Wednesday, May 16, 2007

How the itch came about: Gorillas and humans get intimately close

While researching hominid evolution I came across this interesting article on the Discovery channel webpage by Jennifer Viegas, titled “Lice Passed from Gorillas to People,” that commented on a study done by David Reed and team, on the evolutionary history of anthropoid primate lice.

According to the study, humans have two genera of lice invading our bodies. These are the body and head lice, Pediculus and Pthirus. The latter, found only on pubic hair, was derived from a common ancestor with the louse found on gorilla. Since louse is very host specific it is possible to analyze the evolution of the host with that of the parasite. Therefore, the study done by Reed and team was in search of discovering the evolutionary history of the human lice. They did this by extracting DNA from both the the species found on gorilla and those found on humans. The study involved the use of PCR studies, alongside phylogenetic and cophylogenetic analyses.

Figure 1. Phylogenetic trees for primate lice and their vertebrate hosts. Trees are shown as cladograms with no branch length information, and are based on molecular and morphological data. Dashed lines between trees represent host-parasite associations. Humans are unique in being parasitized by two genera (Pediculus and Pthirus). Photo credits: J. W. Demastes, T. Choe, and V. Smith. (click on image for larger version).
So what happened? Well the studies showed that the human Pthirus diverged from the gorillas species a lot more recent than the actual divergence of human from gorillas. Humans and gorillas separated about 7 million years ago, while the Pthirus species has a common ancestor at about 3 million years ago. It is a well known fact that pubic lice are introduced by sexual contact - does this then imply that humans were having sex with gorillas 3 million years ago? As impossible as this may sound, it is a potential hypothesis, but there are others as well that can explain the relatively recent divergence of the two species. Such as host switching, duplication and extinction among the lice. Overall, this study is an example of coevolution among parasites and their host. How exactly the human host came to acquire pubic lice from gorillas is still an open question, and somewhat impossible situations cannot necessarily be ruled out completely.

--contributed by Patricia Torres



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