New cosmogenic burial ages for Sterkfontein Member 2 Australopithecusand Member 5 Oldowan

Darryl E. Granger
Ryan J. Gibbon
Kathleen Kumanm

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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’)1,2,3,4 in its lower section, as well as early stone tools5,6,7 in higher sections. However, the chronology of the site remains controversial8,9,10,11,12,13,14 owing to the complex history of cave infilling. Much of the existing chronology based on uranium–lead dating10,11 and palaeomagnetic stratigraphy8,12 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 sequence4,14. Earlier dating with cosmogenic nuclides9 suffered a high degree of uncertainty and has been questioned on grounds of sediment reworking10,11,13. 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 it10,11. The skeleton is thus coeval with early Australopithecus afarensis in eastern Africa15,16. 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 Swartkans17 and Wonderwerk18.