OF LICE AND MEN

Why and how did humankind become “unusually successful”? And what, to an evolutionary biologist, does “success” mean, if self-destruction is part of the definition? Does that self-destruction include the rest of the biosphere? What are human beings in the grand scheme of things anyway, and where are we headed? What is human nature, if there is such a thing, and how did we acquire it? What does that nature portend for our interactions with the environment? With 7 billion of us crowding the planet, it’s hard to imagine more vital questions.

One way to begin answering them came to Mark Stoneking in 1999, when he received a notice from his son’s school warning of a potential lice outbreak in the classroom. Stoneking is a researcher at the Max Planck Institute for Evolutionary Biology in Leipzig, Germany. He didn’t know much about lice. As a biologist, it was natural for him to noodle around for information about them. The most common louse found on human bodies, he discovered, is Pediculus humanus. P. humanus has two subspecies: P. humanus capitis—head lice, which feed and live on the scalp—and P. humanus corporis—body lice, which feed on skin but live in clothing. In fact, Stoneking learned, body lice are so dependent on the protection of clothing that they cannot survive more than a few hours away from it.

It occurred to him that the two louse subspecies could be used as an evolutionary probe. P. humanus capitis, the head louse, could be an ancient annoyance, because human beings have always had hair for it to infest. But P. humanus corporis, the body louse, must not be especially old, because its need for clothing meant that it could not have existed while humans went naked. Humankind’s great coverup had created a new ecological niche, and some head lice had rushed to fill it. Evolution then worked its magic; a new subspecies,P. humanus corporis, arose. Stoneking couldn’t be sure that this scenario had taken place, though it seemed likely. But if his idea were correct, discovering when the body louse diverged from the head louse would provide a rough date for when people first invented and wore clothing.

The subject was anything but frivolous: donning a garment is a complicated act. Clothing has practical uses—warming the body in cold places, shielding it from the sun in hot places—but it also transforms the appearance of the wearer, something that has proven to be of inescapable interest to Homo sapiens. Clothing is ornament and emblem; it separates human beings from their earlier, un-self-conscious state. (Animals run, swim, and fly without clothing, but only people can be naked.) The invention of clothing was a sign that a mental shift had occurred. The human world had become a realm of complex, symbolic artifacts.

With two colleagues, Stoneking measured the difference between snippets of DNA in the two louse subspecies. Because DNA is thought to pick up small, random mutations at a roughly constant rate, scientists use the number of differences between two populations to tell how long ago they diverged from a common ancestor—the greater the number of differences, the longer the separation. In this case, the body louse had separated from the head louse about 70,000 years ago. Which meant, Stoneking hypothesized, that clothing also dated from about 70,000 years ago.

And not just clothing. As scientists have established, a host of remarkable things occurred to our species at about that time. It marked a dividing line in our history, one that made us who we are, and pointed us, for better and worse, toward the world we now have created for ourselves.

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