Origin of life
It is generally agreed that all life today evolved by common descent from a single primitive lifeform. We do not know how this early form came about, but scientists think it was a natural process which took place perhaps 3,900 million years ago. This is in accord with a philosophy called naturalism. In this philosophy only natural causes are admitted.
We do not know whether metabolism or genetics comes earlier. The main hypothesis which supports genetics first is RNA world hypothesis, and the one which supports metabolism first is Protein world hypothesis.
Another big problem is how cells develop. All existing forms of life are built out of cells.
RNA world hypothesis [change]
The RNA world hypothesis proposes that life based on ribonucleic acid (RNA) pre-dates the current world of life based on deoxyribonucleic acid (DNA), RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme. It may have supported pre-cellular life and been a major step towards cellular life.
There are some pieces of evidence which support this idea:
Protein world hypothesis [change]
This idea also has some evidences which supports this.
- Protein as enzyme is essential for today's lives.
- Some amino acids are formed from more basic chemicals in the Miller-Urey experiment.
Some also deny this idea because of this reason:
- Proteins cannot copy themselves.
Relevant pages [change]
- For a much longer discussion, see Abiogenesis.
- Steel, Mike; Penny, David (2010). "Origins of life: common ancestry put to the test". Nature 465 (7295): 168–9. doi:10.1038/465168a. PMID 20463725.
- Calvin, Melvin. Chemical evolution: molecular evolution towards the origin of living systems on the earth and elsewhere. Oxford: Clarendon Press, 1969. ISBN 0198553420.