The Sixth Extinction: An Unnatural History

another would materialize. “It was an emotional roller coaster,” recalled
Ed Green, a biomolecular engineer from the University of California-Santa
Cruz, who worked on the project for several years.
The project was finally generating useful results—essentially, long
lists of A’s, T’s, G’s, and C’s—when one of the members of Pääbo’s team,
David Reich, a geneticist at Harvard Medical School, noticed something
odd. The Neanderthal sequences were, as expected, very similar to human
sequences. But they were more similar to some humans than to others.
Specifically, Europeans and Asians shared more DNA with Neanderthals
than did Africans. “We tried to make this result go away,” Reich told me.
“We thought, ‘This must be wrong.’”
For the past twenty-five years or so, the study of human evolution has
been dominated by the theory known in the popular press as “Out of
Africa” and in academic circles as the “recent single-origin” or
“replacement” hypothesis. This theory holds that all modern humans are
descended from a small population that lived in Africa roughly two
hundred thousand years ago. Around a hundred and twenty thousand
years ago, a subset of that population migrated into the Middle East, and
from there, further subsets eventually pushed northwest in Europe, east
into Asia, and all the way east to Australia. As they moved north and east,
modern humans encountered Neanderthals and other so-called archaic
humans, who already inhabited those regions. The modern humans
“replaced” the archaic humans, which is a nice way of saying they drove
them to extinction. This model of migration and “replacement” implies
that the relationship between Neanderthals and humans should be the
same for all people alive today, regardless of where they come from.
Many members of Pääbo’s team suspected that the Eurasian bias was a
sign of contamination. At various points, the samples had been handled
by Europeans and Asians; perhaps these people had got their DNA mixed
in with the Neanderthals’. Several tests were run to assess this possibility.
The results were all negative. “We kept seeing this pattern, and the more
data we got, the more statistically overwhelming it became,” Reich said.