The broad arc of islands north of Australia, extending from Indonesia east towards the central Pacific, is home to a set of human populations whose diversity is unsurpassed elsewhere. Approximately 20% of the world's languages are spoken here, and the biological and genetic heterogeneity among the groups is also extraordinary. This book describes the origins of the genetic and linguistic variation there. It lays out the very complex structure of the variation within and among the islands in this relatively small but important region. This book applies genetic analyses to an intensively sampled set of populations, and subjects these and complementary linguistic data to a variety of phylogenetic analyses. This reveals a number of heretofore unknown ancient Pleistocene genetic variants that are only found in these island populations, and identifies the genetic footprints of more recent migrants from Southeast Asia who were the ancestors of the Polynesians. Finally, a number of explanatory models are tested to see which best account for the observed pattern of genetic variation. The results indicate that a number of commonly used models of evolutionary divergence and biogeography are overly simple in their assumptions, and that human diversity often has accumulated in very complex ways.