[4] David Reich is a geneticist who has studied ancient human genomes, comparing their patterns of mutations to discover which populations migrated and mixed throughout prehistory.

[5] He was mentored by the population geneticist Luca Cavalli-Sforza, who from 1960 pioneered the attempt to match the study of human prehistory by archaeology and linguistics, using the limited genetic data available at that time.

That early work gave what Reich calls the "exciting" result that all modern humans are related to "Mitochondrial Eve", a woman who lived in Africa only about 160,000 years ago.

Forbes writes that Reich explains how ancient DNA teaches a single general lesson, that the human population of any particular place has repeatedly changed since the last ice age.

Any supposed "mystical, longstanding" link between some people and a place based on some kind of racial purity (as reflected in the Nazi slogan of "blood and soil"), is in Reich's words "flying in the face of hard science".

Cookson calls the book[1] a marvelous synthesis of the field: the technology for purifying and decoding DNA from old bones; what the findings tell us about the origins and movements of people on every inhabited continent; and the ethical and political implications of the research.

[1]Cookson notes that Reich dismisses worries that DNA evidence of differences between populations is "racism in genetic clothing",[1] and that on the contrary, the "unsuspected degree of mixing"[1] in every part of human history makes old ideas of racial purity "absurd".

King notes that Reich's group helped to show that "Neanderthals interbred with the ancestors of all modern humans descended from Europeans, Asians and other non-Africans."

She accepts that the book is "scientifically solid and comprehensive",[12] but argues that it "raises some concerns",[12] since in her view Reich overlooks the conflict of interest between the needs of researchers to gather ancient human DNA specimens and the rights of the often marginalised populations that descend from those ancestors.

[13] In Hawks's view, Reich "seems a contradiction: Rising to the pinnacle of human genetics, he nonetheless exhibits incredible naiveté"[13] in affairs such as when German scientists withdrew from a paper that seemed to them "uncomfortably close to pre-Nazi racial theories",[13] or when Native American specimen donors wished to limit the use of their DNA.

[13] The scholar of humanities Barry Wood writes that the book is "densely-packed", not sparing on explanations of advanced techniques, but allowing the determined reader to grasp the methods of genomic analysis.

"[3] According to computational biologist Hussein Mohsen, Reich's examples "conflated ancestry with race, relied on the quantitative reductionism of averages, and failed to address the everyday ramifications of pathologizing socially constructed (racial or ancestral) categories to which millions of human beings belong or were historically forced to.

"[16] In contrast to Reich's analysis, Mohsen cites the open letter as an accurate representation of "the complexity of the race concept and stark limitations of quantitative genetics to grasp its fluid histories.