New dating questions previous ideas about early hominins

The Sterkfontein cave 40 km northwest of Johannesburg in South Africa first sprang to the attention of scientists in 1936, with the discovery there of an adult hominin skull. This showed clear affinities with the discovery 400 km to the SW in 1924 of the fossil skull of a juvenile primate, which Raymond Dart claimed to be ancestral to modern humans, naming it Australopithecus africanus. Sterkfontein has since yielded more than 500 hominin fossils, many of which are Au. africanus.

Limestone cave deposits are difficult to date precisely, unlike sediments that are interbedded with volcanic rocks, the most amenable material being that deposited by water flowing through the cave to form flowstone or speleothem. Using the U-Pb method of radiometric dating yielded an age of between 2.1 to 2.6 Ma for flowstone that cements the breccia in which the Au. africanus fossils occur. Clearly, the flowstone formed after burial so that was a minimum age for them, awaiting the use of a different chronological tool to suggest when burial of the bones took place

The face of an Australopithecus africanus: ‘Mrs Ples’. (Credit University of Zurich)

An almost complete skeleton of another australopithecine found in another part of the Sterkfontein cave system was dated in 2015 by a different approach. This used the decay of 10Be and 26Al isotopes that high-energy cosmic rays produce in quartz grains while they are exposed at the surface. Burial of irradiated sedimentary grains protects them from such bombardment, and the two isotopes  then steadily decay at a known rate. Quartz grains associated with this specimen (fondly known as ‘Little Foot’) turned out to be far older than the flowstone U-Pb age, with a cosmogenic burial age of about 3.7 Ma. Its much greater antiquity prompted scientists to regard ‘Little Foot’ as a different species – Au. prometheus – despite being similar to Au. africanus.

Since that success, much the same team from South Africa, the US and France has been working on sedimentary grains buried with the abundant Au. africanus specimens from Sterkfontein (Granger D.E. et al. 2022. Cosmogenic nuclide dating of Australopithecus at Sterkfontein, South AfricaProceedings of the National Academy of Sciences, v. 119, article e2123516119; DOI: 10.1073/pnas.2123516119). Their newly published efforts show that “Little Foot’s” burial took place between 3.41 and 3.49 Ma, more than a million years earlier than suggested by the flowstone U-Pb dating and just ~200 ka younger than the ‘Little Foot’ skeleton. More surprising is that Au. africanus lived during the same period (3.4 to 3.7 Ma) as did Au. afarensis – the species to which ‘Lucy’ belonged – 3500 km to the north in Ethiopia.

So it is no longer justifiable  to suggest that the first known human species (Homo habilis ~2.3 to 1.65 M) is either a more ‘advanced’ australopithecine or a direct descendant from that genus, for the new dating opens a million-year gap in the history of human evolution. That age range does contain stone tools but no plausible candidates for an australopithecine-human evolutionary connection. One of the most recently suggested link is Au. sediba (see: Another candidate for earliest, direct human ancestor, October 2011; and Australopithecus sediba: is she or is she not a human ancestor? April 2013). The snag with that candidate is that the well-established age (2.0 Ma) of known specimens falls in the middle of the range for H. habilis. The two may have been cohabiters of Africa but are very different.

The million years that separated Au. africanus together with afarensis from H. habilis is the period when the defining character of humans, tool making, evolved. So the hunt is on for hominins associated with stone tools in that huge stratigraphic gap. One of the drawbacks with famous sites, such as the ‘Cradle of Humankind’ that includes Sterkfontein, is that they almost become clichés so that scientists return to them again and again, while the key that they seek may well lie elsewhere.