Reich begins in Part I with a bit of background genetics. This, and other technical details presented throughout the book, is not terribly involved — and readers who find it too much can fairly easily skip to his conclusions without getting stuck in the methods.
Two chapters consider the "deep history" of our species, looking at interbreeding between Neanderthals, Siberian Denisovans and Australo-Denisovans. A possible lineage of "super-archaic" humans split off 1.4 to 0.9 million years ago and later interbred with Denisovans. And Eurasia is a hothouse of human evolution, suggesting the possibility of an "out of Eurasia" event preceding the "out of Africa" one.
Part II begins with now vanished populations of modern humans, such as "Ancient North Eurasians" and "Basal Eurasians", and the traces of five population events in West Eurasia.
European populations derive from ancient hunter-gatherers, farmers from the Near East, additional hunter-gatherer contributions, and pastoralists from the steppes. With complications. (Reich argues for a steppe origin for the major Indo-European migrations, but a South Caucasian origin for Indo-European languages. He also considers the confusion of ideological appeals to "purity" in origins.)
India has a north-south genetic cline, roughly matching the Indo European - Dravidian language divide, resulting from the mixing of genetically distinct "Ancestral North Indian" and "Ancestral South Indian" populations. And there is genetic evidence for the antiquity of endogenous castes: unlike China, India is "composed of a large number of small populations", not one large population.
There were at least four prehistoric migrations to the Americas: a population that split from the closest Eurasians about 21,000 years ago and settled most of the Americas, probably following both coastal and inland routes; a mysterious Population Y whose signs are found in Amazonia; and later Paleo- and Neo-Eskimo migrations out of Asia around 5000 and 1000 years ago.
In East Asia, there is evidence for "ghost" populations in the Yangtze and Yellow river valleys, with admixture as those spread. And Reich finds "not one, not two, but at least three major migrations into the open Pacific, with the first bringing East Asian ancestry and the Lapita pottery culture, and the later migrations bringing at least two different types of Papuan ancestry".
There is evidence for an archaic mixture of African populations: "perhaps all present-day humans are a mixture of two highly divergent ancestral groups". Four modern expansions, associated with agricultural innovations, have left their traces in language families: Bantu from West Africa; Nilo-Saharan accompanying cattle-herding; Afroasiatic mostly likely into northeastern Africa from the Near East; and Khoe-Kwadi in southern Africa. And we can determine something of the pre-Bantu population structure of southern and eastern Africa.
There are no surprises in any of this, at least to anyone who has been following the popular stories and scientific studies appearing in recent years. But Reich provides an accessible summary, bringing all of that together.
In a few places some of this is politically controversial, notably in India, where some Hindu nationalists reject any suggestion that present Indians might trace their descent to anywhere else, and North America, where consent for DNA sequencing is fraught, and often associated with requests for the return of remains. In Part III Reich considers some of the broader, potentially "disruptive" aspects of paleogenomics.
It may be possible to determine something of ancient social structure from genetics. West Eurasian populations, for example, show a Y chromosome bottleneck around 5000 years ago, putatively derived from the social dominance of elites among the Yamnaya (possibly supporting some of Gimbutas' ideas). And Reich suggests that, while genomics doesn't support anything like popular or traditional conceptions of race, we need to be prepared to face the reality of genetic variation between populations. (Here I suspect most complex human traits being polygenic, and the extent of gene-environment interactions, will make this less of a problem than he fears.)
Jean Manco's Ancestral Journeys restricts itself to Europe, but is in other ways broader in scope, drawing on the findings from genetics but delving more into archaeology and linguistics, and covering more recent history. The geographical restriction also allows more detail: there are separate chapters on Indo-European linguistics, the Germanic "Great Wandering", the Slavs, the Magyars, and the Vikings.
Our understanding of human genetic history has progressed a long way in the quarter century since Cavalli-Sforza's classic studies, and paleogenomics is producing new findings at a rapid rate. Who We Are and How We Got Here is so cutting-edge that much of its detail will date quite quickly, if indeed it hasn't already. And parts of Ancestral Journeys could be updated following the work Reich describes. But the broad conclusions of both books are likely to remain untouched.