JOHANNESBURG—At a tea party earlier today for a research team at the University of the Witwatersrand that has grown accustomed to making stunning discoveries of human fossils, a curious excitement erupted when Kristian Carlson unveiled a seemingly modest find: a rib bone from Australopithecus sediba. In fact, it wasn't even an actual fossil—just a resin replica. But despite its humble appearance , the little white rib represents a major scientific advance.

When scientists found the nearly two million-year-old fossil rib, which hails from a site in South Africa called Malapa, they could only see the tip of it. The rest was encased in a rocky matrix called breccia. And it was pressing against another bone--a shoulder blade. Extracting the rib from the breccia would damage or destroy the delicate shoulder blade. So the team imaged the block containing the bones using a micro-CT scanner, which yielded very high-resolution 3D images of the bones. Carlson virtually removed the surrounding breccia and separated the rib from the scapula on his computer. He then created a 3D computer aided design of the bone and printed it out using a 3D prototyper. As a result of this virtual preparation of the rib, says team leader Lee Berger, “you have a fossil in your hand that would a few years ago would have been unrecoverable.”

According to Berger, this specimen from A. sediba is the first complete first rib bone in the fossil record of early humans. As such, it stands to help answer an enduring question about human evolution: namely, whether the thoraxes of early humans were cone-shaped like those of apes or barrel-shaped like ours. The shape of the thorax is important because it can reveal lung capacity and cast light on how a creature got around.

Going forward micro-CT imaging will play a crucial role in the discovery, recovery and analysis of additional fossil remains of A. sediba. The team has collected from the site numerous breccia blocks whose contents are currently unknown. Rather than blindly looking for bones by painstakingly removing the breccia bit by bit, they will be able to scan the blocks to see if they contain fossils and then use the images to guide the preparation of the fossils. One such fossil they will be looking for is the skull that goes with the partial skeleton of the adult female they found. But if and when they find it with the scanner, they may not even bother to remove the breccia at all—opting instead to do what they did with the rib and print a replica of the fossil that they can then analyze without ever freeing the actual skull from its rocky tomb.