Guess where humans got crabs from?

The science blogosphere has been buzzing about new published research that has focused studying the origins of a sexually transmitted disease, crabs which also known as pubic lice.

Before I talk about this paper I wanna thank Carl Zimmer, who opened up a public discussion with a question of the day: How Do You Get Crabs From A Gorilla? And that has been followed up with John Wilkins and Reed A. Cartwright throwing in some commentary. But that’s not it, some of the major news agencies like EurekAlert, Discovery Channel, and ScienceNOW Daily News have articles about this paper too.

The open access paper is titled, “Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice,” and follows a simple premise, one that I’ve discussed about before. The authors basically take the concepts of genetic drift and integrate it into a study that reconstructs the evolutionary history of primate lice. They take this reconstruction and,

“infer the historical events that explain the current distribution of these lice on their primate hosts.”

Lice are a parasite that are limited in mobility. They do not have wings nor really effective bodies to move about. Their small legs are fit to latch onto a host and basically just stay put. Ultimately, the host becomes a village to these lice and a group of hosts become an island. Lice, therefore, begin to share the same evolutionary history as their hosts. For example, if a population of hosts splits into two and each of the isolated populations begins to evolve into separate species, then the parasites evolve too.

Two common types of lice are found on humans, head and pubic lice. Entomologists, or bug scientists, classified human head lice and human pubic lice in two separate genera because they are awfully different. Human head lice resemble chimpanzee head lice but human public lice resemble that of their gorilla counterpart.

This discontinuity between the two types of lice was curious to the authors of this paper. One would expect both types of lice to be more similar to chimpanzees since they are the closest evolutionary ancestor to us. So the authors of this paper, as Carl says it,

“set out to recover the evolutionary tree of pubic lice, just as they had done with head lice. They analyzed DNA from human head lice, human pubic lice, as well as other species from the same genera that live on chimpanzees and gorillas. They also analyzed DNA from lice that live on monkeys and on rodents so that they could get a better sense of how pubic lice had evolved from a common ancestor with other species. The scientists not only drew branches for each species, but also estimated when those branches split over the course of history.

Their conclusion… We did not get pubic lice from other hominids. We got them from the ancestors of gorillas.”

…Which is even more mind-boggling, this means gorillas and some hominid did the nasty. It seems that the authors took their curiosity and opened a Pandora’s box of disgusting-ness, but it shouldn’t be all too surprising.

Actually, in the last hundred years ago or so, humans transmitted what is now HIV from our chimpanzee counterparts, probably from some sort of sexual contact or blood exchange… And why that even happens in modern days opens a whole new discussion on gene flow and species concepts one that Wilkins addresses,

“Cross-species sex is a widespread phenomenon in vertebrate biology. We have been misled by our “intuitions”, based on the one hand on an over-strict application of reproductive isolation concepts of sex, and on the other of projection of moral standards.

Alan Templeton’s classic paper on species concepts has a section entitled “Too much sex”. It’s worth reading just for that.

Templeton, Alan R. 1989. The meaning of species and speciation: A genetic perspective. In Speciation and its consequences, edited by D. Otte and J. Endler. Sunderland, MA: Sinauer.”

3 thoughts on “Guess where humans got crabs from?

  1. Interesting.
    I was under the impression, though, that HIV was pretty well confirmed to have been transferred to humans from chimpanzees when someone was butchering a chimp – if that’s not a lot of blood with a lot of potential for the human doing the chopping to get cut, I don’t know what is. Along the same lines as ebola transferring to humans?

  2. Hi Erin,

    Thanks for commenting.

    HIV’s origins, to the best of my knowledge coulda come from a either sexual contact or blood exchange of a chimp and a human, maybe thru the butchering process you suggested. I haven’t found a definitive answer, and I don’t think we ever will. It is hard to track down the first HIV patient, especially since he or she most likely died undiagnosed. It is even of a challenge to figure out how it was transmitted. But since bush meat is prevalent it is a disturbing possibility, as is some interspecies sexual relations. There are other theories as the the origins, some people also hypothesize that somehow someway some chimp-human needle exchange coulda happened.

    What is known was published in a paper in a 1999 issue of Nature that concluded that wild chimps had been infected simultaneously with two different simian immunodeficiency viruses which had “viral sex” to form a third virus. This third virus could be passed on to other chimps and, more significantly, was capable of infecting humans and causing AIDS.

    And about Ebola, it is a zoonotic disease like SIV/HIV and so it does transfer from one species of animal to another via blood or other bodily fluids, but where Ebola and HIV/SIV differ, is that Ebola can go thru air or sit dormant on an object. HIV, as you may know is destroyed when it is exposed to air or when it is not inside blood.

    You maybe interested in checking out this collection of references on the origins of HIVvtopic:

    Bailes et al. (2003) “Hybrid Origin of SIV in Chimpanzees.” Science, Vol. 300, p. 1713

    Gao, F; Bailes, E; Robertson, DL; Chen, Y; et al. (1999) “Origin of HIV-1 in the chimpanzee Pan troglodytes troglodytes.” Nature, Vol. 397, p. 436-44

    Keele, Brandon F. et al. “Chimpanzee Reservoirs of Pandemic and Nonpandemic HIV-1.” Science, Vol. 313. No. 5786, pp. 523 – 526

    Vandamme, A-M et al. “Tracing the origin and history of the HIV-2 epidemic.” PNAS, Vol. 100, No. 11, 27 May 2003

    Wolfe, ND; Switzer, WM; Carr, JK; et al. (20 March 2004) “Naturally acquired simian retrovirus infections in Central African Hunters.” The Lancet, Vol. 363, p. 932


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