Surprise Human-Ancestor Find—Key Fossils Hidden in Lab Rock

Posing with an A. sediba skull, student Justin Mukanku points to the tooth he found in a fossil-filled rock.

Single tooth tipped researchers off to a bonanza right under their noses.

Last month a prehistoric tooth protruding from a boulder tipped off researchers to hidden evolutionary treasure: remarkably complete human-ancestor fossils trapped in a rock that had been sitting in their lab for years.

Scans later showed that the rock contains two-million-year-old fossils that will "almost certainly" make one Australopithecus sediba specimen "the most complete early human ancestor skeleton ever discovered," anthropologist Lee Berger said in a statement Thursday.

(See pictures of Australopithecus sediba fossils.)

The bones are nearly invisible from the outside, and were discovered only after a technician noticed the small tooth in the three-foot-wide (meter-wide) rock, which was retrieved from a South African cave in 2008 and brought to a lab at the University of the Witwatersrand in Johannesburg.

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