Today the Upper Thames Valley is a region of green pastures and well-managed farmland, interspersed with pretty villages and intersected by a meandering river. The discovery in 1989 of a mammoth tusk in river gravels at Stanton Harcourt, Oxfordshire, revealed the very different ancient past of this landscape. Here, some 200,000 years ago, mammoths, straight-tusked elephants, lions, and other animals roamed across grasslands with scattered trees, occasionally disturbed by small bands of Neanderthals. The pit where the tusk was discovered, destined to become a waste disposal site,...