Whole-genome sequencing reveals the British swallowtail butterfly *Papilio machaon britannicus* split from its European relatives 200,000 to 1.7 million years ago, replacing the previous view that the subspecies diverged only 8,000 years ago after the flooding of Doggerland. The study, published in *Insect Conservation and Diversity*, found a genetically distinct relict of a once wider wetland distribution with no evidence of damaging mutations despite some inbreeding, feeding solely on scarce milk parsley in the Norfolk Broads and facing threats from rising sea levels and potential hybridisation with warmer-climate continental subspecies now reaching Britain. Co-author Mark Collins argues the genetic distinctiveness warrants renewed conservation efforts, including translocation to wetland sites protected from salination.