Did lefty molecules seed life? (as reported in The Scientist today): "Amino acids come in left-handed and right-handed forms, which, like a pair of human hands, are mirror images that cannot be superimposed onto each other. Yet living organisms use only the left-handed version, which presents a conundrum: There's no biochemical reason why one mirror image should be better than the other, so scientists have long debated whether life's left-handed leaning arose because of random processes or whether rocks from outer space seeded a southpaw solar system."
...and suggests an explanation: that there was a left-bias in the amino acids that rained onto early earth from our solar system!
The current study argues for the latter possibility by showing that some extraterrestrial meteorites contain an abundance of left-handed molecules. "The implications are that all life in our solar system could be the same handedness as life on Earth," Jeffrey Bada, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif., who was not involved in the research, told The Scientist.
Daniel Glavin and Jason Dworkin, astrobiologists at NASA's Goddard Space Flight Center in Greenbelt, MD, compared the ratio of left- and right-handed 5-carbon amino acids found in six primitive, carbon-rich meteorites that have an elemental composition similar to that presumably found in the early solar system. Three of these rocks were heavily left-skewed, while the remaining three showed equal handedness, or chirality, the researchers found. Of the lefty rocks, the meteorite that fell on Murchison, Australia, in 1969 -- arguably the most widely studied carbonaceous meteorite in the world -- contained the largest imbalance ever observed: a 18.5% excess of the left-handed form of the amino acid isovaline.
Or, in their own words from the abstract of the PNAS paper:
The large asymmetry in isovaline and other α-dialkyl amino acids found in altered CI and CM meteorites suggests that amino acids delivered by asteroids, comets, and their fragments would have biased the Earth's prebiotic organic inventory with left-handed molecules before the origin of life.
Amino acids found in these and other meteorites landing on earth tend to exhibit a bias favoring left chirality, suggesting our entire solar system has the same left-bias, setting life as we know it on its lefty path. And Glavin & Dworkin think they know what favored left chirality in the meteorites:
The mechanism that Glavin and Dworkin propose to explain the observed left-handed excess is that polarized light -- which is twisted and can rotate molecules -- probably set the imbalance in motion. Then, once the balance was slightly askew, water within the meteorites further drove an enrichment of left-handed amino acids in the liquid phase and relegated right-handed molecules to the solid phase. "The whole amplification is due to this process of aqueous alteration," said Dworkin.
Not everyone is convinced, yet, but the search for answers to these puzzles about life's origin continues. (Or we could simply say goddidit, and stop asking any further questions, couldn't we? but where's the fun in that?)
Reference:
Glavin, D. P., & Dworkin, J. P. (2009). Enrichment of the amino acid l-isovaline by aqueous alteration on CI and CM meteorite parent bodies Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0811618106
As the semester winds down, we have a special treat on campus this Monday, when the Biology colloquium series winds down for the semester with a lecture by paleoanthropologist Dr. Kaye Reed, of the Institute of Human Origin at Arizona State University, on "The Life and Times of Australopithecus" . This lecture is co-sponsored by the Tri-Beta Biological Honors Society and the Consortium for Evolutionary Studies at CSU-Fresno, and is part of our ongoing series celebrating Darwin Bicentennial year. Dr. Reed has been working in Ethiopia and other parts of Africa, places where some of the most important and exciting hominin fossils (such as the famous Australopithecine, Lucy seen in the reconstructed portrait here) have been discovered. Her specialization is paleoecology with a focus on reconstructing the ecological communities within which our own lineage evolved. So it should be a very exciting lecture - I hope to see you there (especially if you are taking one of my classes!)!
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