Precambrian Research 106 (2001) 35 – 45 Review The stereochemistry of amino acids in the Murchison meteorite M.H. Engel a, *, S.A. Macko b a School of Geology & Geophysics, Uniersity of Oklahoma, Norman, OK 73019, USA b Department of Enironmental Sciences, Uniersity of Virginia, Charlottesille, VA 22903, USA Abstract The questions of how, where and when life originated in our solar system remain largely unanswered. Some advances have been made with respect to abiotic synthesis of the key molecules deemed essential for the construction of a living cell. In particular, a variety of plausible mechanisms have been suggested for the synthesis of amino acids, the building blocks of peptides and proteins. Laboratory simulation experiments result in the synthesis of racemic amino acids. However, life as we know it is based almost exclusively on L-enantiomers rather than racemic mixtures of D- and L-enantiomers. A partial solution to this problem may be that the L-enantiomer excess essential for life’s origin on Earth was introduced from elsewhere in the solar system. For the past 20 years we have investigated the stereochemistry of amino acids in stones of the Murchison meteorite. Many of the common protein amino acids in Murchison are not racemic (L-enantiomer excess) and, based on their overall distribution and respective stable isotope compositions, do not appear to be artifacts of terrestrial contaminants (i.e. L-amino acids) introduced subsequent to impact. We hypothesize that comet and meteorite impacts during the early stages of Earth’s formation provided at least some of the essential components with the correct stereochemistry for the origin of life. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Amino acids; Stable isotopes; Murchison meteorite; Stereochemistry; Racemization www.elsevier.com/locate/precamres 1. Introduction For as far back in time as the terrestrial rock record extends, it appears that microbial life may have existed on Earth. Pflug and colleagues (e.g. Pflug and Heinz, 1997 and references therein) provided possible microscopic and spectroscopic evidence for primitive cells in banded ironstones of the 3.8 Ga old Isua supracrustal belt, south- west Greenland. These findings have been the topic of some debate, because amino acids and hydrocarbons that could be extracted from these rocks appeared to be modern contaminants (e.g. Nagy et al., 1981). It should be noted, however, that stable carbon isotope analyses of carbona- ceous materials (e.g. Schidlowski, 1993 and refer- * Corresponding author. 0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0301-9268(00)00123-6