It’s flattering, having a biographer, one more sign that you’ve been living large.
Caesar had Tranquillus. Cleopatra had Plutarch. The human race has a devotee, too, an attentive scribe who has been recording our story since long before we started keeping track of it ourselves.
If only our alter ego weren’t so icky.
Hamlet had Horatio. Don Quixote had Sancho Panza. All we get is a creepy little louse.
A BIG SURPRISE
The drama of human evolution was the furthest thing from David Reed’s mind last year when he and five collaborators embarked on a study of a rust-colored, parasitic insect the size of a grain of salt.
Reed, curator of mammals at the Florida Museum of Natural History at the University of Florida, was still a post-doctorate fellow at the University of Utah when he became the lead investigator in a study of pediculus humanis capitis: human head lice.
Best known for creeping out the parents of school-age children, the crawling pests make a living out of gluing their eggs to hair follicles and grazing on the scalp for tiny blood meals. Anybody who has ever feared the prospect of taking a note home from the school nurse knows about cooties. Yuck.
But who knew they had so much to say?
Even the investigators were surprised to discover that the heads of American schoolchildren bore clues about the behavior of our prehistoric ancestors.
“We stumbled onto a gold mine,” says Reed, 35.
His small group of researchers hadn’t been looking for cave-man secrets when they set out to examine the genetic differences between head and body lice. They were biologists, not anthropologists. And they were looking to the future, not the past.
They knew that three kinds of lice can live on human beings. Each type stakes a claim on one part of the body. Head lice dwell only on the head. Body lice patrol the torso. Pubic lice are loyal to their nethermost neighborhood.
Only one of the three can be anything more than a source of itching and embarrassment: Body lice can transmit diseases, from typhus to trench fever. That was what prompted Reed’s study.
What if head lice, which are far more prevalent, began carrying diseases? “We thought, ‘What if terrorists found a way to introduce disease-carrying head lice to schoolchildren?’ ” he says. “It made the issue even more important to look into.”
So researchers collected samples of lice from throughout the world and began analyzing their DNA, which not only dictates their behavior but records their family lines and evolutionary history. That was when they stumbled onto a surprising discovery.
In most of the world, people have just one kind of head lice. But schoolchildren and adults in North and South America have two — the worldwide variety and another.
When the researchers did further DNA tests, they found that the worldwide type has been sitting atop the human head for more than a million years. That made sense. Paleo-anthropologists, who study the evolution of early man, think it was around then when the ancestral line that evolved into Homo sapiens — modern man — parted ways with other primates. The lice on human bodies would have begun evolving separately at that point. As our species gradually shed the full body-hair look, the lice had to bid goodbye to the free-ranging good old days and migrate to the hairy enclaves that remained, just as larger animals adapt when the topography shifts in their world.
What the DNA clock showed about the worldwide strain of head lice dovetailed precisely with what we know of human evolution.
But what about the other one? According to the DNA dating, it had come from the same pediculus family tree, going back a million years. But it had spent most of those years hitchhiking elsewhere. It had appeared on Homo sapiens only recently, probably within the last 50,000 years.
Where had it come from?
The researchers knew enough to disallow one possibility: other animals. Unlike fleas, which hop from one species to another, lice are what biologists call species-specific. Like most parasites, they pick one animal and stay with it for life. Reed and his research group found themselves confronting the same basic question that all those freaked-out parents have when their little ones come home with uninvited guests:
Where did you get them?
CLOSE ENCOUNTER?
Last month, in answer to that question, Reed and his research group published a study in the Public Library of Science journal PLoS Biology.
They theorized that the head lice must be evidence of a dramatic meeting, probably in Asia, between modern man and a much more primitive cousin.
If they are right, it is the first evidence we have of such a meeting.
“It’s amazing,” says Reed. “We either battled with them, or lived with them, or even mated with them. Regardless, we touched them.”
Most anthropologists think that four types of humans coexisted on the Earth about 100,000 years ago.
