WOLLONGONG, AUSTRALIA—When University of Pennsylvania archaeologist Harold Dibble was reopening excavations at the Grotte des Contrebandiers (Smuggler’s Cave) in Morocco a few years ago, he looked for experts to help him figure out when prehistoric humans had occupied the cave, which is a key site for understanding the spread of Homo sapiens. Dibble knew our species had been there more than 50,000 years ago—beyond the practical limit of radiocarbon dating. So he recruited two dating aces from the University of Wollongong in Australia, Zenobia Jacobs and Richard Roberts, experts in the technique of optically stimulated luminescence (OSL) dating. That method determines how long buried sand grains have been hidden from sunlight and can peer back 200,000 or more years in time.
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