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Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past by David Reich, Pantheon, US $28.95, Pp 368, March 2018, ISBN 978-1101870327

The science of genetics or genomics is relatively a new scientific discipline but it is probably the fastest growing branch of science. The advances in genetics or genomics are bringing about a kind of scientific revolution. The studies in the ancient DNA are rapidly disrupting our assumptions about the past. Yet there is at present no book by a working geneticist that lays out the impact of the new science and explains how it can be used to establish compelling new facts. In Who We Are and How We Got HereDavid Reich does exactly that. Reich tells how the genome revolution is transforming the way we look at the history of human beings. Reich shows that the human genome contains the hidden history of humankind and ancient DNA carries the story of ancient people.

David Reich writes that Who We Are and How We Got Here is about the genome revolution in the study of the human past. This revolution consists of the avalanche of discoveries based on data taken from the whole genome – the entire genome analyzed at once instead of just small stretches of it such as mitochondrial DNA. The revolution has been made far more powerful by the new technologies for extracting whole genomes’ worth of DNA from ancient humans.

The ancient DNA revolution has been highly Eurocentric. Of 551 published samples with genome-wide ancient DNA data as of late 2017, almost 90 percent are from West Eurasia. Reich says that the focus on West Eurasia is a reflection of the fact that it is in Europe that most of the technology for ancient DNA analysis was developed, and it is in Europe that archaeologists have been studying their own backyards and collecting remains for the longest period of time. But the ancient DNA revolution is spreading, and has already produced several startling discoveries about human history outside of West Eurasia, most probably about the peopling of the Americas and of the remote Pacific islands.

As technical improvements have made it possible to get ancient DNA from warm and even tropical places, ancient DNA from central Asia, South Asia, East Asia, and Africa will reveal equally great surprises within the next decade. The product of this effort will be an ancient DNA atlas of humanity, sampled densely through time and space. David Reich says that this atlas will not answer every question about population history, but also provide a framework, a baseline to which we will always return when studying new archaeological sites.

David Reich says that ancient DNA studies with large numbers of samples also offer the promise of being able to estimate human population sizes at different times in the past, a topic about which we have almost no reliable information from the period earlier than the invention of writing, but which is important for understanding not just human history and evolution but also economics and ecology. In a population of many hundreds of millions (like the Chinese), a pair of randomly chosen people is expected to have few if any shared segments of DNA within the last forty generations because they descend from almost entirely different ancestors over this period. By contrast, in a small population (like the indigenous people of little Andaman Island who have a census of fewer than one hundred), all pairs of individuals are closely related and will show evidence of relatedness through many shared segments of DNA. Applying this approach to ancient DNA will provide rich insight into how populations changed in size over time.

Ancient DNA in principle has just as much insight to offer about how human biology has changed over time as it does about human migrations and mixtures. And yet while the power of ancient DNA to reveal population transformations has been a runaway success, so far the insights into human biology have been limited. David Reich says that a key reason is that to track human biology over time, it is important to be able to study how mutations frequencies change. Currently, we do not have enough data on this and it will change with the availability of ancient DNA samples.

Who We Are and How We Got Here is the first draft of our more than 100,000-year old history. It not only tells you about your ancestors 100,000 years ago but also how our planet was peopled over the millennia. Who We Are and How We Got Here is about the revolution genetics or genomics are bringing about in our understanding our origins. David Reich, Professor of Genetics at Harvard Medical School, tells the story of revolution brought by the study of ancient DNA. It is a book that will benefit experts and lay people alike.

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