WHEN and how did the first people arrive in the Americas?

For many decades, archaeologists have agreed on an explanation known as the Clovis model. The theory holds that about 13,500 years ago, bands of big-game hunters in Asia followed their prey across an exposed ribbon of land linking Siberia and Alaska and found themselves on a vast, unexplored continent. The route back was later blocked by rising sea levels that swamped the land bridge. Those pioneers were the first Americans....

 In the past five years, however, a number of discoveries have posed major challenges to the Clovis model. Taken together, they are turning our understanding of American prehistory on its head.