S African ancestors got earlier

Australopithecus fossils from South Africa’s Sterkfontein Cave may be over one million years older than previously thought.

Sterkfontein in South Africa has yielded Australopithecus fossils since 1936 and is the most prolific source of such hominins. Most of them come from a heavily eroded cave 'breccia' known as Member 4 that is weathering at the modern ground surface. Only a few fossils come from subterranean contexts. Hardly surprisingly, dating these finds is thwart with challenges.

Until recently, the generally agreed dates for these fossils, such as ‘Mrs Ples’ pictured above, was between 2.6 and 2.1 million years ago. However, the latest research shows that the entire Australopithecus collection from Sterkfontein dates to c. 3.4 to 3.7 million years ago – potentially over one million years earlier than previously believed.

That age places these fossils toward the beginning of the Australopithecus era, rather than near the end. This means that the South African Sterkfontein australopithecines were early representatives of the genus, who overlapped with a diverse series of Middle Pliocene hominins, including Australopithecus afarensis in East Africa. This challenges the widely accepted concept that Australopithecus africanus, which is well represented at Sterkfontein, descended from A. Afarensis. They also predate robust australopithecines, early Homo, and the mysterious Australopithecus sediba. In short, the new dates provide an emerging portrait of a far more complex diversity of hominin evolution than was previously assumed.

In terms of the dating, scientists at Sterkfontein had previously used other animal fossils to estimate the hominins’ age, also calcite flowstone deposited in the cave. But bones can shift, and young flowstone can be deposited on old sediment making those methods potentially incorrect. A more accurate method would be to date the actual rocks in which the fossils were found. The current team therefore used accelerator mass spectrometry to measure radioactive nuclides in the rocks, as well as geologic mapping and an intimate understanding of how cave sediments accumulate to determine the age of the Australopithecus-bearing sediments at Sterkfontein.

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Image: The A. africanus known as ‘Mrs Ples’ (possibly a Mr) found in Sterkfontein and previously dated to 2.5 Ma. It is now dated to over a million years earlier.

Credit: Ditsong National Museum of Natural History