Contempt is never wise in biology. The creature that you look down on as lowly, degenerate, or disgusting may actually turn out to be sophisticated, successful, and--in some cases--waiting to tell you a lot about yourself. That's certainly the case for lice.

The human body louse, Pediculus humanus, has two ways of making a living--either dwelling on the scalp, feeding on blood, or snuggling into our clothes and come out once or twice a day to graze on our bodies. For lice, we humans are the world. They cannot live for more than a few hours away from our bodies.Only by crawling from one host to the next does their species escape extinction.

A group of louse specialists recently decided to find out where human lice came from. Have they been riding on our bodies since before we were human? A comparison of the lice that live on different primates shows that they certainly can be very loyal. If you draw an evolutionary tree of primates, and then draw a tree of their lice, they are almost identical. On the other hand, some lice can live on more than one species. And a side-by-side comparison of trees reveals that in some cases they don't form a perfect mirror. In other words, sometimes lice can make an evolutionary leap.

As the researchers report today in Public Library of Biology, they compared human lice to the lice of primates, looking at both their DNA and their anatomy. As earlier research had shown, they found a major split among lice species that live on apes and on monkeys and other primates. That reflects an ancient split in the primates themselves: our ape ancestors diverged from other primates 20-25 million years ago. The variation in louse DNA turns out to act like a sort of molecular clock, showing when they split into different lineages. The molecular clock puts the split between lice that live on humans and chimps at 5.6 million years ago--exquisitely close to the age that's been estimated for humans from studies on both DNA and fossils.

The research suggested that we've carried our lice for millions of years, since before the time of our common ancestor with chimpanzees. But after we parted company with the chimps, the lice have a remarkable story to tell. Human lice split into two lineages. One lineages is found around the world. The second is found only in North America. The worldwide branch all share a common ancestor that lived 540,000 years ago. The North American branch shares a common ancestor that lived 150,000 years ago. And finally, the two branches share a far older common ancestor, which lived a 1,180,000 years ago.

So how did these two strains of the same species become separated and then wind up back on our bodies? The researchers argue that human evolution holds the key. Paleoanthropologists and geneticists still debate over the origins of modern humans, but the rough outlines are becoming clear. The first hominids to emerge that were tall, big-brained bipeds--that weren't just upright apes, in other words--lived about 2 million years ago. They very quickly began to spread out of their birthplace in Africa to other parts of the world. They were in the Caucusus mountains 1.8 million years ago and China 1.66 million years ago. These hominids are generally called Homo erectus, although they may well have consisted of several species, rather than one. And the ranks of Asian Homo erectus may have been boosted by fresh migrations of African hominids when ecological conditions favored another journey out of Africa. But it does appear that Asian populations became pretty isolated from African hominids. The fossils of Homo erectus from a few hundred thousand years ago look pretty distinct from both African hominids and Neanderthals, with very thick skull walls and other peculiar anatomical details. Thirty years ago, most paleoanthropologists would have told you that these Asian hominids probably were the ancestors of living Asians. But that's not what the evidence gathered since then suggests. Instead, it now looks pretty clear that Homo erectus was a very distinct species than Homo sapiens, and became extinct perhaps as recently as 30,000 years ago.

Our own roots can be found in Africa. The oldest clear cut examples of Homo sapiens fossils, found in Ethiopia, date back 160,000 years. By about 100,000 years ago, our species was beginning to diverge into different populations, and these differences can still be found in the DNA of various African groups, such as the Khoisan of Southern Africa (sometimes called bushmen). By 50,000 years ago, humans were moving out of Africa. In Europe, they moved into territory occupied by Neanderthals and their ancestors for some 300,000 years. Neanderthals disappeared by 28,000 years ago. They seem to have been driven into mountainous refuges by the booming population of humans. The story in Asia has always been a bit fuzzier. Humans appear to have gotten to Australia by at least 40,000 years ago, and perhaps much earlier. By 15,000 years ago, some Asian populations of Homo sapiens made their way into the New World through Alaska. Exactly where Homo erectus was on their arrival in Asia, and how long they survived, has never been clear. It hasn't even been clear whether the two species came into contact or not.

You may be able to guess how the louse scientists interpret the data from their parasitic charges. When Homo erectus moved into Asia and became isolated from our own ancestors, their lice became isolated as well. When our own ancestors burst out of Africa around 50,000 years ago, they carried the African lice with them. The most sensational part of the story comes when humans arrive in Asia. The researchers argue that a population of humans encountered Homo erectus and picked up their lice. Their descendants then passed into North America, where they--and their lice--live today. One of the many intriguing implications of this research is that the contact may have occurred in one limited regions--the same region where Native Americans originated in Asia.

This is not the first case where our parasites have preserved our own hidden history. Our tapeworms, for example, can tell us about how our ancestors began eating meat. Malaria reveals how agriculture brought new diseases to humans over the past few thousand years. Helicobacter pylori, the bacteria that trigger stomach ulcers, maps the spread of modern humans. (I go into more detail on some of these examples in my book Parasite Rex.) And the lice probably have more to tell us.

For example, the scientists can't say for sure how humans most likely picked up Homo erectus's lice. The contact definitely had to be intimate. But did it occur when humans drove Homo erectus away from their kills? Or did these two species make love, rather than war? Although the genetic evidence indicates that Homo erectus could not have contributed a significant number of genes to our species, it's possible that they contributed a few. The answer to this question may help show how Homo erectus became extinct, leaving us as the sole hominids left on Earth.

One way to test that possibility will be to look at the other species of lice that live on humans--crabs, or Pthirus pubis. If our ancestors got body lice from Homo erectus during sex, they probably got crabs as well. Somehow, though, I'm guessing that putting together a global collection of crabs may take a little bit longer than the body lice. But it will definitely be worth the wait.

UPDATE: 10/4 9:50 PM: A question occurs to me: why didn't we pick up Neanderthal lice?

UPDATE: 10/5 6:20 PM: The link to the paper is fixed (and the paper is free--bless PLOS!)