Early Archean (3.3-billion to 3.5-billion-year-old) microfossils from Warrawoona Group, Australia.

Entrez PubMed: "Cellularly preserved filamentous and colonial fossil microorganisms have been discovered in bedded carbonaceous cherts from the Early Archean Apex Basalt and Towers Formation of northwestern Western Australia. The cell types detected suggest that cyanobacteria, and therefore oxygen-producing photosynthesis, may have been extant as early as 3.3 billion to 3.5 billion years ago. These fossils are among the oldest now known from the geologic record; their discovery substantiates previous reports of Early Archean microfossils in Warrawoona Group strata."

Early Archean (3.3-billion to 3.5-billion-year-old) microfossils from Warrawoona Group, Australia. Schopf JW, Packer BM. Science. 1987 Jul 3;237:70-3.