January 19, 2019
To the Editor:
Gideon Lewis-Kraus (Jan. 17) profiles the nascent field of ancient DNA, which in the last few years has contributed to a transformation in our understanding of the deep human past. His article touches on important issues that we, as a field, have yet to deal with fully: including how to handle ancient remains ethically and in a way that preserves them for future generations; how geneticists and archaeologists can work in equal partnerships that reflect true respect for the insights of different disciplines; and how ancient DNA technology, which at present is applied efficiently only in large labs, can be made accessible to a wider group of scholars.
But Lewis-Kraus misunderstands several basic issues. First, he suggests that competition to publish is so extreme that standards become relaxed. As evidence, he cites a paper by my lab that was accepted on appeal after initial rejection, and another that was reviewed rapidly. In fact, mechanisms for appeal and expedited review when journals feel they are warranted are signs of healthy science, and both processes were carried out rigorously.
Second, he contends that ancient DNA specialists favor simplistic and sweeping claims. As evidence, he suggests that in 2015 I argued that the population of Europe was “almost entirely” replaced by people from the Eastern European Steppe. On the contrary, the paper he references and indeed my whole body of work argues for complex mixture, not simple replacement. Lewis-Kraus also suggests that I claimed that our first study of the people of the Pacific island chain of Vanuatu “conclusively demonstrated” no Papuan ancestry. But the paper in question was crystal-clear that these people could have had some Papuan ancestry – and indeed, to support his claim, Lewis-Kraus could only cite his own notes from an interview I gave him long after I had published a second paper proving that there was indeed a small proportion of Papuan ancestry.
Lewis-Kraus also suggests that I use small sample sizes to make unjustifiable sweeping claims. In fact, small sample sizes can be definitive when they yield results that are incompatible with prevailing theories, as when my colleagues and I described two samples that proved the existence of the Denisovans, a previously undocumented archaic human population. In my papers, I am careful to only make claims that can be supported by the data I have. In small-sample size studies, I emphasize that more samples are needed to flesh out the details of the initial findings. A major focus of my lab is generating the large data sets needed to do this.
Lewis-Kraus’s critiques are based on incomplete facts and largely anonymous sources whose motivations are impossible to assess. Curiously, he did not ask me about the great majority of his concerns. Had he done so, the evidence underlying his thesis that my work is “indistinguishable from the racialized notions of the swashbuckling imperial era” would have fallen apart. The truth, and the main theme of my 2018 book Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past, is exactly the opposite - namely, that ancient DNA findings have rendered racist and colonialist narratives untenable by showing that no human population is “pure” or unmixed. It is incumbent on scientists to avoid advocating for simplistic theories, and instead to pay attention to all available facts and come to nuanced conclusions. The same holds true for journalists reporting on science.
David Reich Harvard Medical School and the Howard Hughes Medical Institute, Boston, Massachusetts