The site of Tlacuachero in southern Mexico is an island in a mangrove swamp made up almost entirely of clamshells. Material recovered from the site shows that it was a place where people harvested shellfish and fish between 5,050 and 4,230 years ago -- long before the great civilizations of Mesoamerica would build their city-states. Over the years, the island grew as clams were harvested from the swamp and the shells were discarded there. While the shell mound was accumulating, the early people at Tlacuachero built several superimposed clay floors at the island center to create smooth surfaces that were...