South African scientists say they’ve discovered the most complete skeleton ever of an early human ancestor – in a rock that had lain unnoticed in a lab for years.

It was only when a technician noticed a tooth protuding from the stone last month that an investigation revealed significant parts of Australopithecus sediba, discovered in 2009 at the Malapa site in South Africa, and identified as a new hominin in 2010.

The new additions to the original find make the skeleton – dubbed Karabo – astonishingly complete. Once the remains, encased in a rock about one metre across, were noticed, they were examined with a CT scanner.

“We have discovered parts of a jaw and critical aspects of the body including what appear to be a complete femur (thigh bone), ribs, vertebrae and other important limb elements, some never before seen in such completeness in the human fossil record,” says Professor Lee Berger of the University of the Witwatersrand in Johannesburg.

“This discovery will almost certainly make Karabo the most complete early human ancestor skeleton ever discovered. We are obviously quite excited as it appears that we now have some of the most critical and complete remains of the skeleton, albeit encased in solid rock. It’s a big day for us as a team and for our field as a whole.”

The university now plans to build a lab at the site where the public can observe archaeology in action, and is in talks with the Shanghai Science and Technology Museum, the Natural History Museum in the United Kingdom and the Smithsonian in Washington over setting up specialist viewing facilities.

“We have already donated casts of Australopithecus sediba to these three institutions, amongst others,” says Berger.