Before the glory that was Greece and Rome, even before the first cities of Mesopotamia or temples along the Nile, there lived in the Lower Danube Valley and the Balkan foothills people who were ahead of their time in art, technology and long-distance trade. For 1,500 years, starting earlier than 5000 B.C., they farmed and built sizable towns, a few with as many as 2,000 dwellings. They mastered large-scale copper smelting, the new technology of the age. Their graves held an impressive array of exquisite headdresses and necklaces and, in one cemetery, the earliest major assemblage of gold artifacts to...