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The origin of life on Earth continues to be one of the greatest mysteries of modern science. The fact that geochemical evidence for life may extend back to the earliest portion of the terrestrial rock record (approx. 4.0 Ga) makes it impossible to state with any certainty whether life originated on Earth or if it was introduced to Earth. Similarly, since a living system has never been synthesized in the laboratory, the requisite conditions and subsequent pathways for that origin remain unknown. It is this lack of definitive information that continues to inspire us to search outside the Earth for clues to life's origins. However, our ability to determine if life as we know it presently exists or existed elsewhere in our solar system or beyond depends on the establishment of criteria that can be used to determine its existence and to distinguish it from present-day life on Earth. The latter is critical, as it is extremely difficult to completely avoid contamination during sample collection and analysis. This is especially true with respect to materials returned to Earth, as is planned for future Mars' missions. Given their ubiquity and unique stable isotope compositions resulting from fractionations associated with their respective abiotic and biotic syntheses, amino acids are one of the classes of compounds best suited for establishing the existence of life as we know it. Criteria will be presented for detecting life and its possible precursors elsewhere in the solar system based on the stable isotope compositions of this important class of compounds.
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