Learning representations by backpropagating errors

delineating the absolute indigeneity of amino acids in fossils. As AMS iechniques are refined to handle smaller samples, it may also become possible to date individual amino acid enantiomers by the °C method. If one enantiomer is entirely derived from the other by racemization during diagenesis, the individual Dp. and L-enantiomers for a given amino acid should have identical “C ages. Older, more poorly preserved fossils may not always prove amenable to the determination of amino acid indigeneity by the stable isotope method, as the prospects for complete replacement of indigenous amino acids with non-indigenous amino acids increases with time. As non-indigenous amino acids undergo racemization, the enantiomers may have identical isotopic compositions and still not be related to the original organisms. Such a circumstance may, however, become easier to recognize as more information becomes available concerning the distribution and stable isotopic composition of the amino acid constituents of modern representatives of fossil organisms. Also, AMS dates on individual amino acid enantiomers may, in some cases, help to clarify indigeneity problems, in particular when stratigraphic controls can be used to estimate a general age range for the fossil in question. Finally, the development of techniques for determining the stable isotopic compasition of amino acid enantiomers may enable us to establish whether non-racemic amino acids in some carbonaceous meteorites” are indigenous, or result in part from terrestrial contamination. M.H.E. thanks the NSF, Division of Earth Sciences (grant | EAR-8352085) and the folowing contributors to his Presidential Young Investigator Award for partial support of this research: LETTERSTONATURE 533