What a Story Lice Can Tell

By NICHOLAS WADE

Published: October 5, 2004

spectator with an especially intimate view of human evolution is beginning to tell its story and has so far divulged two quite unexpected findings.

The human louse finds people so delicious that it will accept no substitutes and cannot live more than a few hours away from the warmth and sustenance of the human body. This devotion to the human cause means that the evolutionary history of human lice dovetails with that of their hosts and reflects several pivotal events that affected both species.

In a finding that seems bound to inspire several science fiction treatments, Dr. David Reed of the Florida Museum of Natural History in Gainesville has reached the startling conclusion that some human lice show signs of having evolved originally on a different human species.

In today's issue of the journal PLoS Biology, he and his colleagues suggest that modern humans may have contracted this strain of lice from an archaic human like Homo erectus.

Last year Dr. Mark Stoneking and colleagues ferreted out another fact from lice, the date when humans first started to wear clothes - about 72,000 years ago according to their calculations. This date has long eluded archaeologists because of the perishability of items like skins, fabrics and the bone needles used to sew them.

Dr. Stoneking, who works at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, hopes that lice may be coaxed to yield a yet more intimate secret, the date when humans shed their ape-like body hair and roamed naked across the scorching savannas of Africa.

This louse's eye view of human evolution rests on analysis of the DNA of lice sampled from populations around the world.

It also depends on the natural history of lice, which have developed some rather specialized habits.

The human body provides three different kinds of ecological niche, from the louse's point of view, and a different type of louse inhabits each. The head louse lives in the hairs of the head. The body louse is adapted to live in clothing, since the human body lacks proper hair. And the pubic louse dwells in the coarse hairs of the groin, a cramped habitat but one that affords a convenient opportunity for switching abodes whenever the host is intimately occupied with a partner.

Six million years ago, before the line leading to humans split from the chimpanzee lineage, matters may have been much simpler. The joint ancestor of humans and chimps had full body hair, like a proper primate, and presumably a single kind of louse ranged freely from its head to its toes.

That louse would have belonged to the genus Pediculus and evolved into two sister species when the human and chimp lineages parted ways.

The human head and body lice are known as Pediculus humanus and the chimpanzee louse is Pediculus schaeffi.

When humans lost their body hair, perhaps 1.8 million years ago, the Pediculus louse would presumably have been restricted to the head, leaving the rest of the body vacant.

This could have been the moment when the pubic louse began its long-term relationship with its human host. Dr. Stoneking believes that if he could establish when the pubic louse arrived, by assessing the amount of genetic variation in today's pubic louse population, he might be able to fix a minimum date for when people attained nakedness.

A perplexity that louse specialists have not yet resolved is that the pubic louse belongs to a different genus from Pediculus, being known to taxonomists as Phthirus pubis.

Bizarrely, its closest relative is Phthirus gorillae, the louse that infects the gorilla.

How it got to humans from gorillas, if that is indeed its origin, is a question that raises some dire possibilities.

But there may be less lurid explanations. Some experts believe that Phthirus lice have infected the primate line for millions of years but have somehow been evicted altogether by chimpanzees and confined to the groin in humans.

"The acquisition of the pubic louse likely has nothing to do with when we lost our body hair," Dr. Reed said.

Dr. Reed's new finding raises the intriguing possibility that humans contracted one strain of the Pediculus louse from another host species.

He and colleagues gathered head and body lice from around the world and analyzed their DNA.

When they tried to construct a family tree for the lice, based on variations in a particular DNA segment, their computer spit out a tree with two quite separate clusters.

One cluster of lice was found only in the New World, in samples from the United States and Honduras. The other cluster occurred worldwide.

Since changes in DNA clock up at a fairly constant rate, geneticists can often estimate the coalescence date - the age at which all branches of a tree merge back into a single root.

Dr. Reed's group calculates that the two clusters of lice split apart some 1.18 million years ago. This is about the time that there was an important split in the humans evolving in Africa, between the lineages that led to archaic and to modern humans.

