Proteins represent the major pool of nitrogen in the biosphere, but undergo rapid biodegradation and efficient microbial recycling so that few recognizable remnants survive into the geological record. If these remnants become incorporated into geopolymers, then their origin becomes increasingly difficult to identify. Despite the relative instability of proteins, sites which exclude enzymolysis reveal that proteins are sufficiently robust to survive in a recognizable form in many burial environments over millennia, and many of their constituent amino acids for millions of years. Decomposition occurs via a series of reactions, the most rapid of which are sulphydryl oxidation, deamidation and dehydration, but some of these reactions can be slowed significantly by reducing the conformational flexibility of the protein. Ultimately, amino acids will decompose completely to produce light hydrocarbons, the pattern of which, in some biominerals, is consistent with a protein origin.