Since 1977, archaeologist Tom Dillehay has been unearthing conclusive evidence of human habitation in the Americas at least 15,000 to 20,000 years ago, settling a bitter debate and demolishing the standard scientific account of the settlement of the Americas. The question of how people first came to the Americas is now thrown wide open: the best guess is that they arrived from a variety of places, at many different times and by many different routes. Dillehay describes who the earliest settlers are likely to have been, where they may have landed, how they dispersed across two continents, what...