The excavation of a Stone Age community center in Ukraine is helping explain why large groups of tens of thousands of people flourished and then fell more than 5,000 years ago. The "megastructure" excavated in Ukraine was large compared with the houses around it, though not particularly huge by modern standards. At 2,045 square feet (190 square meters), the structure was the size of a modest American home. However, some Eastern European megastructures were up to 18,000 square feet (1,680 square m) in size. Archaeologists have puzzled over these buildings, many of which have been discovered through methods that use...