Louse
Phylogeny

Lice have been the subject of significant DNA research in the 2000s that led to discoveries on human evolution. The three species of sucking lice that parasitize human beings belong to two genera, Pediculus and Pthirus: head lice (Pediculus humanus capitis), body lice (Pediculus humanus humanus), and pubic lice (Pthirus pubis). Human head and body lice (genus Pediculus) share a common ancestor with chimpanzee lice, while pubic lice (genus Pthirus) share a common ancestor with gorilla lice. Using phylogenetic and cophylogenetic analysis, Reed et al. hypothesized that Pediculus and Pthirus are sister taxa and monophyletic. In other words, the two genera descended from the same common ancestor. The age of divergence between Pediculus and its common ancestor is estimated to be 6-7 million years ago, which matches the age predicted by chimpanzee-hominid divergence. Because parasites rely on their hosts, host – parasite cospeciation events are likely.

Genetic evidence suggests that human ancestors acquired pubic lice from gorillas approximately 3-4 million years ago. Unlike the genus Pediculus, the divergence in Pthirus does not match the age of host divergence that likely occurred 7 million years ago. Reed et al. propose a Pthirus 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).

Additionally, the DNA differences between head lice and body lice provide corroborating evidence that humans used clothing between 80,000 and 170,000 years ago, before leaving Africa. Human head and body lice occupy distinct ecological zones: head lice live and feed on the scalp, while body lice live on clothing and feed on the body. Because body lice require clothing to survive, the divergence of head and body lice from their common ancestor provides an estimate of the date of introduction of clothing in human evolutionary history.

The mitochondrial genome of the human species of the body louse (Pediculus humanus humanus), the head louse (Pediculus humanus capitis) and the pubic louse (Pthirus pubis) fragmented into a number of minichromosomes, at least seven million years ago. Analysis of mitochondrial DNA in human body and hair lice reveals that greater genetic diversity existed in African than in non-African lice. Human lice can also shed light on human migratory patterns in prehistory. The dominating theory of anthropologists regarding human migration is the Out of Africa Hypothesis. Genetic diversity accumulates over time, and mutations occur at a relatively constant rate. Because there is more genetic diversity in African lice, the lice and their human hosts must have existed in Africa before anywhere else.
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