LIFE × EVOLUTION

The Cambrian Explosion

~540–518 million years ago

Phanerozoic · Paleozoic · Cambrian

Between roughly 540 and 520 million years ago, nearly every major way of building an animal appears in the rocks: arthropods, molluscs, segmented worms, the first chordates. Nothing since has added a fundamentally new design. Trilobites make the point: dated by their own rate of change, they evolve no faster during the Cambrian than after it, which means that by the time the ordinary Cambrian fossil record begins, around 521 million years ago, the burst was already over.

Why it matters

Every animal alive today, ourselves included, is a variation on an architecture settled in this window. But the word explosion flatters it. The macrofossil record runs back to 571 million years, the environment was changing throughout, and this was one radiation among several rather than a single event with a start and a finish.

Dating & uncertainty

Calling it an explosion overstates the suddenness. Diversification begins with the Ediacaran macrofossils around 571 million years ago and runs on into the Ordovician; what is compressed is the appearance of the body plans, not the radiation as a whole. Rates of trilobite evolution have been used to test how fast that compression could have run.

Sources

1. Wood et al. (2019), Nature Ecology & Evolution · Integrated records of environmental change and evolution challenge the Cambrian Explosion
2. Paterson et al. (2019), PNAS · Trilobite evolutionary rates constrain the duration of the Cambrian explosion
3. Zhang & Shu (2021), PalZ · Current understanding on the Cambrian Explosion: questions and answers

Earth history as one day

21:09:212 h 51 min before midnight

if Earth’s 4.54 billion years were compressed into a single day