The Cambrian explosion and the slow burning fuse

The rapid appearance of animal phyla in the fossil record during the 'Cambrian explosion' ca 543 Myr ago marks the most conspicuous turning point in earth history. This 'explosion' was preceded by a 'slow burning fuse', from the start of the prokaryote fossil record at ca 3450 Myr BP to endosymbiotic assembly of the eukaryote cell between ca 2,700 and 1,000 Myr. Research is beginning to put these events into their environmental context. Very long periods of environmental stability are suggested by the carbon isotopic and palaeoclimatic record prior to ca 1,000 Myr. Such stasis may have nurtured endosymbioses to the point at which eukaryotic organization and sexual reproduction became embedded in the genome. This steady state world was chaotically disrupted in the prelude to the Cambrian explosion. Strontium, sulphur and carbon isotopes attained maximal values during this time, and the latter show chaotic oscillations coincident with flips between extreme, low latitude glaciations and possible supergreenhouse conditions. These chaotic bifurcations may have been caused by tectonically driven increases in nutrient flux to the oceans and/or by the impact of multicellularity on the carbon cycle. Whatever the cause, high rates of biotic turnover during these times of stress could have radically redirected and/or accelarated the path of evolution towards new animal body plans.