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.

Renfrew--Lord Renfrew of ­Kaimsthorn since he was made a British life peer in 1991 to honor his many contributions to archaeology--was then a graduate student at Cambridge. As an undergradu­ate, he'd first studied natural sciences before moving on to archaeology; thus, seeking a means to determine the provenance of the obsidian that prehistoric ­peoples favored for toolmaking, he tried the novel tactic of using optical emission spectroscopy to analyze its trace elements.