Proterozoic Life

The first advanced single-celled, eukaryotes and multi-cellular life, preserved as the Francevillian biota, roughly coincides with the start of the accumulation of free oxygen. This may have been due to an increase in the oxidized nitrates that eukaryotes use, as opposed to cyanobacteria. It was also during the Proterozoic that the first symbiotic relationships between mitochondria(found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved.

The blossoming of eukaryotes such as acritarchs did not preclude the expansion of cyanobacteria; in fact, stromatolites reached their greatest abundance and diversity during the Proterozoic, peaking roughly 1200 million years ago.

The earliest fossils possessing features typical of fungi date to the Paleoproterozoic era, some 2,400 million years ago; these multicellular benthic organisms had filamentous structures capable of anastomosis.

Classically, the boundary between the Proterozoic and the Phanerozoic eons was set at the base of the Cambrian Period when the first fossils of animals, including trilobites and archeocyathids, as well as the animal-like Caveasphaera, appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the upper boundary of the Proterozoic has remained fixed at the base of the Cambrian, which is currently placed at 541 Ma.