What Was the Cambrian Explosion?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The Cambrian explosion was a burst of evolutionary innovation around 538 million years ago in which most major animal body plans appear in the fossil record within a geologically short window. Before it, life was mostly simple and soft. After it, the basic architecture of modern animals was in place.
The timing
The main phase spans roughly twenty to twenty five million years starting around 538 million years ago. That sounds enormous, but against a backdrop of three billion years of largely microbial life it is a very sharp change.
In that window the fossil record goes from soft bodied simple organisms to animals with eyes, legs, shells, guts and mouths. Almost every major group of animals alive today, including our own chordate lineage, has its first known representatives from this period.
The Burgess Shale
Most of what we know comes from a handful of exceptional fossil sites, above all the Burgess Shale in the Canadian Rockies. Conditions there preserved soft tissue, so we see whole animals rather than just hard parts.
The creatures found are genuinely strange. Opabinia had five eyes and a grasping nozzle. Hallucigenia was so odd it was originally reconstructed upside down. Anomalocaris was a metre long predator with grasping appendages, the largest animal of its time.
What may have caused it
No single explanation is agreed on, and most researchers now favour a combination of factors:
- •Rising oxygen levels, allowing larger and more active bodies
- •The evolution of eyes, which made predation and escape possible
- •An evolutionary arms race between predators and prey driving armour and speed
- •The development of genetic toolkits that made complex body plans possible
- •Chemical changes in seawater allowing shells and skeletons to form
Was it really an explosion?
The word is doing a lot of work. Molecular evidence suggests several animal lineages had already split well before the Cambrian, meaning the divergence began earlier than the fossils show.
What may have exploded is not animal life itself but hard parts. Shells and skeletons fossilise easily, soft bodies almost never do, so the sudden appearance in the record could partly reflect animals becoming preservable rather than animals suddenly existing.
Why it matters
The Cambrian explosion is where the basic body plans of animals were established, and remarkably few new ones have appeared since. Evolution has produced enormous variety within those plans but has rarely invented a genuinely new one.
That makes this period a kind of foundation for everything that followed. The arrangement of a body with a front and back, a gut running through it and paired limbs along the sides was settled half a billion years ago and has held ever since.
What came before it
The Cambrian did not begin from nothing. The preceding Ediacaran period, from about 635 to 538 million years ago, has its own strange fossil record of soft bodied organisms with frond, disc and quilted shapes.
Researchers still argue about what these were. Some may be early animals, some may belong to groups with no living relatives at all, and some may not be animals in any sense. Most of them vanish at the start of the Cambrian, so the explosion may have involved a replacement of one experiment in multicellular life by another.
The takeaway
The Cambrian explosion was the rapid appearance of complex animals with eyes, shells and skeletons around 538 million years ago, and it set the basic body plans that almost all animals still use today.