LIFE × EVOLUTION

Warm Blood

~233 million years ago

Phanerozoic · Mesozoic · Triassic

Somewhere in the Late Triassic, the ancestors of mammals raised their body temperature by five to nine degrees and began holding it there. The evidence is an odd one: warm blood thins the fluid in the inner ear, which would have degraded the balance organ if its shape had not changed to match. Measured across fifty-six extinct species, that shape changes abruptly, and the earlier relatives on our branch were most probably cold-blooded. On the bird line, the same shift seems to have happened separately.

Why it matters

Warm blood costs about ten times more food and buys the night, the winter and the mountain. Two lineages paid it, each apparently on its own, and those two lineages are the ones that run the land today.

Dating & uncertaintyDisputed

Bone growth marks, isotopes and posture have all been read as evidence, and none of them settles the question. The inner ear gives a sharper signal, and the date shown here follows it: an abrupt shift in the Late Triassic. Others read the same evidence as a slower emergence, beginning in the Late Permian and accelerating in the Early Triassic after the end-Permian extinction, in which case there is no single date to give.

Sources

1. Araújo et al. (2022), Nature · Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy
2. Benton (2021), Gondwana Research · The origin of endothermy in synapsids and archosaurs and arms races in the Triassic
3. Benton et al. (2019), Trends in Ecology and Evolution · The early origin of feathers

Earth history as one day

22:46:051 h 14 min before midnight

if Earth’s 4.54 billion years were compressed into a single day