The heterotrophic hypothesis asserts
the first living beings the
fermenting heterotrophs. Fermentation
carbon dioxide (CO₂) and
the atmosphere became enriched
this gas. By mutation natural
selection organisms capable using
carbon dioxide and to synthesize
organic material . These would
have been first photosynthetic
beings (that also fermenting
beings since was no abundance of
).

Since photosynthesis is a that
releases molecular oxygen, the
emergence of fermenting
this gas became available.
organisms then developed aerobic
using O₂, a highly
method to produce energy.