Cophylogenetic reconstructions with the host phylogeny for humans, chimpanzees, gorillas, and Old World monkeys indicated by thick grey lines, and the louse phylogeny indicated by thin red and blue lines. (A) Reconstruction showing perfect cospeciation between with hosts and parasites with the exception of a single host switch of Pthirus sp. from gorillas to humans (marked by an arrow). (B) Cophylogenetic reconstruction showing an ancient duplication creating two evolutionarily distinct lineages (Pediculus and Pthirus), each having cospeciated with gorillas, chimps, and humans with two extinction events (marked with daggers). The reconciliation shown in panel A requires one evolutionary step (the host switch), whereas reconciliation B requires three steps (one duplication and two extinctions).