David Reich, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past (2018)

This is a fascinating look at the new science of ancient DNA by one of the field's pioneers. In the 1980s the female mitochondrial and male Y chromosomes were first sequenced, followed in 2001 by the entire genome. 2010 was the breakthrough year for ancient DNA when Svante Paabo and colleagues at the Max Planck Institute in Leipzig first sequenced ancient Neanderthal DNA. Five years later, Reich's lab at Harvard built on Paabo's work to "industrialize" the sequencing of ancient DNA: by now thousands of ancient individuals' DNA going back more than 50,000 years and including "modern humans," Neanderthals, and species newly discovered by these techniques, including Denisovans and Homo Floriensis ("Hobbits"), have been analyzed. Interestingly, Reich says that the sequencing of the whole genome has not yet shed much light on the genetic sources of behavior. Given the fact that usually dozens of genes contribute to individual behaviors, Reich admits, it may never do so. However, whole genome sequencing, including of ancient individuals, has made a major, indeed revolutionary, contribution to the understanding of the deep human past. Reich thinks this revolution is so fundamental that is best grasped not by comparison with the dedoding of DNA by Crick and Watson in 1953; rather, it's more like the invention of the microscope in the seventeenth century: a whole new world is coming into view, much of it at odds with previously held theories. The mixing of modern humans and Neanderthals turns out to be much more complex than previously thought. Several "ghost populations" - no longer extant but the source of some contemporary populations - can be identified. A much more detailed, and radically new, picture of the peopling of Europe, India, the Americas, and East Asia has become possible. For example, ancient DNA has settled the debate about the origin of the earliest Indo-Europeans - they came from the Pontic steppe, not Anatolia. As they spread to the west, they merged with neolithic farming population, who in turn had merged with indigenous hunter-gatherers. The precise contribution of these three source-populations to contemporary Europeans can be measured. Something similar happened in India, where Reich's techniques have allowed him to identify the emergence of numerous endogamous populations - castes - starting around 2,000 years ago. Four distinct waves settled the Americas, one of which, however, remains hard to pin down. As Reich repeatedly acknowleged, ancient DNA is a rapidly developing field. Many of the results he included in his 2018 book will now have been modified, overturned, or superceded.