Nonpareil science writer David Quammen explains how recent discoveries in molecular biology can change our understanding of evolution and life's history, with powerful implications for human health and even our own human nature.In the mid-1970s, scientists began using DNA sequences to reexamine the history of all life. Perhaps the most startling discovery to come out of this new field—the study of life's diversity and relatedness at the molecular level—is horizontal gene transfer (HGT), or the movement of genes across species lines. It turns out that HGT has been widespread and important. For instance, we now know that roughly eight percent of the human genome arrived not through traditional inheritance from directly ancestral forms, but sideways by viral infection—a type of HGT.InTheTangled TreeDavid Quammen, "one of that rare breed of science journalists who blends exploration with a talent for synthesis and storytelling"...