By calculating the rate of mutation in both cells and mitochondria, geneticists can establish the broad sweep of our evolutionary history as primates.

The story can be filled in by exploring the history of the other creatures in our ecology, particularly those with which we have an intimate relationship. The evolution of the grasses, for instance, tells us about our food sources, and therefore the kind of social oportunities we had. The spread of eucalypts in southern Australia suggests the the incursion of Aborigines and fire-stick farming.

But our most intimate relationships with other species occurs with our parasites. Enter the louse. Apparently humans as lice farms is a rich study -

“Humans are unique among primates in that we host two different kinds of lice—one on our heads and bodies (Pediculus), the bane of many schoolchildren, and pubic lice (Pthirus). In comparison, chimpanzees have only head lice and gorillas only pubic lice.”

It turns out that crabs have only been part of our evolutionary journey for the last three million years, and we inherited them from gorillas. We picked them up by sleeping in gorilla nests, and/or eating gorillas, eons before we learnt to cook meat, which destroys parasites.

The research, led by David Reed of the Florida Museum of Natural History, relied on live parasites and the fossil record. The gorilla lice came from members of the Mountain Gorilla Veterinary Project.

“The researchers expected humans simply developed pubic lice on their own since this was a simpler explanation than acquiring the parasites from gorillas. However, they were proved wrong.

The evidence suggests gorilla lice began infesting humans about 3.3 million years ago. In contrast, humans and gorillas diverged in evolutionary time about 7 million years ago. The fact the lice took up residence where they did may have coincided with human loss of most hair on the rest of their bodies and the lack of any other suitable niche to live, Reed said.”

Is it fair to think that the history of human lice might have been different if some proto-hominid smeared a raw gorilla chop on his head rather than his crotch? Of course not – the structure of hair on the head is different from the pubis, and would have presented a different environment for the creatures. The scientists are merely saying our heads resemble chimp bodies and our crotches provide a gorilla-like habitat. Something in the difference is enough to keep the populations distinct. This post is getting ikkier and ikkier.

While this story makes me look at our near evolutionary neighbours in a somewhat different way, the historians are excited about something different entirely. The research suggests that humans and gorillas shared a habitat 3.3 million years ago, a time which is not well described in the fossil record.

The New York Times managed to take this story further, in what looks like a subterranean competition between science journalists. Nicholas Wade points out that the number of louse species on people is actually three – the extra one is the body louse which lives on our clothes. You can see where this is going. If we know when that arrived, we know when people started to frock up.

“Archaeologists contend that human ancestors lost their standard ape body hair when they left the shade of the forests for the hot, open savanna and needed bare skin for efficient sweating. Adaptation to the savanna was well in place by 1.7 million years ago. But loss of body hair could have begun earlier, and Dr. Reed’s result suggests a time for when people first became naked.

If people first became nudists 3.3 million years ago, when did they start to wear clothes? Surprisingly, lice once again furnish the answer. Though humans may long have worn loose garments like animal skin cloaks, the first tailored clothing would have been close-fitting enough to tempt the head louse to expand its territory. It evolved a new variety, the body louse, with claws adapted for clinging to fabric, not hairs.

In 2003, Mark Stoneking, a geneticist at the Max Planck Institute in Leipzig, Germany, estimated from DNA differences that the body louse evolved from the head louse about 107,000 years ago. The first sewn clothes were presumably made shortly before this time.

Probing back even earlier in louse evolution, Dr. Reed and his colleagues report that the two species of primate lice, Pediculus and Phthirus, probably diverged from each other on an ape host 13 million years ago. The divergence may have happened after the lice started to specialize in different parts of the body.

Some seven million years ago, this ancient ape species split into gorillas and the ancestors of humans and chimps, with both lineages infected by both species of lice. But Pediculus then fell extinct in its gorilla hosts, according to Dr. Reed’s reconstruction, and Phthirus vanished from the chimp-human ancestor. Next, chimps and humans diverged, and their joint louse diverged with them into Pediculus humanus and Pediculus schaeffi.

The last event in this history of human-louse cohabitation was the transfer of the gorilla’s Phthirus louse to people.

Dr. Stoneking said Dr. Reed’s reconstruction was “pretty reasonable” and said he agreed that acquisition of the gorilla’s louse indicated people had lost their body hair by then. “The transfer doesn’t have to be sexual,” he said, “but presumably it does require reasonably close contact.”

Being a cunning scientist, Reed knows well that his research is not just about the particular reasons why proto-homids scratched themselves around a campfire on some Ur-Serengeti. As his university’s website says:

“Understanding the history of lice is important because the tiny insects give clues about the lifestyles of early hominids and evolution of modern humans, Reed said. Because the human fossil record is patchy and finding early DNA samples is extremely difficult, parasites such as head lice, pubic lice, tapeworms and pinworms that have existed for millions of years provide valuable clues, he said.

“These lice really give us the potential to learn how humans evolved when so many parts of our evolutionary history are obscure,” he said. Lice also can serve as a model in understanding how parasites move from one species to another, Reed said.

“If you look at emerging infectious diseases that affect humans all over the world, most have their origins on some other host before threatening humans,” he said. “Studying what it takes for a parasite to be successful in switching hosts adds to our knowledge about what makes a good host for the spread of disease.”

This entry was posted on Saturday, March 10th, 2007 at 7:00 pm and is filed under science, strangely compelling. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.