Summary
A rapid diversification of animal body plans about 540 million years ago.
In depth
Over perhaps 13 to 25 million years starting about 540 mya, most major animal phyla appear in the fossil record. Explanations include rising oxygen, ecological arms races, evolution of Hox genes and environmental change. Exceptional sites such as the Burgess Shale preserve soft-bodied animals.
Precambrian Ediacaran fossils and molecular clocks show animals had earlier origins.
The explosion, seen in sites such as the Burgess Shale in Canada and Chengjiang in China, includes arthropods, molluscs, chordates (Pikaia) and strange forms such as Anomalocaris and Hallucigenia. Hard parts such as shells and skeletons appeared, as did eyes and predation. Precambrian Ediacaran organisms, soft-bodied fronds and discs, preceded it from about 575 mya.
Molecular clocks suggest that animal lineages started to diverge earlier than the fossil record shows. Explanations combine environmental factors (oxygen and calcium rise), developmental factors (evolution of Hox and regulatory networks), and ecological factors (arms races). Rather than an instant event, it was a burst over tens of millions of years.
Remember: the Cambrian explosion is not a challenge to evolution, since the ancestors have been found in earlier rocks and it was rapid only on a geological scale.
Frequently asked questions
What is Cambrian Explosion?
A rapid diversification of animal body plans about 540 million years ago.
Which area of biology covers Cambrian Explosion, and how advanced is it?
Cambrian Explosion is one of the 53 topics in Evolution. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Molecular Clock, Punctuated Equilibrium, Fossils.
Can you give an example of Cambrian Explosion?
The first eyes and hard shells appear.
What should I learn before Cambrian Explosion?
Work through the Beginner topics in Evolution first, such as Evolution, Natural Selection, Charles Darwin, then return to Cambrian Explosion. Advanced topics assume the Intermediate vocabulary.
What topics are related to Cambrian Explosion?
The closest topics to Cambrian Explosion are Molecular Clock, Punctuated Equilibrium, Fossils, Experimental Evolution. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Cambrian Explosion?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Cambrian Explosion. To practise the whole of Evolution, use the Evolution quiz.
How is Cambrian Explosion different from Molecular Clock?
Cambrian Explosion: A rapid diversification of animal body plans about 540 million years ago. Molecular Clock: Uses the steady rate of neutral mutations to estimate when lineages diverged. Read both side by side and note what each one explains.
How should I study Cambrian Explosion step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Molecular Clock and explain the difference aloud. Finally, take the Evolution quiz and come back to this page for any question you miss.