The overriding lesson ancient DNA teaches is that the population in any one place has changed dramatically many times since the great human post-ice age expansion, and that recognition of the essentially mongrel nature of humanity should override any notion of some mystical, longstanding connection between people and place. We are all, to use Theresa May’s derisive label, “citizens of nowhere”. The Beaker people replaced 90% of the population of Britain around 4,500 years ago. Related to this is the knowledge that all life, from its beginnings, has been an essentially improvisatory, impure process. As Reich puts it: “Ideologies that seek a return to a mythical purity are flying in the face of hard science.”

His findings have important medical implications, too. For instance, his research in India has shown the profound consequences of caste-system inbreeding. There is an abundance of recessive diseases – both partners in some couples carry mutant genes from way back in the lineage. In fact, such populations make gene hunting easier because genes causing recessive conditions come with characteristic markers.

Reich’s overall picture will, in time, acquire much greater detail – just as Darwin’s great study was a beginning not an end – but we should be grateful to him and his large team of co-workers (including his wife, science writer Eugenie Reich, who had a big role in the book’s creation) for putting the essential story before us now. It is thrilling in its clarity and its scope.