February 14, 1997: Amino Acid Asymmetry in the Murchison Meteorite! What'sNEW...Asymmetric molecules; I could not point out any more profound distinction between the products formed under the influence of life and all others. — Louis Pasteur (1) Some unusual amino acids present in the Murchison Meteorite apparently do have small excesses of the L enantiomers.... — Jeffrey L. Bada (2) Most amino acids can exist in either a right-handed or left-handed form. In biology, however, only the left-handed forms are used. The original reason for this anomaly is not known. If life originates from nonliving chemicals there is no convincing reason for one form to be selected and not the other. Amino acids produced nonbiologically would have no obvious reason to accumulate excesses of either form. Now two biochemists at the University of Arizona have reported in Science that they found more left-handed than right-handed versions of certain amino acids in the Murchison meteorite (3).
Other scientists, before 1997, have demonstrated that there are more left-handed than right-handed versions of some amino acids found in the Murchison meteorite (5). Advocates of panspermia have cited this result as additional evidence for life on comets. Although astronomical processes have been hypothesized, no nonbiological process is known to produce this asymmetry. Instead, the case for life forms or life's chemistry in meteorites has always been open to the suggestion that meteorites are easily contaminated with biological products after reaching Earth (6). However, even before the new work by Cronin and Pizarello was reported, analyses of isotope ratios showed that the excess of left-handed amino acids in the meteorite was not the result of earthly contamination (7). The new analysis by Cronin and Pizarello bypasses the contamination problem by testing for amino acids that are extremely rare on Earth — they couldn't be contaminants because they're not otherwise found here. Furthermore, the new tests used more sophisticated methods than the ones published earlier. Now we can be even more confident that the excess of left-handed amino acids in the Murchison meteorite was there before it struck Earth. Cronin and Pizarello make no mention whatsoever of the possibility that the left-handed amino acids could be byproducts of biological processes in the meteorite's parent body. Nor does Bada mention this possibility in his comments. Perhaps one should forgive this omission because the tested amino acids are not among those used in life on Earth today. But the relationship between these amino acids and Earthly life is exactly why we're all interested. That relationship could be the reverse of what Bada and the two researchers consider. Life could have produced the asymmetry Cronin and Pizarello observed. This possibility undermines their conclusion, "the results are indicative of an asymmetric influence on organic chemical evolution before the origin of life." The results increase the weight of the previous argument for panspermia based on biological amino acids from Murchison. The results are entirely consistent with Cosmic Ancestry. The failure by Science to even mention this obvious interpretation is puzzling. A comment by John Horgan in the May, 1997, issue of Scientific American also doesn't mention it (8). But Horgan admits, "The research raises at least as many questions as it answers about life's murky beginnings."
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Zita Martins et al., "Extraterrestrial nucleobases in the Murchison meteorite" [
14 Jun 2008
Murchison CM2 Carbonaceous Chondrite