All living organisms, from microbes to humans, are made of the same basic building blocks, consisting mainly of sugars, amino acids, fatty acids, and nitrogenous bases, altogether little more than fifty distinct, small chemical species, of molecular weights rarely exceeding 200. What largely differentiates organisms chemically is the manner in which these building blocks join into larger assemblages, mostly polysaccharides, proteins, lipids, and nucleic acids. A number of additional compounds peculiar to certain sets of organisms exist – chlorophyll in plants is an example – but these are most likely products of later evolution. In its earliest forms, life probably was made of little more than the universal building blocks found in all living organisms today.

Prebiotic Chemistry

This remarkable singularity goes back to the very beginnings of life, conveying a central message whose meaning did not catch the attention of biologists until 1953, when Stanley Miller observed the spontaneous formation of a number of amino acids and other typical biological constituents in a laboratory situation he and his mentor, the celebrated chemist Harold Urey, believed to be representative of the conditions that prevailed on the primitive Earth at the time life first appeared (Miller, 1953). Even though serious doubts have since been voiced about its underlying assumption, Miller's achievement remains a major landmark in the history of biology. It opened the new field of prebiotic chemistry and sparked a large number of interesting experiments.