The fundamental and universal nature of life's exploitation of peptides suggests they must have played a vital role during the onset of life, but their spontaneous chemoselective synthesis in water remains unknown. Aminonitriles () are widely accepted as prebiotic precursors of both amino acids and peptides, but they do not spontaneously polymerize in water to yield peptides. Here, we demonstrate that the simple prebiotically plausible aminothiol, cysteamine (), participates in Strecker chemistry to furnish β-mercaptoethyl-α-aminonitriles () and β-mercaptoethyl-amino acids (), which are predisposed to spontaneously form peptides in water. Intramolecular thiol catalyzed ligation is faster, higher-yielding, and more α-selective than previously reported prebiotic peptide ligation chemistries, enabling, for example, the highly regioselective α-ligation of Asp- and Glu-dinitriles in quantitative yields. Our findings suggest that cysteamine (), the thiol bearing moiety of the universal thiol cofactor coenzyme A, may have played an important role in the selective chemical synthesis of prebiotic α-peptides.