Dr. Reed believes that ancestors of the New World cluster of lice must have been carried out of the Africa by archaic hominids more than a million years ago. When modern humans left Africa much later, around 50,000 years ago, they would have encountered the descendants of the archaics, who had evolved into Homo erectus in East Asia and into the Neanderthals in Europe.

When modern humans and Homo erectus met in Asia, whether in social or hostile circumstances, lice on the archaic humans seized the chance to switch over to moderns, Dr. Reed believes.

The New World was first colonized by people from Siberia who first crossed the now sunken land bridge between Siberia and Alaska, probably about 15,000 years ago. These colonists may have carried the Homo erectus brand of lice, Dr. Reed suggests, which perhaps was the only kind in the New World until the Europeans arrived, bringing the standard Old World lice with them.

It is because the two brands of lice have such long separate histories that they show up as two separate clusters on the genealogical tree derived from their DNA.

Contact between different human species is not the only story that lice have to tell. Another is the invention of clothing.

When humans lost their body hair, the Pediculus louse was confined to the head. But it was able to recolonize much of its lost domain, except the enclave occupied by the pubic louse, when humans decided they had been naked for long enough. The head louse developed an adaptation, the body louse, which lives in clothing.

A dispute has long raged among louse specialists as to whether the head and body louse are distinct populations or can switch back and forth between the two forms. The body louse is somewhat larger but otherwise very like the head louse.

The two can interbreed in captivity. But they differ in behavior.

Head lice that are placed on clothing will migrate quickly back to the head, as if totally averse to the thought of living on cloth. Natalie Leo, a parasitologist at the University of Queensland in Australia, has addressed the issue by studying lice from people in China and Nepal who were infected with both types.

"It's hard to find double infestations so usually you have to go to conditions where people don't change clothes very regularly," she said.

Ms. Leo and her colleagues have found that the head and body lice on a single person do not interbreed. From genetic analysis to be published in the journal Heredity, she believes that the two types are distinct and that body lice evolved only recently from head lice.

It was based on the assumption that body lice evolved when clothing was invented that Dr. Stoneking and his colleagues, after analyzing the DNA of a worldwide collection of head and body lice, last year set the inception of the rag trade at 72,000 years ago.

Genetic clocks have to be set by some outside reference point, and Dr. Stoneking chose the chimpanzee louse as his base for assessing the number of mutations in its cousin species, the lice of people.

Dr. Reed believes Dr. Stoneking's chimpanzee louse has so odd a DNA sequence that it may have been misidentified. If Dr. Stoneking had used a real chimp louse sequence, Dr. Reed says, he would have gotten a date closer to 500,000 years for the head/body louse split.

Dr. Stoneking said he did not think his chimp louse was misidentified but that there could have been an error in the DNA sequencing, which he would correct if necessary.

Could clothing have been invented as early as 500,000 years ago? Humans may well have protected themselves against the elements by this date, with fire and loose-fitting garments like capes, but it seems unlikely they had mastered the tailor's art by so early a date.

"Regarding the timing for clothing, I think 500,000 years ago is as good as any other date, assuming that the clothing doesn't have to be tightly tailored with seams,'' said Dr. Richard Klein, an archaeologist at Stanford University. The body louse may have needed snugly fitting clothing so as to share human body heat, in which case it probably evolved more recently than 500,000 years ago.

Dr. Stoneking said that he too had seen signs of a second cluster of human lice, but the age of cluster, according to his data, suggested it had evolved on another human species only some 500,000 to 600,000 years ago. If so the host switch would probably have taken place in Africa, from some other branch of the human family to the branch that became modern humans.

For eons the louse has done nothing but suck human blood and, worse than that, the body louse has been the distributor of three scourges: typhus, trench fever and relapsing fever.

Now at last these parasites are giving a little back. "I really think we are only scratching the surface of what lice can tell us,'' Dr. Reed said.

-- send this page to a friend --