What should you expect when you read this book? Don’t expect an oversimplified story. Population genetics is a complicated, fast-moving field with many uncertainties of interpretation. To tell that story to the broad public, and not just to scientists reading specialty journals, is a big challenge. Reich explains these complications as well as any geneticist could; others rarely even try. Some of Reich’s conclusions will surely be superseded within a few years. But few subjects fascinate us as much as human origins. Humans have been on the move not just since Columbus’s voyage of 1492, but throughout our long history. The ancestors of modern Japanese, Indians and Native Americans didn’t become fixed in their modern locations in ancient times and simply stop moving. If you want to understand our origins over the course of the last 100,000 years, this book will be the best up-to-date account for you.

Who We Are and How We Got Here chronicles Mr. Reich’s work in five regions of the world. The stories are varied ... Understandably, a few details in Mr. Reich’s book are already out of date. In such a novel area of science, individual research results are unlikely to persist for long. What will endure are the larger themes: People in many parts of the world today have little DNA from the first peoples who lived in the same region. Most living groups are mixtures of ancient groups, which used to be much more genetically different from one another. Modern human ancestry is checkered with the traces of archaic ghosts ... for the great human story to matter to us, we must each see our own place in it. The dead cannot speak, but science can help us to see their humanity nonetheless. Doing so helps us maintain the humanity of our science.

Reich’s work can finally answer the tantalising question first posed by an British civil servant, Sir William Jones. In 1786, he discovered the kinship of Sanskrit and ancient Greek. This led to the recognition of the vast Indo-European language family – which includes the Germanic, Celtic, Italic, near eastern (Iranian) and north Indian languages (Hindi, Urdu, Bengali, Punjabi, Marathi, etc) – but not to any consensus on how this might have occurred. Reich has now shown that the Indo-European languages and the largest single component of the genetic makeup of Europe and north India today stem from migrations around 5,000 years ago from the vast Steppe, the grass plains bordering the Black and Caspian seas ... 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.

Reality, it turns out, is more complex and interesting than scientists ever imagined. But Who We Are and How We Got Here only begins by exploring our relationship to ancient lineages of humans gone by. The meat of the book deals with the human populations that are present today and how they — we — got here ... Who We Are and How We Got Here is less than 300 pages of text, but it is packed with startling facts and novel revelations that overturn the conventional expectations of both science and common sense. And after chapter upon chapter of dense scientific results, Reich concludes by making some observations about the social history of our species that is implied by ancient DNA ... Who we are is humanity. And how we got here is interesting, astounding, but at the end of the day incidental. We are here, and have been for a very long time.

People love to read about human origins, so many recent findings of ancient DNA research have been reported extensively in the media. But David Reich, professor of genetics at Harvard, is the first leading practitioner to pull everything together into a popular book. Who We Are and How We Got Here provides a marvellous synthesis of the field: the technology for purifying and decoding DNA from old bones; what the findings tell us about the origins and movements of people on every inhabited continent; and the ethical and political implications of the research. The overall conclusion is that there has been far more mobility and mixing of populations around the world, through migration and interbreeding, than palaeontologists had ever imagined.

Surprise follows surprise in the new science, as it does in the pages of Reich’s book. Sometimes the surprise is in discovering just how much we don’t know ... Reich’s book isn’t just a collection of stories about the histories of human populations. It is a fascinating case study of scientific revolution: the role of colleagues, of conferences, chance meetings, discoveries, technological innovations; what we do not yet sufficiently understand (notably, the rate of mutation); what new methods are required, and so on. Reich also has interesting things to say about the way his discipline has over the years been caught up in politics ... The big message of Reich’s book is that through our evolution and, even more important, our history, populations have continually mixed with each other. Tiny pockets of persistent endogamy apart, we are blends of past populations, which were themselves blends of those who went before.

This is a compendious book and insofar as it has a theme it is captured by that word 'admixture'. All humans, in spite of countless local mythologies, are a hopeless genetic stew. DNA tells us that no purist claim of special heredity, ancient or modern, makes any sense whatsoever ... The book is, frankly, a bit of a slog. Reich recounts his work but does not really elucidate or dramatise it effectively, so this feels like a semi-academic text. It is also unclear in its organisation — I thought I’d reached the last page on at last half a dozen occasions. But its importance cannot be overstated and neither can some of its best stories.

Throughout the book, Reich includes numerous timelines, graphs, maps, and diagrams to assist readers in visualizing his material, but those who are not scientifically inclined may find the narrative difficult to follow—though ultimately rewarding. Not an easy read, but an eye-opening account of significant scientific advances that throw a spectacular, often unexpected light on human prehistory.

The three sections of researcher Reich’s summary report on genomic analysis of ancient DNA lay out how the gleanings of such research reveal the variety and the dispersal of prehistoric humans throughout the world, how ancient DNA discloses humanity’s development in different parts of the world, and the implications of ancient DNA research for the future, especially for dispelling race-based conceptions of differences among modern humanity. Though probably not the easiest reading of the year, Who We Are and How We Got Here may be the most rewarding for those enthralled by humanity’s long prehistory.