The
evolutionary history of life and origin of life are fields on ongoing geological and biological research. Although not necessary conditions for the acceptance of evolution by
natural selection, the
origin of life and its evolutionary history can nonetheless help shed light on evolutionary processes. The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions, but it is unclear how this occurred. Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any
last universal common ancestor or ancestral gene pool. Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as
RNA, and the assembly of simple cells. The first simple, sea dwelling organic structures appeared about 3,400 million years ago. It is considered that they may have formed when certain chemical (organic) molecules joined together.
Prokaryotes, single-celled micro-organisms like
blue green algae, were able to photosynthesize and produce oxygen. Around thousand million years later, sufficient oxygen had built up in the atmosphere and hence it allow multicellular organisms to proliferate in the
Precambrian seas.
