"The technique takes samples from the layers, extracts uranium and lead from them, and measures the proportion of the daughter lead isotope compared to the remaining uranium," Cliff said.
"The older the sample, the more daughter product there will be and the less uranium."
Uranium decays very slowly, however, through a series of intermediate stages. This makes it difficult to use the technique on relatively young rocks such as those in Sterkfontein.
Further complicating matters, the rocks in the cave contain unusual background levels of another element that also decays into lead.
The scientists took these background traces into account to arrive at the new date for the fossil. They report their findings in the current issue of the journal Science.
Paul Renne of the University of California, Berkeley's Geochronology Center, says the new study appears to have "convincingly resolved" a longstanding issue, opening the door for similar work on other South African fossils.
"It is very exciting," he said by email.
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