|28 July 2011
Part ape, part human
The eroded limestone cave called Malapa - about 40km (25 miles) northwest of Johannesburg - is in a region famous for its ancient human fossils. Paleo-anthropologist Lee Berger believes Malapa may hold the key to the origin of the first species enough like us to be called human - Australopithecus sediba.
The evidence includes an australopith's little brain (with some curiously modern features), apelike shoulders and arms adapted to climbing in trees, attached to a bizarrely modern hand with the precision grip of a toolmaker. According to the researchers, a adult female's foot presents an even odder melange; her mostly modern ankle is connected to a heel bone more primitive than that of A. afarensis - Lucy's species - which is at least a million years older.
Two million years ago, a cave-studded aquifer lay beneath an undulating plain of shallow, wooded valleys and rolling hills. Some of the caves were open to the surface through steep entryways or vertical shafts stretching up to 50 metres (160 feet).
In wet periods, animals could easily drink from seepage ponds near the surface. During drier times they would risk a plunge down a hidden shaft. After death, their bodies would rapidly become entombed in a single, thick layer of sand and clay - preserving the skeletons, right down to tiny bones of the hands and feet.
The brain of A. sediba is a chimp-like 420 cubic centimetres - not at all unusual for something called Australopithecus. The shape, however, is. "The frontal lobes... appear to be different sizes," notes paleo-anthropologist Kristian Carlson. Pronounced asymmetry between right and left brain hemispheres is a hallmark of humans, because our cerebrum has become specialised, with the left side more involved in language.
Anatomically, the species shows a mix of primitive and advanced traits. The long legs and modern ankle are key elements on the human side, as is the surprisingly human-like pelvis built for a fully bipedal stride, smaller teeth and chewing muscles, a projecting nose and some other features of the face, and that remarkable precision-grip hand. These traits are enough for the team to propose it as the species most likely to have given rise to Homo.
Edited from National Geographic (August 2011)
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