DAVID POGUE (New York Times Technology Columnist): Around 30,000 years ago, there were at least four different kinds of hominids coexisting on the planet, including the Neanderthals. More than a million years ago, some of our ancestors began making their way from Africa to Europe, while the rest stayed behind. Those who remained in Africa evolved into us, modern humans. The group that went to Europe eventually developed into Neanderthals.

You know the Neanderthals, otherwise known as "cavemen." Most people think of them as being a bunch of brutes. Well, it turns out, that the more we learn about them, the closer they seem to us.

Their bones were first identified in Germany, in the Neander Valley, thus, Neanderthals. Since that discovery, about a hundred and fifty years ago, scores of Neanderthal sites have been uncovered, from the British Isles to Western Asia. We now know that Neanderthals used fire, wore clothing and made stone tools.

They thrived as skilled hunter/gatherers for at least 200,000 years. But then, thirty thousand years ago, they vanished. Why didn't they survive? And why aren't they living among us today?

Daniel Lieberman, of Harvard, looks for answers in the way human heads have evolved.

Why the head?

DANIEL LIEBERMAN (Harvard University): Well, heads are one of the most important things about our bodies, and they're actually what makes us special, for the most part. If you were to meet a Neanderthal on, say, the subway, from the neck down you'd be basically the same. And what really makes you different from a Neanderthal is above the neck.

DAVID POGUE: It turns out our skulls and Neanderthal skulls are pretty different. All right, so this is a mockup of a modern human?

DANIEL LIEBERMAN: Right.

DAVID POGUE: "Hi, Dan".

And this is our Neanderthal guy. If we wanted to establish the differences between the two, specifically what would you point out?

DANIEL LIEBERMAN: There are really two basic essential differences that matter. And the first is that a modern human head is much more globular. It's much more spherical, more like a soccer ball, right? It's rounded. Right? But the Neanderthal head, if you can see here, is much longer and lower. It's more like a lemon or an American football, right?

DAVID POGUE: If our skulls are the main difference between modern humans and Neanderthals, perhaps that's the key to why Neanderthals died out and we survived.