Researchers from South Africa, the US, Canada and France today announced new dates pertaining to the internationally famous Sterkfontein Caves in the Cradle of Humankind World Heritage Site in Gauteng, South Africa.
The announcement was made at the University of the Witwatersrand in Johannesburg following the publication of their paper, titled: New cosmogenic burial ages for Sterkfontein Member 2 Australophithecus and Member 5 Oldowan, in the prestigious scientific journal, Nature, on 1 April 2015.
The researchers announced:
- A new date for StW 573, the Little Foot skeleton of Australopithecus prometheus, of 3.67 ± 0.16 million years (My);
- A cosmogenic burial date for the Sterkfontein Oldowan of 2.18 ± 0.21 My.
Sterkfontein has been internationally famous since 1936 for its key hominid and palaeontological discoveries and since the 1950s for its early archaeological finds.
Until now, however, no direct dating of the deposits has been without controversy. In particular, there has been much confusion surrounding the dating of StW 573, the 'Little Foot' skeleton of Australopithecus prometheus.
Palaeomagnetic dating of flowstones published in 1999 suggested an age near 3.3 million years (My), but this was not widely accepted. Cosmogenic nuclide burial dating by Professor Darryl Granger (Purdue University, US) and colleagues published in 2003 suggested an age near 4 My for the cave sediments containing the fossil.
Subsequent uranium-lead dating of calcite flowstones indicated a much younger age of 2.2 My, calling the cosmogenic dates into question. Although Professor Ronald (Ron) Clarke (Wits University, South Africa) had recognised as early as 2002 that the calcite flowstones are younger than the skeleton, the major discrepancy in ages left the age of the skeleton in doubt.
Recently, in 2014, Dr Laurent Bruxelles (INRAP, France), Clarke and colleagues published a detailed map of the cave sediments and their stratigraphy and showed beyond doubt that the dated flowstones had formed within voids opened by collapse of the cave sediments. The flowstones, in fact, separate conjoining parts of the skeleton and must be younger than the skeleton itself. This opens the possibility that the sediment and the skeleton within it could be far older than the 2.2 My old flowstones.
In 2010, Wits alumnus Dr Ryan Gibbon (University of New Brunswick, Canada) took up postdoctoral research with Granger and worked extensively on cosmogenic dating at Sterkfontein and elsewhere.
In this week's issue of Nature, the researchers report on a new date for Little Foot of 3.67 ± 0.16 My. This successful cosmogenic dating represents the collaborative work of Granger and Gibbon. The new date is made possible by two major advances in methodology.
The first is the development of the 'isochron' method for burial dating with cosmogenic 26Al and 10Be. An isochron uses multiple samples from the same site to check that all assumptions required for cosmogenic burial dating are met, including that the sediment has not been reworked.
However, successful dating required a second major advance, the development of the gas-filled-magnet at Purdue University's PRIME Lab, the accelerator mass spectrometry facility directed by Professor Marc Caffee where the cosmogenic nuclides were measured. The gas-filled-magnet allows a much better measurement of 26Al, and therefore more precise burial dating. The equipment became operational in mid-2014, and samples from Sterkfontein were among the first to be re-analysed.
The results were stunning. Out of 11 samples collected over the past decade, nine fell onto a single isochron curve, giving a robust age for the deposit. The other dating reported here is of a quartz cobble found at a higher level in the cave sequence.
The researchers used a manuport (a cobble carried into the site by Oldowan hominids) to eliminate the issue of sediment reworking, so a simple burial date rather than an isochron suffices. The age of the Oldowan at Sterkfontein is about 2 My, similar to three other sites with Oldowan tools recently dated with cosmogenic nuclides in South Africa.
The process of discovery and excavation of StW 573, the Little Foot hominid fossil, has been ongoing since 1994 when the first four footbones were found in a box of animal fossils by Clarke. In July 1997, Clarke and his assistants, Stephen Motsumi and Nkwane Molefe, located the position of the end of the shin bone of the skeleton in a deep deposit of the Sterkfontein caves known as Member 2 in the Silberberg Grotto.
Due to the arduous conditions of excavation in this cavern, the concrete-like deposits, and the displaced and fragile nature of the bones, it was August 2010 before Clarke and his team had exposed the whole skeleton and began lifting it within blocks of breccia to the surface.
This process continued until December 2011, when the last elements were freed. Today the process of cleaning the matrix from the bones and reconstructing the fossil is still ongoing, but over half of the skeleton has been MicroCT scanned at Wits University and the more detailed interpretation has begun.
Clarke has assigned the skeleton to the species Australopithecus prometheus. It differs from the site's better- known fossils of Australopithecus africanus in being of larger body size, with a skull that has a flatter, longer face, and larger, bulbous-cusped cheek teeth. The new dating of this skeleton at 3.67 My places it as a contemporary of early Australopithecus afarensis from Laetoli in Tanzania and Woranso-Mille in Ethiopia.
However, A. prometheus is morphologically very different from A. afarensis and has more similarities to the younger, flat-faced Paranthropus, with its large bulbous-cusped cheek teeth. Hence the date of 3.67 My for A. prometheus at Sterkfontein poses new questions about the diversity, geographic spread, and relationships of early hominid species in Africa.
THE OLDOWAN TOOLS
In the early 1990s, Clarke and Professor Kathleen Kuman (Wits University) excavated Member 5 East, a younger and higher portion of the cave deposits exposed at the surface by erosion of the cave roof. A new tool industry of great antiquity was discovered in the lower levels of this infill and was the first of its kind in southern Africa.
Initially estimated by fauna to ca 1.7 to 2 My, the industry was announced by Kuman in 1994 as Oldowan and the details of 3 500 pieces published in 2009. The Oldowan is a simple flaked stone tool technology, occurring in East Africa as early as 2.6 My, focused on flake tools knapped from cobbles. It lacks the handaxes and cleavers which signal the advent of the Acheulean industry, which first appears ca 1.7 My in both East and South Africa (the latter occurring at Sterkfontein and in the Northern Cape Province).
Today the researchers announce a cosmogenic burial date for the Sterkfontein Oldowan of 2.18 ± 0.21 My. It is similar to the recently published date of 2.19 ± 0.08 My for Oldowan tools at the neighbouring site of Swartkrans and shows that South Africa's Cradle of Humankind was home to tool-making hominids by 2 My or earlier.
A third assemblage of similar age (also dated by Gibbon and Granger) is currently under excavation at Canteen Kopje in the Northern Cape, and a fourth Oldowan assemblage ca 1.8 My has recently been published from Wonderwerk Cave, also in the Northern Cape.
Thus we now know that the Oldowan is consistently present in South Africa by 2 My, a much earlier age for tool-bearing hominids than previously anticipated in this part of Africa. It is now clear that the small number of Oldowan sites in southern Africa is due only to limited research and not to the absence of these hominids.
The maker of this industry is much debated, but many researchers accept that it is the handy work of a species of early Homo, such as Homo habilis that is dated in Malawi and East Africa from 2.4 to 1.8 My and at Swartkrans by 1.8 My or more.