GÖBEKLI TEPE, TURKEY—Dawn was still half an hour away when Klaus Schmidt arrived at his hilltop dig site here, but a half-dozen workers from a nearby village were already waiting for him. It was the end of the dig season, and the enthusiastic, white-haired German was there for one last look. He picked his way down steep excavation layers toward a massive, T-shaped slab of rock rising 3 meters out of the ground.