New cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and Member 5 Oldowan

The cave infills at Sterkfontein contain one of the richest assemblages of Australopithecus fossils in the world, including the nearly complete skeleton StW 573 ('Little Foot')(1-4) in its lower section, as well as early stone tools(5-7) in higher sections. However, the chronology of the site remains controversial(8-14) owing to the complex history of cave infilling. Much of the existing chronology based on uranium-lead dating(10,11) and palaeomagnetic stratigraphy(8,12) has recently been called into question by the recognition that dated flowstones fill cavities formed within previously cemented breccias and therefore do not form a stratigraphic sequence(4,14). Earlier dating with cosmogenic nuclides(9) suffered a high degree of uncertainty and has been questioned on grounds of sediment reworking(10,11,13). Here we use isochron burial dating with cosmogenic aluminium-26 and beryllium-10 to show that the breccia containing StW 573 did not undergo significant reworking, and that it was deposited 3.67 +/- 0.16 million years ago, far earlier than the 2.2 million year flowstones found within it(10,11). The skeleton is thus coeval with early Australopithecus afar-ensis in eastern Africa(15,16). We also date the earliest stone tools at Sterkfontein to 2.18 +/- 0.21 million years ago, placing them in the Oldowan at a time similar to that found elsewhere in South Africa at Swartkans(17) and Wonderwerk(18).