Pushing back the time things could have happened just by accident (a million monkeys typing)…
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.”University of Queensland, “Warm-bodied ties between mammals and birds more ancient than previously recognized” at ScienceDaily (December 13, 2021)
The paper is open access.
This is the current theory, for what it is worth: