The Cambrian explosion is known as the Big Bang in biology. It began 542 million years ago and ended about 40 million years later. This period is characterized by the first appearance ofabundant skeletonized metazoans, a sudden increase in biodiversity, and the emergence of most modern lines. In the Vendian (0.56-0.54 Gyr ago) first animals with soft bodies appeared announcing the Cambrian explosion. Before the Vendian period life was microscopic, vegetative, and mainly prokaryotic and eukaryotic.
There is still a lot ofspeculation about what caused the Cambrian explosion and why it happened when it did after 3 billion years of potential evolutionary time. The approaches that have been put forward to solve the puzzle of what triggered the explosion can be split into extrinsic (environmental) factors, intrinsic (biological) factors, or a mixture of both. Extrinsic factors are physical changes in the Precambrian environment. Among these changes are the breakup of the supercontinent Rodinia and the Neoproterozoic glaciations known as snowball Earth events. The snowball Earth events and the continental breakup are associated with genetic isolation but also with a reorganization of oceanic flow patterns causing upwel-ling, with increasing primary production, and with a consequently higher atmospheric oxygen level. Another cause is given by the rise of atmospheric oxygen as a trigger of the Cambrian explosion. This higher oxygen level can be caused by an intensified phosphorus flux into the ocean. The phosphorus is released by weathering rates biotically enhanced by the first colonization of continents. Intrinsic causes involve some mechanisms within the Precambrian biosphere itself, which enabled evolution and diversification to start. An example is the finding in developmental genetics that the mutation of an ancestral metazoan could potentially initiate a large morphological change in the animal.
The dashed line in Figure 5a shows a possible second evolutionary path. A cooling event can cause a premature rising of complex life. Up to 1.75 Gyr ago there is only a unique evolutionary path. After that time more than one stable state of the Earth system exists (bistability). It depends on the environmental conditions which state (with or without complex life forms) is realized.
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