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In fact, two amino acids, lysine and ornithine, became more abundant in the gut after infection.

By using stable isotope tracers and analyzing gene expression, the researchers showed that S. gordonii releases a substance called ornithine that prompts F. nucleatum to produce more of a molecule called polyamine.

Tawfik and Metanis demonstrated that functional proteins could have come into being "from short and simple sequences" including an amino acid, ornithine, that does not exist in modern proteins.

One possible way to explain this is if the core apoptotic machinery prevents ornithine from entering the mitochondrion and instead diverts it towards spermidine production.

Lysine and ornithine have very similar molecular structures, and the authors found that Spe1, the enzyme that decarboxylates ornithine, can also decarboxylate lysine to generate cadaverine.