Lice, Humans, and Microbes

Lice are small, wingless, minor ectoparasites of mammals and birds. More than 540  blood-sucking lice (Phthiraptera: Anoplura) have been described with each host having its own type of louse, suggesting the cospeciation of the lice species with their host. Among these, two lice species from two different genera infest humans: Pediculus humanus and Phthirus pubis (pubic “crab” lice ).  The former constitutes two morphotypes, P. humanus morphotype capitis (head lice) and P. humanus morphotype corporis (clothing “body” lice). Head, body, and pubic lice live on the head, in clothing, and in the pubic areas, respectively. These tiny creatures have been humans' and primates' close companions for millions of years and have played a significant role in human history, as they became the source of inspiration for many novelists. Human lice cannot survive on their target hosts for extended periods of time and die of starvation within 24-48 h. However, recent evidence implies human lice can shift hosts and adapt to other closely related species to evade death. This “host infidelity” has enabled these obligate, host-specific creatures to possibly survive the potential extinction of their host.  This phenomenon has shed light on a novel way for scientists to study the history of human evolution, which previously relied on the fossil records, the human genome, and archaeological findings.