Peptides and proteins are the most diversified and abundant class of “biochemicals” which exist in animals and in the human body. Furthermore, all biocatalysts that are active in plants contain (poly)peptide chains. Hence, the origin of peptides and proteins is of paramount importance for any theory about the emergence of life. The existence of protein α-amino acids (prAAs) on the early Earth may be explained by two quite different hypotheses. Hypothesis I assumes that all prAAs originate from space delivered to the Earth by comets, meteorites and, above all, by large amounts of micrometeorites (s. Chap. 9 ). The alternative hypothesis II says that the prAAs were formed on Earth by efficient reaction pathways starting out from simple chemicals, such as ammonia, aldehydes, cyanide ions, and water. These different origins are not necessarily alternatives, but might have coexisted and even had a synergistic interaction, when optically active AAs imported from space (s. Chap. 9 ) have favored a catalytic formation of prAAs having an excess of one enantiomer (s. Chap. 10 ).