By sequencing and analyzing 15 ancient genomes found throughout the Americas—six of which were older than 10,000 years—these researchers determined that, around 8,000 years ago, the ancestors of Native Americans were still on the move, migrating away from Mesoamerica (what is today Mexico and Central America) toward both North and South America. These groups moved rapidly and unevenly, sometimes interbreeding with local populations, complicating the genetic—and historical—picture even further. The close genetic similarity observed between some of the groups studied suggests rapid migratory speed through North and South America. The Meltzer and Willerslev team, which included dozens of researchers from...