Scientists have pinpointed the moment in time our earliest ancestors evolved to be warm-blooded, and it happened much later and far more quickly than the researchers expected.
The discovery, made by studying the minuscule tubes of the inner ear, places the evolution of mammalian warm-bloodedness at around 233 million years ago — 19 million years later than scientists previously thought.
These semicircular canals are filled with a viscous fluid, called endolymph, that tickles tiny hairs lining the canals as the fluid sloshes around. These hairs transmit messages to the brain, giving it instructions for how to keep the body balanced. Like some fluids, the honey-like endolymph gets runnier the hotter it is, requiring the semicircular canals to change their shape so the fluid can still do its job. In ectothermic, or cold-blooded, animals, this ear fluid is colder and thus behaves more like molasses and needs wider spaces in which to flow. But for endothermic, or warm-blooded, animals, the fluid is more watery and small spaces suffice.
