LIFE × EVOLUTION

Warm Blood

~233 million years ago

Phanerozoic · Mesozoic · Triassic

Somewhere in the Late Triassic, the ancestors of mammals began holding their body temperature five to nine degrees above their surroundings. The evidence is an odd one: warm blood thins the fluid in the inner ear, which would have wrecked the balance organ if its shape had not changed to match. Measured across fifty-six extinct species, that shape changes abruptly, and everything earlier on our branch was cold-blooded. The ancestors of birds did the same thing, 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, independently, 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 all are ambiguous. The inner ear gives a sharper signal: warm blood thins the fluid in the semicircular ducts, and their shape had to change with it. That method dates the shift to the Late Triassic, and finds earlier relatives cold-blooded.

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