Beyond Stones & Bones

Unlike teeth and skulls and other bones, hair is no match for the pitiless ravages of weather, geologic upheaval and time. So although skulls from millions of years ago testify to the increase in brain size as one species of human ancestor evolved into the next, and although the architecture of spine and hips shows when our ancestors first stood erect, the fossil record is silent on when they fully lost their body hair and replaced it with clothing. Which makes it fortunate that Mark Stoneking thought of lice.

Head lice live in the hair on the head. But body lice, a larger variety, are misnamed: they live in clothing. Head lice, as a species, go back millions of years, while body lice are a more recent arrival. Stoneking, an evolutionary anthropologist, had a hunch that he could calculate when body lice evolved from head lice by comparing the two varieties' DNA, which accumulates changes at a regular rate. (It's like calculating how long it took a typist to produce a document if you know he makes six typos per minute.) That fork in the louse's family tree, he and colleagues at Germany's Max Planck Institute for Evolutionary Anthropology concluded, occurred no more than 114,000 years ago. Since new kinds of creatures tend to appear when a new habitat does, that's when human ancestors must have lost their body hair for good—and made up for it with clothing that, besides keeping them warm, provided a home for the newly evolved louse.

If you had asked paleoanthropologists a generation ago what lice DNA might reveal about how we became human, they would have laughed you out of the room. But research into our origins and evolution has come a long way. Starting with the first discovery of a fossil suggesting that a different sort of human once lived on this planet—it was a Neanderthal skull, unearthed in a mine in Germany's Neander Valley in 1856—our species' genealogy was inferred from stones and bones. Fossils and tools testified to our ancestors' origins in Africa, the emergence of their ability to walk upright, the development of toolmaking and more. But now two new storytellers have begun speaking: DNA and brains.

The science of human evolution is undergoing its own revolution. Although we tend to see the march of species down through time as a single-file parade, with descendant succeeding ancestor in a neat line, the emerging science shows that the story of our species is far more complicated than Biblical literalists would have it—but also more complex than secular science suspected. By analyzing the DNA of today's humans as well as chimps and other species (even lice), scientists are zeroing in on turning points in evolution, such as when and how language and speech developed, and when our ancestors left Africa. DNA can even reveal how many pilgrims made that trek. At the new Hall of Human Origins at the American Museum of Natural History in New York, DNA gets equal billing with fossils. And by comparing the impressions that brains left on the inside of skulls, "paleoneurology" is documenting when structures that power the human mind arose, shedding light on how our ancestors lived and thought. Whether or not you believe the hand of God was guiding these changes, the discoveries are overturning longstanding ideas about how we became human.

Not that fossils are passé. new discoveries are pruning and reshaping humankind's family tree as radically as bonsai. The neat traditional model in which one species gave rise to another like Biblical "begats" has been replaced by a profusion of branches, representing species that lived at the same time as our direct ancestors but whose lines died out. It's like discovering that your great-great-grandfather was not an only child as you'd thought, but had a number of siblings who, for unknown reasons, left no descendants. New research also shows that "progress" and "human evolution" are only occasional partners. More than once in human prehistory, evolution created a modern trait such as a face without jutting, apelike brows and jaws, only to let it go extinct, before trying again a few million years later. Our species' travels through time proceeded in fits and starts, with long periods when "nothing much happened," punctuated by bursts of dizzying change, says paleontologist Ian Tattersall, co-curator of the American Museum's new hall.

As its exhibits show, humankind's roots are sunk deep in the East African savanna. There, the last creature ancestral to humans as well as chimps—our closest living cousins—lived, standing at a fork in the family tree as momentous as it is contentious. Fossils never resolved when the lineages split. DNA might. Human DNA and chimp DNA differ by no more than 1.2 percent, and DNA changes at a fairly regular rate. That lets scientists use this rate to calibrate a "molecular clock" whose tick-tocks measure how long ago a genetic change occurred. The fact that the DNA of living chimps and humans differ by about 35 million chemical "letters," for instance, implies that the two lineages split 5 million to 6 million years ago. That fits with the discovery that Earth became cruelly colder and drier 6.5 million years ago, just the sort of climate change that coaxes new species into being. The apes that stayed in the forests hardly changed; they are the ancestors of today's chimps. Those that ventured into the newly formed habitat of dry grasslands had taken the first steps toward becoming human.