Surprise human-ancestor find

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.

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.

The tooth turned out to be just the tip of the fossil iceberg, National Geographic reports.

A CT scan revealed other bones inside, including parts of a jaw, a complete thigh bone, ribs, vertebrae, and possibly hand and foot bones.

"What we can see inside of the rock using state-of-the-art scanners," Berger said, "are some of the most complete bones that we've ever discovered in early hominins"-human species and their close evolutionary ancestors.

Scientists think the fossils are the missing remains of "Karabo," a partial A. sediba skeleton belonging to a male child that Berger's team had previously unearthed.

"What we found inside of this enormous rock is what appears to be the remainder of either the skeleton of Karabo or another skeleton entirely," Berger, of the University of the Witwatersrand in Johannesburg, told National Geographic News.

"The More Bones, the Better"

A. sediba is one of the newest additions to the list of early human ancestors and also one of the most enigmatic hominins ever discovered.

Studies have shown that the species possessed an unusual mix of primitive and or humanlike traits. For example, scientists think A. sediba could stand and walk upright but that it still spent some time in trees.

Additionally, the creature had a relatively large brain and surprisingly agile hands, suggesting the species may have been capable of crafting and using stone tools.

But there are still a lot of lingering questions about A. sediba and early hominins in general-questions that paleontologist Scott Simpson thinks the new bones could help resolve.

"One of the things we're still working out is the shape of [A. sediba's] rib cage. It's unclear how the shape of the rib cage evolved in hominins, and also what the relationship of the rib cage shape is to upper limb function," said Simpson, of Case Western Reserve University in Ohio, who's not involved in the project.

The vertebrae are also valuable finds, Simpson added, because they're relatively fragile and tend to degrade quickly or get eaten before the bones can fossilize.

"It would be nice to get these rare bones," he said. "The more bones, the better."

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