page_title: Cambrian explosion
section_title: Life a billion years ago
hierarchical_section_title: Cambrian explosion / Life a billion years ago
caption_attribution_description: English: Stromatolites growing in Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia. Français&#160;: Stromatolithes dans la réserve naturelle marine de Hamelin Pool (Shark Bay, Australie).
context_page_description: The Cambrian explosion is when many animal phyla first appeared in the fossil record. It happened 541 million years ago. Probably most had evolved before then, but that was their first appearance as fossils.
  Before about 580 mya it seems that most organisms were simple. They were made of individual cells, occasionally organized into colonies. Over the following 70 or 80 million years the rate of evolution seemed to accelerate. By the end of the Cambrian most phyla we know today existed.
  The Cambrian explosion has caused much scientific debate. The seemingly rapid appearance of fossils in the 'primordial strata' was noted as early as the mid 19th century, and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.
context_section_description: Stromatolites, stubby pillars built by colonies of cyanobacteria and other microorganisms, are the first fossils. Their record starts about 3.5 billion years ago, and they were very common from about 2700 mya. They declined steeply after about 1250 mya, and this decline was probably caused by grazing and burrowing animals.
  Precambrian marine diversity was dominated by small fossils known as acritarchs. This term describes almost any small organic walled fossil—from the egg cases of small metazoans to resting cysts of many different kinds of green algae. After appearing around 2000 mya, acritarchs had a boom around 1000 mya, increasing in number, diversity, size, complexity of shape and especially size and number of spines. Their increasingly spiny forms in the last billion years may indicate an increased need for defence against predation. Other groups of small organisms from the Neoproterozoic era also show signs of anti-predator defences. Measuring taxon longevity  seems to show an increase in predation around this time.
  However, in general, the rate of evolution in the Precambrian was very slow, with many cyanobacterial species lasting unchanged for billions of years. Of course, bacteria are defined mainly by their biochemistry, especially their genomes. Changes in their biochemistry would usually leave no trace in the fossil record.
  If the predatory organisms which grazed on bacteria and acritarchs really were metazoans, this means that Cambrian animals did not appear "from nowhere" at the base of the Cambrian; their ancestors had existed for hundreds of millions of years.
caption: Modern stromatolites in Hamelin Pool Marine Nature Reserve, Western Australia.
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