The Cambrian explosion begins
What happened
The Cambrian Period opened about 538.8 million years ago, and within a geologically short window, perhaps only its first 20 million years, animal life diversified into nearly every major body plan alive today. The Natural History Museum dates the period from 539 to 485 million years ago and credits this opening burst with producing the body plans that define almost 40 animal phyla, the broadest categories life is sorted into. Before this, the Ediacaran world held soft-bodied, largely immobile organisms. After it, oceans held fast-moving predators, armored prey, and burrowing scavengers, the entire architecture of animal life compressed into a sliver of geological time.
Why it matters
Every animal alive today, vertebrate or invertebrate, insect or mollusc, traces its basic body plan to a lineage that appeared or was already present in this one narrow window. No comparable burst of new body plans has happened since.
How we know
The 538.8-million-year start is fixed by a golden spike, an internationally agreed reference point in a rock section at Fortune Head, Newfoundland, where the first trace fossils of a burrowing worm mark the base of the Cambrian. The pace and range of the diversification come from thousands of fossil-bearing rock layers worldwide, cross-checked against each other's ages.
Sources
- Natural History Museum, London. The Cambrian Period · Reputable sourcenhm.ac.uk · The domain "nhm.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- Big Bang to Now → · Zoomed out: this is the spine's 'The Cambrian explosion'