Through Lluís Quintana-Murci’s window, fog obscures the Eiffel Tower. The Mallorcan geneticist, long a fixture of the Institut Pasteur, still hasn’t adapted to Paris’s greyness. He is a migrant, appropriately. His book Human Peoples, about this century’s breakthroughs in genetics, explains that humans have always been migrants. “This is not champagne socialism,” he says. “It’s science.” And migrant humans mated (“admixed”, say geneticists) with other groups they encountered.

Genetics has advanced unimaginably since the 1990s, when Quintana-Murci did his PhD. The Human Genome Project, which sequenced the 3.2 billion nucleotides in our genome, was completed in 2003. From the 2010s, advances in studying ancient DNA allowed geneticists to compare prehistoric and modern genomes. Their findings are reframing our history. Now we can map human migrations since Homo sapiens first left Africa about 60,000 years ago. The past, says Quintana-Murci, is “the most natural experiment ever”, because it was carried out by nature itself. Genetic history helps explain who we are. 

A fundamental finding: because Homo sapiens is a comparatively recent species, about 200,000 to 300,000 years old, people are all genetically quite similar. The world’s eight billion humans are much less diverse than our 200,000 chimpanzee cousins, writes Quintana-Murci. “The differences between the genomes of two humans chosen at random, whatever their ethnic or geographic origins, are of about three million nucleotides, in other words, 0.1 per cent of the genome.”