Why did humans take over the world while our closest relatives, the Neanderthals, became extinct? It's possible we were just smarter, but there's surprisingly little evidence that's true.

Neanderthals had big brains, language and sophisticated tools. They made art and jewelry. They were smart, suggesting a curious possibility. Maybe the crucial differences weren't at the individual level, but in our societies.

Two hundred and fifty thousand years ago, Europe and western Asia were Neanderthal lands. Homo sapiens inhabited southern Africa. Estimates vary but perhaps 100,000 years ago, modern humans migrated out of Africa.

Forty thousand years ago Neanderthals disappeared from Asia and Europe, replaced by humans. Their slow, inevitable replacement suggests humans had some advantage, but not what it was.