History › Complex life
Cambrian explosion
c. 539–515 million years ago· Event· Life & early humans
Well documentedSolid records, physical evidence or scientific consensus.
Within roughly 25 million years, most major animal body plans appeared in the fossil record.
See it on the History map →What happened
From about 539 million years ago, animals with shells, eyes, legs and guts appeared in quick succession, geologically speaking. Ancestors of insects, crabs, snails and vertebrates (animals with backbones) all show up. Predators and prey drove an arms race of armor and speed. Likely triggers include rising oxygen, new body-building genes, and predation itself. The Curious Life map covers it in depth.
Why it matters
Almost every major animal group alive today, including our own backboned line, traces back to this burst.
How we know
Exceptionally preserved fossils at the Burgess Shale in Canada (found 1909) and Chengjiang in China (found 1984), which record soft parts as well as shells.
Still debated
How sudden it really was is debated: genetic clocks suggest animal groups split earlier but left few fossils.
Inside this (3)
Connections (3)
▸Snowball Earth may have set the stage for Cambrian explosionDebated
The connection
The Snowball Earth episodes reshaped ocean chemistry and may have raised oxygen levels. Some scientists argue this set up conditions, more oxygen and nutrients, that later let animals grow larger and more complex during the Cambrian explosion.
When and how
Following the glaciations of about 720–635 million years ago, roughly 100 million years before the Cambrian began.
How sure are we?
The order of events fits, but about 100 million years separate them, so a direct causal link is speculative.
Debates and open questions
Whether Snowball Earth was a key trigger or simply an earlier event is debated; other causes, such as new genes and predation, are also proposed.
▸Ediacaran biota: the first large animals gave way to Cambrian explosionWell documented
The connection
Most Ediacaran organisms disappeared around 539 million years ago, possibly pushed out by new burrowing and predatory animals. Their decline opened space for the far more varied Cambrian animals, with shells, eyes and legs, that rapidly took over the seas.
When and how
On seafloors worldwide, at the start of the Cambrian Period, about 539 million years ago.
How we know
Fossils show Ediacaran forms vanishing as Cambrian animals and their burrows increase; the transition is well recorded in rock layers.
Debates and open questions
Whether the Ediacarans were outcompeted, eaten, or wiped out by environmental change is debated.
▸Cambrian explosion was followed by Life's move onto landWell documented
The connection
The Cambrian explosion produced the animal groups that later moved ashore, especially arthropods (jointed-leg animals) and early vertebrates. Meanwhile green algae in fresh water gave rise to the first land plants. Within about 70 million years, plants led the way onto bare continents and animals followed.
When and how
From shallow seas and shorelines, beginning about 470 million years ago, some 70 million years after the Cambrian began.
How we know
Fossils show the order clearly: sea animals first, then land plant spores, then land arthropods and four-legged vertebrates.
Debates and open questions
Trackways suggest some arthropods ventured onto tidal flats during the Cambrian, before plants; how early animals first left water is debated.