About 6,000 years ago, at the start of the Archaic Age, humans first settled in the islands of the Caribbean. Three thousand to four thousand years later, stone tools gave way to clay pottery and the Ceramic Age began. Another two millennia passed before Europeans sailed across the Atlantic and made first contact. Those who study and those who live in the region have long wondered: Where did these stone tool-using and clay-crafting populations come from? Were they related to each other? How many people lived in the Caribbean when the Spanish first arrived? How much, if any, ancestry can today...