Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past by David Reich (review)

David Reich's book Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past (New York: Pantheon Books, 2018; xxv+335 pp.) is a landmark synthesis of findings from studies on DNA sequences collected from the bones of ancient people. Twenty-five years ago the idea of DNA extracted from the remains of long dead beings was science fiction. Today, these data are flooding in, and they are contributing a new dimension to our investigations of prehistory. Real science is surpassing fiction. The author, David Reich, is a primary innovator in this new field of study. He has written a personalized account that will interest a wide audience that includes both professionals and the lay public. His prose is readable, and he has avoided overuse of jargon and highly technical terms. However, Reich is prone to overstate his case in ways that will infuriate some readers; for example, Chapter 1 is titled "How the Genome Explains Who We Are."

The book is divided into three main sections. The first begins by reviewing the basics of DNA's structure and the genetic principles that allow inferences about ancestry from variations in the sequences of nucleotides in DNA. This section then describes the Neanderthal and Denisovan genomes. Our knowledge of both genomes resulted from the work of a research team led by the evolutionary geneticist Savante Pääbo. As a core member of this team, Reich contributed statistical analysis. The possession of a Neanderthal genome sequence is an awesome achievement insofar as Neanderthals were the first fossils to shed light on human evolution. The Denisovan genome shows how an ancient genome can reveal a previously unknown, and unanticipated, member of our lineage. The importance of discovering the Denisovans approaches that of Raymond Dart's discovery of australopithecine fossils. Australopithecines were also unanticipated, and this finding ultimately restructured our thinking about human origins.

The second section of the book presents a historical survey of people living in different continental regions. The inferences come from combining data from genome sequences of contemporary people with those extracted from skeletal remains. Many of the skeletons are several thousand years old. The author's laboratory has contributed an impressive number of the primary DNA sequences. He presents a detailed historical scenario for each continental region and skillfully uses maps and diagrams to summarize his interpretations. Reich repeatedly found that the current inhabitants of a particular region have not descended from the ancient people in the region. Thus, large-scale migrations and population shifts must have been the norm in human prehistory. He also found evidence that modern populations often arose from admixtures of highly differentiated ancient groups that no longer exist. Presently, the only traces of these "ghost" populations are embodied in our DNA. While the author does an admirable job of explaining the underlying logic that allows these inferences, he does less to show the actual patterns of variation in the DNA that are the bases of his conclusions. This gap makes it difficult to critically evaluate the conclusions that he presents. However, this book is a synthesis of peer-reviewed publications, and the sophisticated reader can refer back to the primary sources.

In the third section, Reich passionately expresses his views and concerns related to hotbutton topics such as health disparities, behavioral genetics, and race. Unfortunately, he presses these buttons with less objectivity and thoroughness than he applies to his primary research. He fears that genomic analysis will reveal dark truths about human diversity that compassionate scientists would rather not face. One mistake that Reich makes is dividing polar views on racial classification into camps populated by anthropologists (who see no value) versus biologists (who see value), a division inconsistent with academic disciplines—Reich names biologists and anthropologists on both sides of the debate. A second mistake is Reich's...