The Woodstock of Evolution -- The World Summit on Evolution (ScientificAmerican.com): "The Proterozoic and Archean Eons extend back to more than 3.6 billion years ago and cover the first microfossils and stromatolite fossils. Mikhail Fedonkin, head of the Laboratory of the Precambrian Organisms at the Paleontological Institute in Moscow suggested that a fall of global temperatures and the oxygenation of the biosphere secondary to photosynthesis played a major role in the dramatic change in the availability of heavy metals, which he believes were crucial in the metabolic processes that led to the evolution of complex life. This metal-rich environment served as a catalyst: 'Over 70 percent of known enzymes contain metal ions as a cofactor of an active site. Fast catalyzed reactions segregated life first dynamically and then structurally from the mineral realm.' Once prokaryotes gave rise to eukaryotes through symbiogenesis, life was off and running, exploding in the Cambrian with complex hard-bodied organisms.