Theories and evidence for chemical biopoieses
PLoS Biology: What Is Life and How Do We Search for It in Other Worlds?: "The obvious diversity of life on Earth overlies a fundamental biochemical and genetic similarity. The three main polymers of biology-the nucleic acids, the proteins, and the polysaccarides�are built from 20 amino acids, five nucleotide bases, and a few sugars, respectively. Together with lipids and fatty acids, these are the main constituents of biomass: the hardware of life (Lehninger 1975, p 21). The DNA and RNA software of life is also common, indicating shared descent (Woese 1987). But with only one example of life�life on Earth�it is not all that surprising that we do not have a fundamental understanding of what life is. We don't know which features of Earth life are essential and which are just accidents of history."