The Cambrian explosion, and possibly its cause(s)?
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Animals are classified into “phyla”, based on their body plans. Examples are chordates (e.g., fish), echinoderms (e.g., sea stars), annelids (e.g., earthworms), molluscs (e.g., snails), and arthropods (e.g., insects).
The phyla are all very different from each other, but seem to have evolved at roughly the same time. Rocks older than 540 million years don’t contain fossils of any of them. Rocks newer than that contain fossils of *all* the phyla.
It’s as if a dozen different lineages of sea sponge, all with fairly similar lifestyles to one another, simultaneously evolved (a) complex shapes, (b) mobility, (c) brains, and (d) vision. They all did it independently of one another, ending up with *different* shapes and *different* methods of locomotion and *different* brain structures and *different* types of eyes, but they all did it at approximately the same time, give or take a few tens of millions of years.
And then it *never happened again*. Some phyla have gone extinct, but no new phyla have arisen. Anything that was still living a sea-sponge-like lifestyle 500 million years ago would *never* evolve a complex shape, mobility, brains, or vision; it would remain a sea sponge for all eternity (or until it went extinct).
Anyway, that’s the Cambrian Explosion: the event during which all the different types of animals *became* different types of animals from one another.
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As for what caused it, well, we can speculate: It was right after a “Snowball Earth” episode when the planet’s entire surface had been frozen over, so maybe there were a bunch of empty environmental niches, giving experimental body plans a chance to succeed even if they weren’t very refined yet. It was during a period when oxygen and ozone levels in the atmosphere were super high, so maybe mobility was less expensive than usual. And once it started, maybe the different phyla ended up in an “arms race” with one another, needing to get smarter and smarter in order to compete successfully.
But the truth is, we don’t know. None of the above speculations quite explain what made that moment in Earth’s history *unique*.
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