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).
The Murchison meteorite is a carbonaceous chondrite. These are generally believed to be remnants of spent comets. There is conclusive evidence that water once flowed through them (4). This one struck Earth on September 28, 1969, scattering fragments across pastures near Murchison, Victoria, Australia.
by Martin Horejsi
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 do not mention 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."
13 Jun 2011: Fragments of the Tagish Lake meteorite contain amino acids that are predominantly left-handed, as in life.
19 Jan 2011: NASA has found left-handed amino acids in many more carbon-rich meteorites.
18 Feb 2010: Extraterrestrial chemodiversity is high .... [>14,000 different compounds in the Murchison meteorite.]
Ronald Breslow and Zhan-Ling Cheng, "On the origin of terrestrial homochirality for nucleosides and amino acids" [abstract], p 9144-9146 v 106, Proc. Natl. Acad. Sci. USA, 9 Jun (online 28 May) 2009.
23 Apr 2009: It's hard to imagine a planet's surface exhibiting handedness without the presence of self assembly, which is an essential component of life — Thom Germer, NIST.
17 Mar 2009: Predominantly left-handed amino acids were found in six out of six meteorites....
Zita Martins et al., "Extraterrestrial nucleobases in the Murchison meteorite" [PDF], p 130-136 v 270 Earth and Planetary Science Letters, 15 Jun (online 20 Mar) 2008.
14 Jun 2008: Organic compounds were present in the early solar system.
5 Mar 2008: A carbonaceous meteorite collected in Antarctica contains amino acids that are predominately left-handed.
Uwe J. Meierhenrich et al., "Identification of diamino acids in the Murchison meteorite" [abstract], Proc. Natl. Acad. Sci. USA online, 11 Jun 2004.
23 Feb 2004: Chirality from space?
2003, November 4: Amino acids in meteorites....
2003, February 12: Amino acids in the Murchison meteorite...
2002, March 12: More on amino acid chirality in Murchison.
2001, December 19: Sugar in Murchison and Murray meteorites...
Pascale Ehrenfreund et al., "Extraterrestrial amino acids in Orgueil and Ivuna: Tracing the parent body of CI type carbonaceous chondrites" [text], p 2138-2141 v 98 Proc. Natl. Acad. Sci. USA, 27 Feb 2001.
Murchison CM2 Carbonaceous Chondrite, brief description and photo, New England Meteoritical Services.
Amino Acids in Meteorites, by Narcinda R. Lerner, NASA Ames Research Center, 1997.
1997, October 23: The amino acids in the Murchison meteorite now weigh even more heavily...
1997, September 18: Michael H. Engel and Stephen A. Macko announce additional evidence...
1. Louis Pasteur, "Recherches sur la dissemètrie molèculaire des produits organiques naturel," p 333 v 1 Oeuvres de Pasteur, ed. Pasteur Vallery-Radot, 1922; cited in The Spontaneous Generation Controversy from Descartes to Oparin by John Farley, The Johns Hopkins University Press, 1974. p 101.
2. Jeffrey L. Bada, "Extraterrestrial Handedness," p 942-943 v 275 Science, 14 February, 1997.
3. John R. Cronin and Sandra Pizarello, "Enantiometric Excesses in Meteoritic Amino Acids," p 951-955 v 275 Science, 14 February, 1997.
4. J.R. Cronin and S. Chang, "Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite," p 209-258 The Chemistry of Life's Origins, J.M. Greenberg, C.X. Mendoza-Gómez and V. Pirronello, eds. Kluwer Academic Publishers, 1993.
5. Michael H. Engel and Bartholomew Nagy, "Distribution and enantiomeric composition of amino acids in the Murchison meteorite," p 837-840 v 296 Nature, 1982.
6. Keith Kvenvolden, James Lawless, Katherine Pering, Etta Peterson, Jose Flores, Cyril Ponnamperuma, I. R. Kaplan and Carleton Moore, "Evidence for Extraterrestrial Amino-acids and Hydrocarbons in the Murchison Meteorite," p 923-926 v 228 Nature, 1970.
7. M.H. Engel, S.A. Macko and J. A. Silfer, "Carbon isotope composition of individual amino acids in the Murchison meteorite," p 47-49 v 348 Nature, November 1, 1990.
8. John Horgan, "The Sinister Cosmos: A meteorite yields clues to life's molecular handedness," p 18 Scientific American, May 1997.