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. "But people actually are living in the course of history." And from the moment they spit, send, and consent, DNA kit customers become actors in a much larger story—told through the massive data sets companies like Ancestry are accumulating from casual genealogists.

Ball's team of geneticists and statisticians started by pulling out subsets of closely related people from their 770,000 spit samples. In that analysis, each person appears as a dot, while their genetic relationships to everyone else in the database are sticks. The result, Ball says, "looks like a giant hairball."

From that hairball her team pulled out more than 60 unique genetic communities—Germans in Iowa and Mennonites in Kansas and Irish Catholics on the Eastern seaboard. Then they mined their way through generations of family trees (also provided by their customers) to build a migratory map. Finally, they paired up with a Harvard historian to understand why communities moved and dispersed the ways they did. Religion and race were powerful deterrents to gene flow. But nothing, it turned out, was stronger than the Mason Dixon line.