A study published in Nature yesterday discusses new evidence that human migrations into the Americas occurred in at least three waves. The study, led by David Reich of Harvard Medical School, sampled DNA from 52 groups of native populations from the Arctic down to South America. Anthropologists generally think that the first human movements into North America occurred around 15,000 years ago, at the height of the last ice age, when groups crossed a now-submerged land bridge linking Siberia to Alaska. This initial group continued along the American coastlines as the generations progressed, with smaller groups splitting off along the way. Meanwhile,  by examining DNA sequences in the native populations, the researchers have uncovered evidence that this initial movement was followed by at least two more major waves which contributed major influxes of new genetic information to some of the groups which had previously migrated to the Americas. Read the press release from University College London here. The paper about the study is available here.