The first, and most dramatic, development in the application of genome sequencing to archaeology was Svante Pääbo's sequencing Neanderthal genome in 2009, which showed that there had been interbreeding between Neanderthals and modern humans. Reich started working with Pääbo in 2007, and in 2013 Pääbo helped him to set up the first laboratory in the USA for the large-scale production of ancient genomes. Similar work is beginning to be carried out in other countries as well.
In this book Reich presents an overview of what has been discovered so far. He emphasises that this cannot be a definitive description; new discoveries are being made continually and much of what he says here will inevitably have to be modified or even contradicted later. Still, enough has been achieved, he insists, to produce a radical transformation in our ideas about prehistory. 'The ancient DNA revolution is rapidly disrupting our assumptions about the past.' This is the first book to provide a popular account of what has been discovered so far.
The central fact to emerge is that it is no use looking at the genetics of people alive today to infer where they come from or what happened in the past. Study of ancient DNA has shown, time and again, that earlier people were much more mobile than many scholars had supposed. Large migrations have occurred repeatedly on a worldwide scale and there has been a vast amount of interbreeding. So the metaphor of an evolutionary tree is misleading; what we have is more like a network.
The book is in three parts. Part I is about interbreeding between modern humans and other species—mainly the Neanderthals but also the Denisovans and other now extinct species. Part II looks at the evolution of modern humans in five regions of the world: Europe, India, America, East Asia, and Africa. Part III is more 'political' and considers the relevance of this work to modern life and ideas of identity.
Part I is mostly a recapitulation of Pääbo's work and adds little to what readers of Neanderthal Man will know already. However, Reich has an interesting discussion of the idea of a retrograde migration from Eurasia to Africa as the source of modern humans.
It is generally supposed that modern humans evolved in Africa from African Homo erectus. But Homo erectus had moved out of Africa and colonised much of the Old World long before this, and it is possible that the ancestral population that gave rise to Neanderthals, Denisovans, and modern humans actually lived in Eurasia (pp.68–71). 'In this scenario, there was later migration back from Eurasia to Africa, providing the primary founders of the population that later evolved into modern humans.' The advantage of this idea is that it requires one less major migration between Africa and Eurasia. At present it is speculative, but it would fit with the discovery of skeletons of 'Homo antecessor' at Atapuerco, in Spain, dated to about a million years ago.
Whatever explains these patterns, it is clear that we have much more to learn. The period before fifty thousand years ago was a busy time in Eurasia, with multiple human populations arriving from Africa beginning at least 1.8 million years ago.
Part II, or at least its first two chapters, was for me the most interesting part of the book. For Europe, a very important event was the westward spread into central Europe of the Yamnaya, people from the steppes of Central Asia, about five thousand years ago. The existing population at the time was mainly derived from farmers who had themselves arrived from the Near East, largely replacing the hunter–gatherers who preceded them.
The Yamnaya, themselves of mixed ancestry, are credited with the introduction of Indo-European languages into Europe, along with Corded Ware pottery. The idea that migration was responsible for these changes had been proposed in the 1920s but fell out of favour after the Second World War as a reaction to the abuse of archaeology by the Nazis. Reich is clear that the genetic evidence makes the idea inescapable.
Our analysis of DNA from the Yamana…showed that they harbored a combination of ancestries that did not previously exist in central Europe. The Yamnaya were the missing ingredient that needed to be added to European farmers and hunter–gatherers to produce populations with the mixture of ancestries observed in Europe today. Our ancient DNA data also allowed us to learn how the Yamnaya themselves had formed from earlier [Armenian and Iranian] populations.
The Yamnaya also spread east, into India, again bringing Indo-European languages as well as the religious ideas we find in the Rig Veda. The genetic evidence for this agrees with what many Western scholars have believed for a long time. The end result is that India contains a mixture, in varying proportions, of two highly divergent populations. But expressing this in scientific terms required careful handling when Reich collaborated with two Indian researchers. They objected on political grounds to the proposed term 'West Eurasians' and to the suggestion that immigration had brought outside ideas into India. They even suggested that there could have been an Indian migration in the opposite direction, to the Near East and Europe. Eventually the issue was fudged, with no reference being made to migrations.
I found the remaining chapters in Part II less satisfactory, probably because less work has been done on America, East Asia, and Africa, so what we get is a number of facts but not much of a coherent story to tie them together. But things are changing fast and if there is a subsequent edition of the book or a sequel, no doubt we will get a more comprehensive picture. In relation to Africa, Reich remarks that most researchers take little interest in what happened after the emigration of modern humans about 50 000 years ago (more recently than the 85,000 cited by Oppenheimer in 2003), yet there is a huge amount to be studied.
Research of this kind cannot be separated from social and political questions, as Reich found in India. It has also cropped up in the USA, with regard to archaeologists' alleged interference with the graves of ancient Native Americans. In Part III Reich discusses these questions with sensitivity and also considers the relevance of his research to 'race'. I found this section to be somewhat peripheral to the main part of the book.