The Earth formed four and a half billion years ago and after a billion years of cooling the crust solidified and life appeared. Earth has been inhabited for most of its existence, but only five hundred million years ago does anything more complex than microbial mats and cyanobacteria appear in the fossil record. Then all the basic body plans that exist today evolved so quickly (in a geological sense of time) that the era is called the Cambrian Explosion. After three billion years of stasis, what changed?
Maybe the earth moved.
Repeatedly, the continents on Earth have clustered together into a single supercontinent, leaving a single superocean. Once again, about five hundred million years ago, tectonic forces broke a huge slab of continent off, creating a shallow ocean in warm equatorial latitudes while dredging up nutrients from deeper waters. The continents have merged and broken apart cyclically over geologic time, but this event was a perfect evolutionary opportunity. Life exploded in an orgy of multicellular diversity.
It’s the sort of unlikely situation that leads to thoughts that, even if life is common in the galaxy, complex life and especially intelligent life may be vanishingly rare. But plate tectonics seem to be related to planetary mass, so any rocky planet Earth-sized or larger may offer life the same chance. Just add water.