Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past
David Reich
Oxford University Press
368pp, 27 black and white illustrations
Hardback, £20

Deduce movements and history of ancient human populations from scattered pottery shards, or bone fragments? That's the way it's been done since the 19th century, with far from satisfying results. But now, Harvard population geneticist David Reich has chronicled recent breakthroughs in what he calls the 'genomic revolution' of the past decade: the use of state-of-the-art technology to map the spread and migrations of ancient peoples. The discipline was founded by the late visionary population geneticist Luca Cavalli-Sforza more than 40 years ago, but the technology he needed was in its infancy.

Only a few years ago, human fossils with discernible DNA thousands of years old were considered fabulous rarities. No longer. Reich and his colleagues have sequenced the genomes of more than 900 ancient individuals (along with those of living populations) to reconstruct key features of humankind's prehistoric past. Conclusions are provisional, but he has been able to produce an interim report of what's new in ancient history. He compares genomics to the invention of a scientific instrument, which makes everything we thought we knew seem new and surprising.

It's impossible to give more than a hint of this book's wide-ranging content in reinterpreting the prehistory of European, South Asian, East Asian, Polynesian, African and Native American peoples. To take one example, the genetic evidence tells us that European hunter-gatherers were invaded by farmers from Anatolia about 9000 years ago, which we thought we knew – but genomic evidence also strongly suggests that these early mixed populations were supplanted by a later, hitherto unsuspected invasion of herders from the Caucasus who probably spoke the Indo-European protolanguage.

On the question of African origins, Reich opines that the scenario of a single lineage of humanity spreading smoothly from central Africa to Europe and Asia is impossibly simple; genomic evidence indicates that there were multiple intertwined lineages in various places, with some local populations even doubling back to Africa after the Neanderthal branch had become extinct in Europe.

All humans outside Africa today carry about 2% of Neanderthal genes, but no modern Africans do. A reasonable interpretation is that after they left Africa, around 100,000 years ago, the early Europeans hybridised with Neanderthals who were already there. And yet modern Europeans carry no more Neanderthal genes than do modern New Guineans or Chinese.

New genomic evidence also suggests that the rather benign idea of populations slowly assimilating and blending seems less likely than a model of many abrupt total or near-total population displacements. In addition, Reich explains the recent discovery of 'ghost population', whose ancestral genes are imbedded in widespread ethnic groups alive today, while the peoples that contributed that DNA no longer exist and have left no cultural or archeological traces behind.

Who We Are and How We Got Here is both comprehensive and exceptionally well-written, but it requires a dedicated reader to plough through its vast global scope as well as its myriad of fascinating details.

Richard Milner