Our body produces lots of enzymes that break down toxic substances. One class of such enzymes are the flavin-containing monooxygenases (FMOs), which are present in all tetrapods. Humans have five different FMO genes, of which the first four display the same activity. However, the fifth FMO gene triggers a different breakdown reaction. University of Groningen biochemists have succeeded in resurrecting the ancestral genes of all tetrapod FMOs to show how this divergence has occurred. Their results appear on 24 February in Nature Communications.