One afternoon in 1947, when German archaeologist Helmut de Terra made an unusual find. The remains of a mammoth had recently been discovered in the nearby town of Santa Isabel, but with stone tools that seemed to have been made by human hands. In a Tepexpan field nearby, Terra found the remains of a human, face down. It was thought to be as much as 10,000 years old, and became known as oldest human remains on the entire continent. (Now many sites show humans to have arrived in the Western Hemisphere thousands…