In 1974, paleoanthropologist Donald Johanson discovered the partial fossilized skeleton of a female of the hominin species Australopithecus afarensis at a dig site in Ethopia. Named “Lucy” because of the Beatles’ song playing continuously on the camp’s loudspeaker system, the early australopithecine was dated to about 3.2 million years ago, making it at the time the oldest human ancestor remains ever found, and helped establish Ethiopia as the top candidate for the birthplace of humanity. That title was previously claimed by South Africa, where in 1947 a nearly complete skull of an Australopithecus africanus was found. Nicknamed Mrs. Ples, it was dated to 2.1 to 2.6 million years ago. That changed recently when new analysis of Mrs. Ples changed her date of birth by a million years – making her about 3.4 to 3.6 million years old, pushing Lucy and Ethiopia back down the list.
“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.”
In a new study published in the journal Proceedings of the National Academy of Sciences (PNAS), co-author Professor Dominic Stratford from South Africa’s University of Witwatersrand explains how Sterkfontein, a set of limestone caves about 25 miles (40 km) northwest of Johannesburg, became known as the Cradle of Humankind – now a World Heritage Site. Limestone mining started there in the late 1890s and miners immediately began finding fossils which they turned over to paleontologists. In 1936, paleontology students from the University of the Witwatersrand found the first adult Australopithecine fossils – a child’s skull determined to be Australopithecus africanus was found earlier in Taung and became known as Taung Child. In 1947, British-South African doctor and palaeontologist Robert Broom uncovered a nearly complete skull of an adult female A. africanus (although there was some speculation it was an adolescent male) which Broom called Plesianthropus transvaalensis (near-man from Transvaal) – the name that was eventually shortened to Mrs. Ples.
In 1997, a nearly complete skeleton of a second species of Australopithecus was discovered in the caves by paleontologist Ronald J. Clarke – named “Little Foot” after the first parts of the skeleton found, its age has been difficult to determine. However, Little Foot kicked off a flurry of paleo activity at Sterkfontein – to date, fossils of approximately 500 hominids have been excavated there, leading it to be called the Cradle of Humankind … despite the fact that most of the fossils have been resistant to accurate dating and those that have are younger than Ethiopia’s Lucy. Until now.
“Sterkfontein has more Australopithecus fossils than anywhere else in the world. But it’s hard to get a good date on them. People have looked at the animal fossils found near them and compared the ages of cave features like flowstones and gotten a range of different dates. What our data does is resolve these controversies. It shows that these fossils are old — much older than we originally thought.”
Co-author Darryl Granger, a professor at Purdue University, explains in Sci-News the new technique developed by the study team to get a more accurate date on Mrs. Ples and other Sterkfontein fossils. It started with the sediments surrounding the fossils in the limestone caves, particularly the cave known as Member 4 where the majority of the Australopithecus fossils have been found. They were looking for radioactive cosmogenic nuclides – which are extremely rare isotopes created by the cosmic rays which are constantly bombarding the Earth as high-energy particles. These cosmogenic nuclides cause nuclear reactions as they hit surface rocks, thus creating new radioactive isotopes within the mineral crystals. In particular, they looked for beryllium-10 and aluminum-26 – aluminum-26 is aluminum that is missing a neutron, causing it to slowly decay into magnesium over millions of years. According to the researchers, finding the two cosmogenic nuclides together was the key.
“Since aluminum-26 is formed when a rock is exposed at the surface, but not after it has been deeply buried in a cave, we can date cave sediments — and the fossils within them — by measuring levels of aluminum-26 in tandem with another cosmogenic nuclide, beryllium-10.”
The cosmogenic nuclides put the date of the sediment and the Australopithecus fossils in it at 3.4 to 3.67 million years ago. That pushed Mrs. Ples ahead of Lucy on the age chart. More importantly, it pushed the australopiths into a more prominent position as a human ancestor. Professor Stratford points out that the previous 2.4 million-year-old age made the South African Australopithecus species too young to be the ancestors of the Homo genus, which appeared starting 2.8 million years ago. By pushing its appearance back one million years, it now overlaps in age with other early hominins in Africa, including Australopithecus afarensis and Australopithecus deyiremeda at Burtele in eastern Africa, Australopithecus bahrelgazali in Chad at the crossroads of North and Central Africa, Kenyanthropus platyops at Lake Turkana in the Kenyan Rift Valley, and Australopithecus anamensis at Woranso-Mille in Ethiopia. This now solidifies South Africa’s place in early hominin evolution, and Sterkfontein’s hold on the title of the Cradle of Humankind.
But wait … there’s more.
The caves of Sterkfontein were used for millions of years by hominins and animals, resulting in a confusing mix that has made it difficult for paleontologists to paint an accurate picture of the historical evolution of the caves and their occupants. This new age determination technique using cosmogenic nuclides helped the research team draw maps of the cave deposits, which clearly showed how the animal fossils of different ages dating back millions of years were mixed with those from excavations in the 1930s and 1940s. While it helped this research team positively date Mrs. Ples and her friends, Dr. Granger hopes it will aid paleontologists around the world facing similar challenges.
“What I hope is that this convinces people that this dating method gives reliable results. Using this method, we can more accurately place ancient humans and their relatives in the correct time periods, in Africa, and elsewhere across the world.”
This could be the key to finding more Denisova hominin fossils in the piles of bones and sediment in the Denisova Cave in Siberia.
We think we know so much … and yet we know so little.