Dating: Cradle of Humankind cracks missing link
It took 13 years, but now scientists are able to date the caves at the fossil-rich site
The proof might be in the pudding for most things in life, but for an international team led by the University of Cape Town, the proof was in the flowstones.
It was thanks to these information-rich layers that the team – about 13 years later and involving 10 researchers – had a major breakthrough and were able to date caves in the fossil-rich Cradle of Humankind near Johannesburg where about 40% of all known human ancestor fossils have been found. By dating the caves, the task of dating the fossils found inside them becomes a lot easier.
The flowstones are hard sheets of calcite and other cave minerals that have formed into hard layers inside the cave. Looking at them from the side one can see each exact layer which is often a different colour from the ones below and above. These occur after water has flowed down the walls of a cave and along its floors. The minerals would have been dissolved in the water, but when the water can no longer hold the minerals in solution, they are deposited – and that’s when you get the sheets of different minerals building up on top of each other.
The research, published last week in the online journal Nature, puts paid to previous theories that the caves in the Cradle were not related to one another, and more than that, it hypothesises that the Cradle caves date to “just six specific time periods”.
Lead researcher Dr Robyn Pickering, an isotope geochemist from UCT, said: “Unlike previous dating work, which often focused on one cave, sometimes even just one chamber of the cave, we are providing direct ages for eight caves and a model to explain the age of all the fossils from the entire region.”
She added: “Now we can link together the findings from separate caves and create a better picture of evolutionary history in Southern Africa."
The researchers used uranium-lead dating to analyse 28 flowstone layers. These were “sandwiched between fossil-rich sediment” in eight caves across the Cradle, and the results showed that the caves all date to between 3.2 and 1.3 million years ago.
Pickering also explains in her piece in The Conversation, that the fossil record in the Cradle has “largely taken a back seat to East African finds” because “we didn’t know how old the caves in the Cradle are and could thus not provide conclusive dates for the many fossils found inside them”.
East Africa has the advantage of a rift system which is home to abundant volcanic ash layers between the fossil beds.
These layers “can be dated”, giving ages to the fossils – compared with SA’s caves which have “no such volcanic layers”.
Using the uranium-lead method to analyse the flowstones, Pickering and the team were able to “narrow down the entire early human record of the Cradle to a few brief time windows between one and three million years ago”.
She says: “One of the things that’s particularly exciting about this research is that we can, for the first time, compare South African hominins with their cousins in East Africa.”
Professor Andy Herries, a co-author of the study at La Trobe University in Australia, concurs that “while the South African record was the first to show Africa as the origin point for humans, the complexity of the caves and difficulty dating them has meant that the South African record has remained difficult to interpret”.
“In this study we show that the flowstones in the caves can act almost like the volcanic layers of East Africa, forming in different caves at the same time, allowing us to directly relate their sequences and fossils into a regional sequence,” he says.
The results “return the Cradle to the forefront and open new opportunities for scientists to answer complex questions about human history in the region”, say the researchers.