Researchers say they've discovered a 'missing link' - a hominid 1.8 million years old that may be our oldest direct human ancestor yet found.
The fossils, of a species dubbed Australopithecus sediba, are the most complete early hominid fossils ever found. They were discovered in newly-exposed cave sediments at the Malapa Cave site in South Africa last year.
"The site is especially exciting because the A. sediba skeletons are nearly complete," says Emory University anthropologist Dietrich Stout.
"We can relate the face to the hand and the body and the brain of a single individual. A. sediba is represented by the most complete hominid skeletons we have, until we get up to the Neanderthals."
Stout examined the cranium of a young A. sediba male, estimated to be 12 to 13 years old at the time of death, with brain growth essentially complete. A virtual endocast gives a three-dimensional view of the interior, revealing bumps, ridges and even impressions from blood vessels.
The brain appears to have been around 420 cubic centimeters in volume, around the size of a grapefruit and similar to the size of a chimpanzee's.
The face, however, was far less protruded. "We don’t fully understand how the human face got smaller and tucked under the brain case, although that may have a lot to do with diet and chewing," Stout says.
Measurements from the brain of A. sediba were compared with humans, chimpanzees and other hominids.
While the A. sediba brain clearly wasn't a human configuration, a surface bump shows possible foreshadowing of Broca’s area, a region of the human brain associated with speech and language, says Stout.
"It’s a big leap, however, to go from a surface bump to really understanding what the cells were doing beneath it," he warns.
Use of simple stone tools by hominids began about 2.5 million years ago, and A. sediba's hands appear to be associated with tool use.
The age of the fossils has been established through uranium lead dating of the surrounding flowstone, combined with palaeomagnetic analysis. This has enabed the team to pin down the age of the fossils to within 3,000 years of 1.98 million years.
"Knowing the age of the fossils is critical to placing them in our family tree, and this new age means that Australopithecus sediba is the current best candidate for our most distant human ancestor," says Dr Robyn Pickering of the University of Melbourne's School of Earth Sciences.