The Cambrian explosion, and possibly its cause(s)?
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In general a sudden spike in diversity of animal groups found in the fossil record – for some reason a lot of new species that are widely different from one another started to show up around that time, much more than during any other period during earth’s history.
We still don’t have a concrete answer as to “why” we see the effects of Cambrian explosion in the fossils, there are only some theories. One possible option is called the edicarian garden hypothesis, which states that the Cambrian explosion started as a result of first ever predators appearing which kick-started an evolutionary arms race, another is bio sustainability concept that proposes that rising oxygen and calcium levels in the oceans allowed for higher diversity of species, and there are also the reduction hypotheses that pose that no sudden evolutionary spike was present but it’s just a result of environment being more friendly to fossilisation at the time allowing preservation of higher count of species.
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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