The core machinery for protein synthesis is universal to cellular life. However, idiosyncrasies exist that differentiate translation across Archaea, Bacteria and Eukarya. An example is found in bacteria, mitochondria and chloroplasts, where a formyl group is added to methionine prior to translation initiation. This formyl group is removed from the nascent polypeptide by peptide deformylase before protein production is complete, and appears to have no clear function. Despite this, it is essential to bacterial translation: interrupting formylation is deleterious. A well-conserved process that, if lost, leads to a severe phenotype is usually associated with important function. Our previous work indicates formylation and deformylation likely evolved from an ancient, plasmid-transmitted, toxin-antitoxin system capable of post-segregational killing (PSK). PSK systems get their name because of how they act: the antitoxin is more labile than the toxin, so segregating daughter cells that do not inherit the gene-pair die through action of the toxin. This creates an ‘addiction’, because cells can no longer lose the genes. We predicted that the formyl group is toxic, and that removal negates this toxicity. A line devoid of the formylase and deformylase genes was produced. While initially very unfit, after 1,500 generations of evolution, we found that growth rates of knockouts were identical to the wild-type lineages. We showed that there were mutational changes to genes involved in translation which enabled the cells to adapt to formylation loss. Moreover, introducing the genes on a plasmid elicited a PSK phenotype, as per our model. To reproduce initial adaptation to formylation, we reintroduced the genes on the E. coli chromosome, and performed a further 3,000-generation evolution experiment. Our results indicate that, despite our lines not requiring formylation, addiction to the gene-pair has reasserted itself, with formylation appearing essential. Our results suggest that formylation invaded and spread via addiction, with these genes becoming ‘essential’ as a result of PSK.