History of the Earth
The first advanced single-celled and multi-cellular life roughly coincides with the oxygen accumulation; this may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria.(Stanley, 325)  It was also during the Proterozoic that the first symbiotic relationship between mitochondria (for animals and protists) and chloroplasts (for plants) and their hosts evolved.p321-2