 For example , genetic evidence suggests that our human ancestors acquired pubic lice from gorillas approximately 3 @-@ 4 million years ago . Unlike the genus Pediculus , the divergence in Phthirus does not match the age of host divergence that likely occurred 7 million years ago . Reed et al. propose a Phthirus species host @-@ switch around 3 @-@ 4 million years ago . While it is difficult to determine if a parasite @-@ host switch occurred in evolutionary history , this explanation is the most parsimonious ( containing the fewest evolutionary changes ) . 
