|University of Bristol|
Many extrinsic theories have been put forward. During the late Precambrian a supercontinent was breaking up into smaller land masses (see The Cambrian environment), producing an increase in favourable habitats such as continental shelf edges and shallow seas. This provided new niches and new opportunities for any 'proto-metazoans', allowing unfettered evolution and possibly driving the explosive radiation seen from the fossil record.
A prolonged glaciation event also occured during the late Precambrian and could have caused a period of oceanic upwelling, increasing primary production and consequently the atmospheric oxygen level. It is thought that a critical level of oxygen may have been reached, firstly enabling animals to construct large bodies without oxygen diffusion becoming a constraint, and secondly allowing the synthesis of biomineralized skeletons.
On to: Intrinsic causes