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September 28, 2015 | By:  Sedeer el-Showk
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Learning from Lice

To most people, lice are just parasites to be exterminated, but their parasitic nature also makes them an exceptionally good window into our past. By studying these little blood-suckers, researchers have learned something about where we came from and how we got here, and even picked up some broader points about evolutionary biology along the way.

Three kinds of lice infest humans: head lice, body lice, and pubic lice. Each makes its living in a different area; while head lice and pubic lice attach their eggs to the hairs growing where they live, body lice attach their eggs to our clothing. (As an aside, I didn't know that lice eggs were called 'nits' — that's where 'nitpicking' comes from!) We've got three different kinds of lice because, unlike our closest relatives, we aren't covered head-to-toe in a furry coat.

Each louse species is adapted to its particular niche in our body; by working out their evolutionary history, we can learn something about when those niches appeared. Reseachers have compared DNA from the mitochondria of head lice and clothing lice to figure out when they diverged. The analysis suggested that the two parted ways somewhere between 30,000 and 110,000 years ago, which gives us an idea of when our ancestors started wearing clothing. Head and clothing lice are closely related and share an ancestory with chimpanzee lice, which makes sense, since chimps are our closest relatives. Pubic lice, however, belong to a different genus, and it turns out their closest relative is gorilla lice. While the common ancestor of humans and gorillas lived around 7 million years ago, pubic lice and gorilla lice share a much more recent common ancestor — somewhere around 3-4 million years ago. The most likely explanation is a host switch, with gorilla lice adapting to live in human pubic hair. This would have happened after we lost our body hair — so head hair and pubic hair were separate niches — and before we started wearing clothing, but that's all we can say about it at the moment.

In addition to everything they can tell us about when our bodies started to look the way they do, lice also offer a great opportunity to study how parasites and hosts evolve together. In general, parasites are thought to evolve more rapidly than their hosts. Several studies support this idea, but so far they've only looked at the rate of change of one gene (or a few genes). We've studied humans, our parasites and our relatives extensively, which means we already have complete genomes for several of the species in this interaction. Since the genomes of humans, chimps, and human body lice had all been sequenced, a research team decided to sequence the chimpanzee louse so they could compare rates of evolution across the whole genome.

Overall, they found that the lice diverged roughly 14 times faster than their hosts. Some genes diverged more quickly than this average and others less quickly. Strikingly, the same genes that evolved quickly in the diverging louse species also evolved quickly as humans and chimps diverged. It's not entirely clear why the same genes should evolve quickly or slowly in such groups as disparate as lice and primates, which are separated by over 600 million years of evolution. The correlation only holds for mutations that change the amino acid encoded by the DNA; when the researchers looked at silent mutations, the correlation disappeared. Based on this, they think the correlated rates of evolution might reflect functional constraints on the genes shaping their evolution. It's an intriguiging idea, and certainly one worth further exploration.

Far from being mere troublesome pests, it turns out that lice can shed light on our origins and reveal mysteries for evolutionary biologists to ponder.

Johnson, KP et al. Rates of genomic divergence in humans, chimpanzees and their lice. Proceedings of the Royal Society B 281:20132174 (2014) doi:10.1098/rspb.2013.2174
Kittler, R et al. Molecular Evolution of Pediculus humanus and the Origin of Clothing. Current Biology 13(16):1414-1417 (2003) doi:10.1016/S0960-9822(03)00507-4
Reed, DL, et al. Pair of lice lost or parasites regained: the evolutionary history of anthropoid primate lice. BMC Biology 5:7 (2007) doi:10.1186/1741-7007-5-7

Image credits
The (amazing!) image of a head louse is by Gilles San Martin and is distributed under a CC-BY-SA license.

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