and lived at alkaline hydrothermal vents or acidic geothermal ponds. Simple biomolecules transported from space was considered to have been either too reduced to have been fermented or too heterogeneous to support microbial growth. Heterotrophic microbes likely originated at low H₂ partial pressures. Bases, amino acids, and ribose are considered to be the first fermentation substrates.

Heterotrophs are currently found in each domain of life: Bacteria, Archaea,