New dates claimed for Australopithecus africanus


July 10, 2022

In the journal PNAS (Proceedings of the National Academy of Science) for June 27 of this year, Granger et al. add to their findings, first reported in the journal Nature in 2015. In 2015 they reported on Sterkfontein member 2. In this PNAS paper, they report on their findings with regard to member 4: Sterkfontein Member 4 bears a much earlier date than previously determined. Sterkfontein Member 4 has yielded a number of Australopithecus africanus fossils, retrieved from subterranean breccia sites, previously dated to 2.1- 2.6 ma; the new dates, derived from Cosmogenic nuclide dating, are considerably older at 3.4 ma. The significance of this paper is that Au. africanus is contemporaneous with Au. afarensis,  a truly startling development.

The abstract of the PNAS paper reads as follows: "Sterkfontein is the most prolific single source of Australopithecus fossils, the vast majority of which were recovered from Member 4, a cave breccia now exposed by erosion and weathering at the landscape surface. A few other Australopithecus fossils, including the StW 573 skeleton, come from subterranean deposits [T. C. Partridge et al., Science 300, 607–612 (2003); R. J. Clarke, K. Kuman, J. Hum. Evol. 134, 102634 (2019)]. Here, we report a cosmogenic nuclide isochron burial date of 3.41 ± 0.11 million years (My) within the lower middle part of Member 4, and simple burial dates of 3.49 ± 0.19 million years. My in the upper middle part of Member 4 and 3.61 ± 0.09 My in Jacovec Cavern. Together with a previously published isochron burial date of 3.67 ± 0.16 My for StW 573 [D. E. Granger et al., Nature 522, 85–88 (2015)], these results place nearly the entire Australopithecus assemblage at Sterkfontein in the mid-Pliocene, contemporaneous with Australopithecus afarensis in East Africa. Our ages for the fossil-bearing breccia in Member 4 are considerably older than the previous ages of ca. 2.1 to 2.6. My interpreted from flowstones associated with the same deposit. We show that these previously dated flowstones are stratigraphically intrusive within Member 4 and that they therefore underestimate the true age of the fossils."

Read the full PNAS paper.