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Large and varied organisms lived before the Cambrian explosion, and some fossils preserve tracks showing that animals could move. The Cambrian explosion therefore looks less like the sudden beginning of complex animal life and more like a later period when existing animal groups spread and changed especially quickly.

After the Permian–Triassic mass extinction, a few tough kinds of life spread quickly. This made recovery look fast, but complex ecosystems took much longer to return. Research places the start of broader recovery around four to six million years later, with some estimates putting full recovery as late as 30 million years afterward.