Rethinking Neanderthals: research suggests the so-called brutes fashioned tools, buried their dead, maybe cared for the sick and even conversed. But why, if they were so smart, did they disappear?

Bruno Maureille unlocks the gate in a chain-link fence, and we walk into the fossil bed past a pile of limestone rubble, the detritus of an earlier dig. We're 280 miles southwest of Paris, in rolling farm country dotted with long-haired cattle and etched by meandering streams. Maureille, an anthropologist at the University of Bordeaux, oversees the excavation of this storied site called Les Pradelles, where for three decades researchers have been uncovering, fleck by fleck, the remains of humanity's most notorious relatives, the Neanderthals.

We clamber 15 feet down a steep embankment into a swimming pool-size pit. Two hollows in the surrounding limestone indicate ...