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One of biology's unanswered questions involves the evolution of the genetic code and the fact it uses just 20 natural amino acids as its building blocks for making proteins. A mathematical analysis of biochemistry by researchers in Canada suggests a possible answer that could have profound implications for our search for life on other planets. Biophysicists Paul Higgs and Ralph Pudritz of the Department of Physics and Astronomy at McMaster University in Hamilton, Ontario, Canada, explain in a paper posted to the physics arXiv preprint server, that of the twenty amino acids used in proteins, ten were formed in the atmospheric discharge experiments carried out by Stanley Miller and Harold Urey in the 1950s. Their experiments were designed to test the theory of Soviet scientist Alexander Oparin's and J. B. S. Haldane's hypothesis that conditions on the primordial Earth allowed prebiotic chemical reactions to synthesize organic compounds from simple precursors, which then organised themselves into simple self-replicating systems and ultimately living things. However, there are now several alternative theories to explain the origins of the organic building blocks of life. One is terrestrial and suggests that rather than life crawling from a primordial soup it emerged from hot chemically rich vents in the oceans. The second, in some ways more controversial theory, is that life on Earth was seeded by the delivery of amino acids formed in outer space and carried to Earth in a cometary impact. This is the idea of panspermia due to the late Leslie Orgel, a British chemist. Higgs and Pudritz have now analysed experimental data on how common each amino acid should be on the basis of the various origins theories and insider information on their thermodynamics. Their work has allowed them to rank these ten original amino acids in order of putative abundance on the prebiotic Earth. The observational work and the thermodynamics analysis both suggest essentially the same order of abundance. The team points out that the different concentrations of each early amino acid in these prebiotic conditions would, coupled with the early stretches of the genetic code, have led to proteins containing amino acids with a similar abundance ranking, and as complexity increased other amino acids would have been sequestered into the system. "The distinction between early and late amino acids is important for theories of the origin of the genetic code (i.e. the mapping between codons and amino acids - a codon is the combination of three bases in a nucleic acid sequence needed to specify an amino acid)," the researchers explain. Higgs is on record regarding the importance of understanding this amino acid ranking. "It is likely that some amino acids were much more frequent than others at the time of the origin of life. Many possible methods of non-biological synthesis of amino acids have been proposed (atmospheric chemistry, deep sea vents, interstellar grains and meteorites)," Higgs has said, "Although these theories are very diverse, there is a consensus that indicates which of the amino acids were frequent before life arose - about 10 of the 20 used in proteins today. It seems likely that these were the first amino acids to be used in proteins, and that the remaining ones were added at a later date as the organisms evolved efficient pathways to synthesize them." However, perhaps more important for those looking for extraterrestrial life, the thermodynamics analysis of amino acids would suggest that the same abundance ranking could be universal. Thermodynamics is, after all, a set of fundamental principles that will hold true on other planets. Given that fact, then one might imagine that life elsewhere would also have emerged using the same amino acid alphabet as the one we see on Earth. "The combined actions of thermodynamics and subsequent natural selection suggest that the genetic code we observe on the Earth today may have significant features in common with life throughout the cosmos," the researchers conclude.
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