There was Homo sapiens, whose population was concentrated in Africa. They were similar enough to modern humans that you could station one of them in front of a television, slip a can of beer in his hand, and never know the difference.
There was Homo neandertalis, the hardy hunters who painted cave walls and buried their dead with flowers. They lived in what is now Europe.
To the east, in Asia, there was the similarly built Homo erectus, stocky and heavy-browed, many of whom lived in seaside encampments as primitive fishermen.
On the tiny island of Flores, near the coast of Australia, there was the diminutive, newly discovered Homo floresiensis — nicknamed “hobbit” by the anthropologist who discovered evidence of them this year. They stood only 3 feet tall but may have been capable of using language and hunting tools.
TINY SOUVENIRS
Up until now, the only evidence that Homo sapiens ever mingled with any of our more primitive cousins had been fossil evidence that showed we coexisted with Neanderthals, who became extinct within 10,000 years of our ancestors’ arrival in Europe.
If Reed’s parasitic paparazzi are to be believed, much the same thing happened in Asia. Small bands of Homo sapiens gradually migrated out of Africa to the east. At some point they met up with Homo erectus. No doubt both sides were as astonished by the sight of the other as we would be if a green-skinned interstellar invader turned up at the mall.
Perhaps they fought. Perhaps their encounters were friendly, even intimate. Because we have no cave-man DNA, there’s no way of telling if they mated.
All that is certain is that a few thousand years later, Homo erectus, like the Neanderthals, was dead and gone. But we had something to remember them by. We carried tiny souvenirs of our encounter with them on our heads, bringing them with us as we continued to migrate, eventually working across the Bering Strait and into the Americas roughly 20,000 years ago.
ALTERNATE VIEW
That, at least, is Reed’s contention. Anthropologists are divided on whether to accept it.
The notion that modern man evolved in Africa and eventually spread across the world, sweeping away his more primitive cousins as he went, is called the “Out of Africa” theory. The head lice study appears to support that theory.
But other anthropologists subscribe to what has been called multiregionalism. They think that mankind did not emerge in one region, but gradually evolved, through interbreeding among the various kinds of early humans, all over the world.
Milford Wolpoff, anthropologist at the University of Michigan, is the most outspoken proponent of multiregionalism.
“We’re all one species,” he says. Wolpoff dismisses the lice study — and anything else that challenges multiculturalism — as “sloppy thinking.”
“Some anthropologists will not accept the validity of genetic analysis at all,” says Chris Stringer, a researcher at the Natural History Museum in London who studies early man. “Others will question the analysis because their preconceived ideas are being challenged.”
BEDBUGS AND BEYOND
Reed wants to keep on challenging. He says the results of the head lice study are so intriguing that he plans on chasing down other parasites, from pinworms to bedbugs, to see what secrets they can share.
Other researchers have used genetic analysis of tapeworms and body lice to determine when humans began eating meat and first wore clothing.
Reed’s next study may be an analysis of the DNA of pubic lice to see if he can help to answer that question about whether Homo sapiens interbred with other human species.
He also envisions another study of head lice from both sides of the Bering Strait, to see if he can pinpoint exactly when humans migrated into North America.
And he may probe further into the evolutionary past. We descended from a hairy species. Wherever our ancestors traveled, they sprinkled their little friends, like bread crumbs along a trail through time and space.
“There are just so many things we can learn from these parasites,” says Reed. “They match up to our evolution, going all the way back to our earliest ancestors. You can see where we split off from chimps. You can see where we split off, even further back, from the old world monkeys.”
His research has captured the public imagination enough that Reed is often asked to give talks about it.
At first, he says, he can gaze out at the audience and tell by the looks on the faces that most of his listeners are put off by the notion of examining blood-sucking creatures.
Gradually, though, they leave their queasiness behind, and become lost in the subject.
Reed knows what comes next. He has grown accustomed to seeing the same phenomenon, time after time: One by one, row by row, the hands rise up as they slowly, thoughtfully, begin to scratch their heads.
