Mystery man: Cro-Magnons emerged about 40,000 years ago and were very similar to us. So why did it take tens of thousands of years for civilization to take hold? Credit: Bettmann/Corbis

How did we become the thinking animals that we are? That's the question at the heart of the study of human prehistory--and the one that Colin Renfrew has been asking since the summer of 1962, when he travelled to Milos, one of the Cycladic Islands in the Aegean Sea, a source of the black obsidian that was the earliest commodity traded by humans.

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