Archean Eon colorful illustrations Bundle: StromatolitesArchaeaBacteriaArchean Eon AtmosphereCyanobacteriaGreenstone BeltsArchean Eon colorful posterThe Archean Eon is the second eon—the time when simple, single-celled organisms— archaea and bacteria—first appeared. Cyanobacteria produced oxygen through photosynthesis. The Archean atmosphere lacked free oxygen, but over time, oxygen levels began to increase. Explore Archean Rocks- Greenstone Belts, and stromatolites- microbial structures in laye