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Science Frontiers ONLINE No. 115: Jan-Feb 1998 |
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Why did life take a left turn?
Life-as-we-know-it is left-handed; that is, our amino acid molecules are levorotatory rather than the mirror-image dextrorotatory versions. Because humans "expect" symmetry in nature, it is taken for granted that everything else in the universe is split equally between leftand right-handed molecules. Earth life is just a fluke -- or is it?
On September 28, 1969, organic-rich stones fell in Victoria, Australia. This was the Murchison meteorite, and it may carry a message. Over a decade ago, M.H. Engel and B. Nagy reported that the organic molecules in the Murchison meteorite were not split 50:50 between left- and right-handed versions. So contrary to expectations was this finding that most scientists assumed that the analysis was contaminated by terrestrial organic molecules. Now, M.H. Engel and S.A. Macko have refined the analytical techniques and apparently avoided any taint of contamination. Their conclusion: the Murchison amino acids still lean to the left.
From all this arise several intriguing possibilities:
- Life on earth started split evenly between left- and right-handed amino acids, but was nudged to the left by the influx of organic-laden meteorites like the Murchison.
- Terrestrial life actually originated elsewhere in the universe where much matter is left-handed, including life, if it exists there. In other words, our philosophical expectation of symmetry in the universe-as-a-whole is incorrect.
- The universe on the average is evenly split between left- and right-handed molecules, but there are "islands" or "pockets" which are left- or right-handed. Earth life is one of these "islands."
- Given the chance, amino acids and other organic molecules would exist evenly split, but physical phenomena, such as circularly polarized light, tip the scales -- to the left, in the case of terrestrial life. But that would mean that some physical phenomena are not symmetrical!
(Engel, M.H., and Macko, S.A.; "Isotopic Evidence for Extraterrestrial Nonracemic Amino Acids in the Murchison Meteorite," Nature, 389:265, 1997. Also: Chyba, Christopher F.; "A Left-Handed Solar System," Nature, 389:234, 1997.)
Comment. The references above state that terrestrial life is almost exclusively left-handed. Are there really righthanded organic molecules in terrestrial life forms? Where?
 | Left- and right-handed versions of the amino acid alanine. |