Current theories on the origin of life reveal significant gaps in our understanding of the mechanisms that allowed simple chemical precursors to coalesce into the complex polymers that are needed to sustain life. The volcanic gas carbonyl sulfide (COS) is known to catalyze the condensation of amino acids under aqueous conditions, but the reported di‐, tri‐, and tetra‐peptides are too short to support a regular tertiary structure. Here, we demonstrate that alanine and valine, two of the proteinogenic amino acids believed to have been among the most abundant on a prebiotic earth, can polymerize into peptides and subsequently assemble into ordered amyloid fibers comprising a cross‐β‐sheet quaternary structure following COS‐activated continuous polymerization of as little as 1 mm amino acid. Furthermore, this spontaneous assembly is not limited to pure amino acids, since mixtures of glycine, alanine, aspartate, and valine yield similar structures.Once upon a time: The aqueous synthesis of peptides under conditions that are relevant to a prebiotic earth leads to the formation of ordered amyloid aggregates. With mixtures of four amino acids, such conditions yield thousands of unique peptides that then undergo a spontaneous selection and self‐assembly process. The inherent ability of simple peptides to form ordered quaternary structures may be relevant to the origins of biological macromolecules.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137528/1/anie201605321_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137528/2/anie201605321.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137528/3/anie201605321-sup-0001-misc_information.pd
