770,000 Tubes of Spit Help Map America's Great Migrations

America is not the great melting pot that poets like Ralph Waldo Emerson once extolled. At least, that's not the story that DNA tells, according to the genealogy company Ancestry. Using more than 770,000 spit samples taken from their customers over the last five years, its researchers mapped how people moved and married in post-colonial America. And their choices--especially the ones that kept communities apart--shaped today's modern genetic landscape. The study, published today in Nature Communications, combines a DNA database with family tree information collected over the company's 34-year history. "We're all living under the assumption that we are individual agents," says Catherine Ball, chief scientific officer at Ancestry and the leader of the study.