About 3.5 billion years ago (Ga), LUCA gave rise to the two prokaryotic domains – Eubacteria (more commonly called bacteria) and Archaea (formerly archaeobacteria, commonly called 'extremophiles' because of their persistence in extreme environments).
Oxygenic photosynthesis first arose in the Cyanobacteria at least 2.8 to 2.7 Ga, and gradually converted the Earth's atmosphere from its primordial reducing state. Oxygen is toxic to organisms that lack the metabolic machinery to rapidly utilize oxygen, so coexistent organisms faced the challenge of coping with rising atmospheric levels of toxic oxygen. Serial Endosymbiosis Theory (SET) is a widely accepted, evidence-based theory of cellular evolution that explains the cellular evoluton of eukaryotes through the endosymbiotic union of engulfed bacteria with a precursor eukaryotic cell. The archaeobacteria are more similiar than are eubacteria to eukaryotic cells, so it is believed that eukaryotes sprang from an archaeobacterial eukaryotic-precursor cell.
Symbiotic unions with bacteria capable of oxidative metabolism would have ensured cellular survival in environments with increased oxygen tension (pO2). Serial endosymbiotic transfers ultimately resulted in mitochondrial organelles, while another union with Cyanobacterial cells ultimately resulted in photosynthetic plastids (chloroplasts).
Cyanobacteria are found in the earliest known microfossils, and built large stromatolite reefs, dominating life for more than 2 billion years (~3.5 Ga to ~1 Ga). Modern stromatolites exist in only a few stressed environments on the planet, though Cyanobacteria remain relatively plentiful among the prokaryotes. ∩ Cyanobacteria and stromatolites ∩ Stromatolite structure
∩ Ancient stromatolite reefs ∩ Imaging fossil cyanobacteria ∩ Stromatolite fossils