Pubic louse (Pthirus pubis)
Biologists wonder about all kinds of things. Dr. Robert Weiss for example wondered why humans are parasitized by two different species of lice, head lice (Pediculus humanus) and pubic lice (Pthirus pubis commonly called crabs). What would have caused this separation of lice into two species, both of which occupy different habitats on the body? After all our ancestors were hairy all over so why specialize in different areas of the body?
Head louse (Pediculus humanus)
It turns out Dr. Weiss discovered pubic lice that infect humans evolved from lice found on gorillas (Pthirus gorillae). The two species of lice diverged about 3.3 million years ago according to David Reed from the University of Florida, however humans and gorillas diverged earlier, about 7 million years ago. Dr. Reed's work also demonstrated that human head lice probably came from our hominid ancestors. Dr. Weiss hypothesizes that later our human ancestors caught the pubic lice ancestor from the gorillas while hunting and butchering them. This close contact would have allowed the gorilla lice to jump onto our ancestors, burrow in and start feeding!
Weiss et al. Apes, lice and prehistory. Journal of Biology, 2009; 8 (2): 20 DOI: 10.1186/jbiol114