From 1.7 billion years ago, for a billion boring years, Earth remained a slimy, near-static world of algae and microbes. The pace picked up 750 million years ago: glaciers spread, complex animals appeared, and by 520 million years ago the Cambrian revolution – an explosion of varied life – was under way. The reason for that long stasis has been a mystery.
We may now have the answer: the gradual cooling of the planet’s interior. Just as turning down a stove burner slows the boiling of a stew pot, cooling of the mantle allowed the “scum” on top to thicken, says Peter Cawood at the University of St Andrews, UK. The resulting surface stability slowed geological change, seemingly stalling evolution for a billion years, until the planet was cool enough for tectonic activity to shift up a gear.
Cawood and Chris Hawkesworth, also at St Andrews, analysed studies of continental motions and geologic processes to see how they lined up with the boring period.