Geochemistry
The most recent Doushantuo rocks show a sharp decrease in the 13C/12C carbon istope ratio. Since this change appears to be worldwide but its timing does not match that of any other known major event such as a mass extinction, it may represent "possible feedback relationships between evolutionary innovation and seawater chemistry" in which metazoans (multi-celled organisms) removed carbon from the water, which increased the concentration of oxygen, and the increased oxygen level made possible the evolution of new metazoans.
Read more about this topic: Doushantuo Formation