Among all the forms of early humans, the Neandertals hold a special place in our imaginations. Thriving through the Ice Age rigors of Europe and western Asia for 150,000 years, they combined enormous physical strength with manifest intelligence. They could not lose.

And then, somehow, they lost. The Neandertals disappeared some 35,000 years ago, just as a new kind of human made its gaudy entrance on the continent: Homo sapiens sapiens, the "double wise" species that left its handprints on the walls of caves and the mark of its mind everywhere on the globe. How did it happen? What part did the Neandertals play? Who were they, and what was their fate?

In recent years, revolutionary developments in fossil dating and the spectacular entrance of genetic research into the origins debate have sent the anthropological establishment into an uproar. The old, comfortable explanations for how and where our species evolved have been utterly destroyed. Left behind is a tangle of new mysteries, not just in Europe but all over the Old World. The key to unraveling them lies with the Neandertals.

A fascination with this vanished race led the distinguished science writer James Shreeve on a journey through Europe, the Middle East, and Africa, searching for insights and evidence. Along the way he began to suspect that the Neandertal enigma could be understood only by a marvelous paradox.

Threading his way through the violently polarized debates surrounding the fate of the Neandertals, Shreeve offers a fascinating theory for what might have allowed two equally human species to share the same landscape at the same moment of evolutionary time, and what led, ultimately, to the triumph of one and the poignant disappearance of the other.