A century ago, the discovery of a juvenile skull in South Africa’s North West province revolutionized our understanding of human origins. This skull, dubbed the Taung Child by Raymond Dart, an anatomist at the University of the Witwatersrand, became a landmark find. While Dart couldn’t pinpoint the exact age in 1924, he declared it belonged to a new species he named Australopithecus africanus. This discovery was groundbreaking, providing the first tangible evidence to support British naturalist Charles Darwin’s assertion that apes and humans shared a distant common ancestor and that humanity originated in Africa.
Following the Taung Child, numerous discoveries of early hominin fossils were made at Sterkfontein, a site located about 70 kilometers southwest of Pretoria. This site falls within the “Cradle of Humankind,” a UNESCO World Heritage Site renowned for its rich fossil record. However, a fierce debate has raged over the geological ages of the Sterkfontein fossils, as well as those from Taung and another site, Makapansgat.
Much of this controversy centers on Sterkfontein. Some researchers place the age of fossils from a specific area, known as “Member 4,” at around 3.4 to 3.7 million years old. Others contend that these fossils are much younger, dating back to between 2 million and 2.6 million years. This discrepancy stems from the different dating methods employed by opposing research teams. Each group has published their findings in scientific journals, adding fuel to the ongoing debate.
Now, a new study may have brought us closer to resolving this long-standing controversy. Working with the late Sue Dykes, the author of this piece has employed a novel dating method, directly applied to the fossil teeth of hominins, to estimate the ages of the Sterkfontein fossils. Their results for Member 4 suggest a wider range of ages, from approximately 2 million to 3.5 million years. This encompasses the age estimates put forward by both opposing groups.
This method also allowed the researchers to date the Taung Child to 2.58 million years ago. While the authors believe their method is accurate, they acknowledge that further studies using different approaches will undoubtedly be conducted. The quest to definitively establish the time period when these ancient members of our family tree existed in South Africa will continue.
One key issue hinging on the answer is the identification of the region from which our genus, Homo, originated: was it in South Africa or east Africa, from an ancestral australopithecine species? Accurately dating the Sterkfontein fossils has been particularly challenging due to the initial discoveries being made during limestone mining operations, which involved the use of dynamite. This destructive method resulted in the loss of the fossils’ original context.
However, both at Sterkfontein and other sites in South Africa, fossils have been found of animal species also present in east Africa. Volcanic deposits in east Africa contain traces of potassium (K) and argon (Ar), allowing for precise K/Ar radiometric dating. Unfortunately, active volcanism did not occur in South Africa during the period of interest, between 2 million and 5 million years ago. Nevertheless, comparisons can be made between fossils of species found in both regions, including bovids (antelope like wildebeest, hartebeest, and kudu), suids (such as warthogs), monkeys, and hominins.
Since the east African fossils can be accurately dated using the K/Ar method, the ages of the same species in South Africa can be estimated. This approach is known as biochronology, and it’s how one group of researchers reached their conclusion: that the Sterkfontein fossils from Member 4 are between 2 million and 2.6 million years old. Essentially the same ages have been obtained from fossil comparisons with east African species.
The group that places the fossils’ ages between 3.4 million and 3.7 million years old, on the other hand, employed a method called cosmogenic nuclide dating. They reached their conclusions by analyzing the elements beryllium and aluminum to estimate the ages of chert (a type of sedimentary rock) found in the Sterkfontein cave deposits associated with hominin fossils from Member 4.
The authors of this new study also used a biochronological approach for dating, but instead of relying on animal teeth, they worked directly with measurements of the hominin fossils’ teeth. They examined the ratios of length and breadth of the lower first molars of east African hominins. Using an equation they developed, they quantified a relationship between these ratios and geological age for their sample of Tanzanian, Kenyan, and Ethiopian fossils, including Paranthropus boisei and early Homo species such as Homo habilis. The dates for these fossils have been well established.
Assuming that the age of South African fossils representing the same genera could be estimated using the same relationship, the authors applied their method to lower first molar teeth from Sterkfontein, particularly those attributed to Australopithecus africanus and early Homo, for which tooth ratios could be determined. This approach allowed them to obtain dates for individual molars. Their method was also applied to molar teeth of the Taung Child, yielding a new result of 2.58 million years for this specimen of Australopithecus africanus.
Two teeth of Australopithecus africanus from Makapansgat were also dated using this method. The specimens are 3.07 million and 3.00 million years old, respectively, aligning well with earlier estimates based on paleomagnetism. The authors further used their method to date teeth attributed to the hominin species known as Australopithecus sediba, found at Malapa near Sterkfontein. Their dates for two teeth representing this species (cataloged as MH1 and MH2) are 2.14 million and 1.93 million years, respectively. This corresponds remarkably well with the age of 1.98 million years obtained through methods utilizing uranium, lead, and paleomagnetism.
