How Life Began – Origin of Biochemicals
Processes That Created the Organic Building Blocks
Oct 31, 2008
The previous article in this series, How Life Began – Formation of the Earth, described an experiment by Miller and Urey showing the laboratory generation of organic compounds known to exist in outer space. The present article discusses the nature and possible sources of such chemicals, as a prelude to their possible utilization for the creation of living cells.
Extraterrestrial Origin of Organic Chemicals
Evidence for extraterrestrial organic chemicals was established when a type II carbonaceous chondrite meteorite fell on September 28th 1969 in Murchison, Australia. It contained common amino acids including glycine, alanine, glutamic acid, valine and proline, together with the atypical isovaline and pseudoleucine.
A complex blend of alkanes was also present, which was similar to that found by Miller and Urey. Importantly, serine and threonine were not present in the samples; these are considered terrestrial contaminants and thus their absence supports an extraterrestrial source.
Further experiments on the Murchison meteorite in 1997 also support the amino acids’ extraterrestrial origin because individual amino-acid enantiomers (stereoisomers that are mirror images of each other) were enhanced in the 15N nitrogen isotope compared to their Earth counterparts.
Sugars in the Murchison Meteorite
Additional research on the meteorite has revealed the presence of organic compounds called polyols. Polyols such as sugars, sugar alcohols and sugar acids are fundamental components of nucleic acids (RNA and DNA) and cell membranes, and act as energy sources for cells. They are crucial to all known forms of life.
Origins of Organic Chemicals in Outer Space
Astronomers have observed short radio wavelength emissions in the Milky Way that are characteristic of a diverse range of molecules, from the simple molecule carbon monoxide to complex long linear molecules. Included are many organic molecular groups that are important in living organisms, for example aldehydes, alcohols, ketones, and ethers.
Infrared spectra of nebulae suggest that the interstellar medium could contain large ringed molecules called polycyclic aromatic hydrocarbons, for example coronene and naphthalene. There is obviously scope for an enormous diversity of organic molecules that could have been available for the creation of the biochemicals essential to life.
The Organic Soup on the Primordial Earth
Amino acids can survive in the harsh conditions of outer space and were probably present when the Earth first formed. The Murchison meteorite provides clear evidence that at least some amino acids and other organic compounds could have been acquired from outer space. These would have dissolved in a complex organic soup cooked under an electrified and fiery maelstrom on the early Earth. In it, chemical reactions could abound, creating a vast range of simple and complex biochemicals, eventually including proteins and possibly self-replicating molecules such as nucleic acids.
There is still a major hurdle to explain the natural creation of cellular life. The next article in this short series, How Life Began – The Origin of Cells, will discuss the first of several controversial theories to describe the origin of biological cells.
References:
Cooper, G., Kimmich, N., Belisle, W., Sarinana, J., Brabham, K. and Garrel, L. (2001). Carbonaceous meteorites as a source of sugar-related organic compounds for the early Earth. Nature 414: 879–883.
Duley, W.W. and Williams, D.A. (1984) Interstellar Chemistry. Academic Press.
Engel, M.H. and Macko, S.A. (1997). Isotopic evidence for extraterrestrial non-racemic amino acids in the Murchison meteorite. Nature 389: 265–268.
Harbquist, T.W. (1990) Molecular Astrophysics. Cambridge University Press.
Kvenvolden, K.A., Lawless, J., Pering, K., Peterson, E., Flores, J., Ponnamperuma, C., Kaplan, I.R. and Moore, C. (1970). Evidence for extraterrestrial amino-acids and hydrocarbons in the Murchison meteorite. Nature 228 (5275): 923–926.
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