And you were probably thinking that people only got crabs from coeds on spring break in Cancun!
Seriously though, I apologize for the sensationalist title, but this story is a really neat demonstration of the application of phylogenetic comparison and evolutionary divergence time estimation. (For a primer of phylogenetics see this post.)
The venereal affliction known as “crabs” is an infestation of lice that specifically reside in the pubic region of humans. Pubic lice, Pthirus pubis, are of a genetically and morphologically distinct genera of lice from the head louse, Pediculus humanus. If you compare the evolutionary history of primates with the history of the lice that parasitize them, an interesting picture emerges.
Phylogenetic cladograms, assembled from morphological and genetic characters, for primates and parasitic lice. Dotted lines indicate parasite-host relationships. Figure from Weiss, 2009; after Reed et al., 2007.
Ignore the numbers positioned at certain nodes of the trees for the moment. Notice how almost perfectly the branching pattern of the mammalian evolutionary tree matches that of the lice. This blatantly suggests coevolution between the parasites and the hosts. As the hosts diverge evolutionarily from their common ancestors, the parasites stranded on them also speciate. Think of the divergent evolution of species residing on different islands, except in this case the islands that the lice are living on are populations of primates.
Now, there is one glaring discrepancy between the phylogeny of primates and lice. That is that, while other primates harbor only a single species of lice, humans are afflicted by two: A head louse, P. humanus, related most closely to chimp lice, and a pubic louse, P. pubis, related most closely to lice associated with our second closest cousin species, gorillas. From coevolution we would suspect that humans should only have P. humanus, the species of lice whose closest cousin lives on our closest cousin. However, we also have P. pubis.
How can this discrepancy be explained? There are two possibilities:
- A: There were two species of lice present in the common ancestor of humans, chimps, and gorillas. The pubic louse lineage went extinct in the chimp lineage, and the head louse lineage went extinct in the gorilla lineage. Both types of lice only avoided extinction in the human lineage, after diverging from gorillas and then chimps. Or…
- B: Humans inherited P. humanus as we diverged from our common ancestor with chimps, and P. pubis jumped to us from the gorilla lineage later.
Figure from Reed et al., 2007.
So, which option is true? Option B, a recent host jump of the pubic lice from gorillas to humans, is the most parsimonious; the simplest possibility is the most likely. However, that is not necessarily enough to settle the matter. I realize that I’ve given away the answer in the title of this post, but let’s find out why Option B is true.
The researchers determined the correct answer by estimating evolutionary divergence times. This is done by sequencing certain genes from each species. One can apply a molecular clock to the degree of evolutionary divergence between the gene sequences of closely related species. These molecular clocks are based on refined models of amino acid substitution rates for specific genes. That is, we can estimate how long two species have been diverged from their common ancestor by observing the amount of evolutionary changes in the amino acid sequences coded for by those genes. For instance (assuming a hypothetical gene and molecular clock); if a gene sequence is 60 percent identical between two species, their common ancestor diverged 10 million years ago. If the sequence is 80 percent identical then they diverged 5 million years ago. The higher the genetic similarity, the more recent the divergence. In addition, divergence times can be further anchored temporally by including the fossil record in the analysis.
Divergence time estimation was applied to the primate and lice lineages. Look again at the first figure. The numbers at branch nodes on the tree represent the time of that divergence in millions of years ago (MYA). Notice, that the gorilla louse diverged from the human pubic louse, P. pubis, only 3.3 MYA. However, the common ancestor of humans and gorillas lived about 11.8 MYA. Therefore, the pubic louse must have recently, about 3.3 MYA, jumped directly from the gorilla lineage to the human lineage. This probably resulted from humans eating, wearing the skins of, or sleeping in the nests of gorillas…
Not from cross-species philandery. How did I give you that idea?
Reed, D.L. et al., 2007. Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice. BMC Biology, 5, 7.
Weiss, R.A., 2009. Apes, lice and prehistory. Journal of Biology, 8(2), 20.