David Reich, a geneticist at the Harvard Medical School, has redrawn our species’ family tree. And today, in his office overlooking Avenue Louis Pasteur in Boston, he picks up a blue marker, walks up to a blank white wall, and shows the result to me. He starts with a pair of lines — one for humans and one for Neanderthals — that split off from a common ancestor no more than 700,000 years ago. The human branch divides into lineages of Africans, Asians, and Europeans, and then into twigs for smaller groups like the people of New Guinea or the residents of the remote Andaman Islands in the Indian Ocean. Reich also creates a branch off the Neanderthal line for the Denisovans, a paleolithic lineage geneticists discovered only a few years ago.
All well and good. This is the sort of picture you’d expect if we and our humanlike relatives diverged neatly through evolution. It looks a lot like the tree of life that Darwin included in The Origin of Species. But then Reich violates his tree.
Instead of making new branches, he starts linking branches together. He inscribes a line that links the Neanderthal lineage to the Europeans and Asians. He joins the Denisovan line and the one leading to the people of New Guinea. He crisscrosses the tree again and again, joining the branches into a thicket of grafts.
Reich steps back and looks over his creation. He has a high forehead, a peregrine profile, and a very soft voice. “So,” he says quietly, “it’s a little bit complicated.”
That’s putting it mildly. Over the past 15 years, Reich has developed a toolbox of sophisticated statistical methods to extract history out of our DNA. And with those methods he has revealed scandalous liaisons dating back tens of thousands of years.
