.
This process is called non-shivering thermogenesis. Brown fat is missing in many mammals. Where is the extra body heat coming from?
The hypothesis coming from biochemical research seems to answer that question. It suggests that the heat for a stable body temperature in mammals comes from the same biochemical mechanism as the heat from brown fat.
We were interested in the fact that heat production in birds is thought to be driven by a very similar process, which made us wonder if birds and mammals have had a common origin. Our central hypothesis was born.
The capacity for endothermy could be found in fossils of the earliest ancestors of birds and mammals. The survey began with a comprehensive survey of the literature.
Professor Grigg said that with only bones to go on, there would seem to be little evidence of their thermoregulatory biology.
The structure of the bones can indicate a rapid and sustained growth rate, and certain bone types are more common in endotherms.
Large body size and erect stature can mean endotherm-like heart structure and high blood pressure, and large diameter entry points for blood vessels entering the limb bones can indicate high blood flow. These are features of endotherms.
Radioisotopic analysis can give information about the body temperature at which bones or teeth were formed.
The research found evidence for the early ancestors of the sauropsid and synapsid lineages, including Pareiasaurs, large reptile-like herbivores shielded by a bony external armor.
Professor Grigg said that the early signs of endothermy are consistent with his hypothesis that endothermy is older than the sauropod and synapsid lineages.
It's exciting that we found multiple occurrences of endothermy throughout both lineages, indicating that it was common throughout both of the major evolutionary lineages.
The question of whether the newly recognized and probably ancient source of supplementary body heat has been replaced by brown fat in placental mammals is one of the questions raised by our discovery. A better understanding of human body temperature regulation should be led by unraveling that.
The research was published.
Gordon Grigg and his colleagues wrote about Wholebody endothermy, an ancient, homologous and widespread disease among the ancestors of mammals, birds and crocodylians. 10.1111/brv.12822 is a book.
The journal has information about Biological Reviews.
There are warm-bodied ties between mammals and birds that are more ancient than before.
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