Several hundred amino acids have been identified in organisms, but only 20 are the building blocks of proteins. As best as can be determined from the fossil record, the 20 protein amino acids have never varied with respect to structure or stereochemistry. Approximately 55 amino acids that have yet to be discovered in modern terrestrial organisms have been identified in carbonaceous meteorites. If life originated on Earth, a fundamental question that remains to be answered is what were the source(s) and mechanisms of formation of the amino acids that preceded life? Laboratory simulation experiments have not resulted in the synthesis of all of the protein amino acids. Also, these experiments always produce racemic amino acids (D/L=1), whereas life as we know it is based almost exclusively on L-amino acids. The alternative to laboratory synthesis has been investigations of ancient rocks, terrestrial and extraterrestrial. Given that life is ubiquitous on present-day Earth and no rock is an entirely closed system, the challenge has been to distinguish ancient, indigenous amino acids from those more recently introduced via contact with the Earth's biosphere. Amino acids in Precambrian rocks are not easily distinguished from modern overprints. However, amino acids in carbonaceous meteorites with short residence times on Earth provide a unique opportunity to begin to assess what the Earth's organic inventory may have been like prior to life's origin. In addition to numerous exotic amino acids, several of the common protein amino acids essential for life occur in the Murchison and Orgueil meteorites. More importantly, these amino acids exhibit the L-enantiomer excess that was, arguably, a necessary precondition for the origin of life. The stable isotope composition of amino acids in the Murchison meteorite confirms their authenticity. It is hypothesized that at least some of the starting materials for life on Earth may have been introduced by impact events. There are several protein amino acids that occur in all living organisms on Earth but have not been synthesized in the laboratory by abiotic mechanisms and have not been detected in carbonaceous meteorites. It is suggested that the presence of these amino acids, e.g.: Phe, Lys, His, and Arg, on other planetary bodies would by evidence for the existence of life as we know it.