Warm-bodied ties between mammals and birds more ancient than previously recognized

Date:
December 13, 2021
Source:
University of Queensland
Summary:
The evolutionary origin of endothermy (the ability to maintain
a warm body and higher energy levels than reptiles), currently
believed to have originated separately in birds and mammals,
could have occurred nearly 300 million years ago.



FULL STORY ==========================================================================
The evolutionary origin of endothermy (the ability to maintain a warm
body and higher energy levels than reptiles), currently believed to have originated separately in birds and mammals, could have occurred nearly
300 million years ago.


========================================================================== Emeritus Professor Gordon Grigg said their hypothesis is that endothermy
is ancient, with a shared origin in the common ancestry of mammals and
birds. One of the reviewers said that is 'a bold hypothesis', but it
is based on very 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, and birds.

"Endothermy permits sustained aerobic work and a favourable body heat,
and contrasts strongly with the low energy, lower and more variable
temperature lifestyles of today's reptiles. We assembled evidence from palaeontology showing that endothermy has been widespread in the two
vertebrate lineages that diverged from the first land-dwelling animals
very early on -- one leading to the living birds, lizards, and crocodiles
(the sauropsida) and the other lineage leading to monotremes, marsupials,
and placental mammals (the synapsida).

"Our main hypothesis is that endothermy in mammals and birds has a
common ancestry and is much older than has been accepted previously,
evolving not long after the first amphibians came onto land," he said.

"This is likely to be controversial, but we think, and hope, that it
will spark some great conversations and it could lead to a change in our understanding of the ways 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 biochemistry, all to do with how
mammals generate body heat (called thermogenesis) when responding to cold.



========================================================================== "Many placental mammals exposed to cold conditions supplement the heat
produced by shivering with additional heat produced by a special tissue,
brown adipose tissue (brown fat)," Professor Grigg said.

"Unsurprisingly, this process is called 'non-shivering thermogenesis'
(NST).

But brown fat is lacking in marsupials, monotremes (platypus and echidnas)
and in many placental mammals too. So where can their extra body heat
be coming from?" The hypothesis coming from biochemical research
appears to answer that question. It suggests that the heat for a stable
body temperature in those mammals comes directly from skeletal muscle, employing a different biochemical mechanism from the one generating the
heat from brown fat.

"Also of great interest to us was the fact that heat production in birds
is thought to be driven by a very similar process -- that made us wonder,
could endothermy in birds and mammals have had a common origin, rather
than evolving independently? And so our central hypothesis was born."
The researchers realised that if the capacity for endothermy really
had evolved so far back in deep time, they might discover evidence of endothermy in fossils of the very earliest ancestors of modern and birds
and mammals. And so began a comprehensive survey of the palaeontological literature.



========================================================================== "With only bones to go on, plus reconstructions based on those bones
suggesting what size and shape their owners may have had in life, there
would seem to be little evidence of their thermoregulatory biology,"
Professor Grigg said.

"However, the microscopic structure of the bones can indicate a rapid
and sustained growth rate, and certain bone types are more common in endotherms.

"Also, large body size and erect stature can imply 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 characteristic features of endotherms.

"Finally, radioisotopic analysis can sometimes provide information about
the body temperature at which bones or teeth were formed." During months
of review the research found evidence for endothermy very early ancestors
of the sauropsid and synapsid lineages; Pareiasaurs, large reptile- like herbivores shielded by a bony external armour, and in several groups of Pelycosaurs, mammal-like reptiles.

"These early signs of endothermy are consistent with our hypothesis that endothermy is very ancient and likely to be older than the divergence
of the sauropod and synapsid lineages," Professor Grigg said.

"What's just as exciting is that we found multiple occurrences of
endothermy throughout both lineages over time, indicating that it
was widespread throughout both of the major evolutionary lineages and characteristic of them both.

"Our discovery also raises a question about whether the newly recognised
and probably ancient source of supplementary body heat has been
replaced by brown fat in placental mammals, or whether both the muscle
NST and NST in brown fat operate together. And unravelling that should
lead to a better understanding about human body temperature regulation." ========================================================================== Story Source: Materials provided by University_of_Queensland. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Gordon Grigg, Julia Nowack, Jose' Eduardo Pereira Wilken Bicudo,
Naresh
Chandra Bal, Holly N. Woodward, Roger S. Seymour. Whole‐body
endothermy: ancient, homologous and widespread among the ancestors
of mammals, birds and crocodylians. Biological Reviews, 2021; DOI:
10.1111/ brv.12822 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211213121837.htm

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