Most researchers agree that the earliest Americans came over from Asia via the Bering Strait between Siberia and Alaska, beginning at least 15,000 years ago. But many have long puzzled over findings that some of the earliest known skeletons—with long skulls and prominent foreheads—do not resemble today's Native Americans, who tend to have rounder skulls and flatter faces. Some have even suggested that at least two migrations into the Americas were involved, one earlier and one later. But the discovery of a nearly 13,000-year-old teenage girl in an underwater cave in Mexico's Yucatán Peninsula argues against that hypothesis. The girl had the skull features of older skeletons, but the genetic profile of some of today's Native Americans—suggesting that the anatomical differences were the result of evolutionary changes after the first Americans left Asia, rather than evidence of separate ancestry.