Early human history breakthrough

New research from an international team of scientists led by University of Cape Town (UCT) isotope geochemist Dr Robyn Pickering is the first to provide a timeline for fossils from the caves within the Cradle of Humankind. It also sheds light on the climate conditions in which our earliest ancestors in the area lived.

Published in the prestigious journal Nature on Wednesday, 21 November, the work corrects assumptions that the region’s fossil-rich caves could never be related to each other. In fact, the research suggests fossils from the Cradle caves date to just six specific time periods.

There is agreement that the results return the Cradle to the forefront of scientific opportunity, and open new opportunities for scientists to answer complex questions about human history in the region.

“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,” said Pickering, of UCT’s Department of Geological Sciences.

“Now we can link together the findings from separate caves and create a better picture of evolutionary history in southern Africa.”

The Cradle of Humankind is a World Heritage Site made up of complex fossil-bearing caves. It is the world’s richest early hominin site and home to nearly 40% of all known human ancestor fossils, including the famous Australopithecus africanus skull nicknamed Mrs Ples.

Using uranium-lead dating, researchers analysed 28 flowstone layers that were found sandwiched between fossil-rich sediment in eight caves across the Cradle. The results revealed that the fossils in these caves date to six narrow time windows between 3.2 and 1.3 million years ago.

Dating the caves

Pickering explained that the flowstones are the key.

“We know they can only grow in caves during wet times, when there is more rain outside the cave. By dating the flowstones, we are picking out these times of increased rainfall. We therefore know that during the times in between, when the caves were open, the climate was drier and more like what we currently experience,” she said.

This means the early hominins living in the Cradle experienced big changes in local climate, from wetter to drier conditions, at least six times between 3 million and 1 million years ago. However, only the drier times are preserved in the caves, skewing the record of early human evolution.

“This is the most important advance to be made since the fossils themselves were discovered.”

Up until now, the lack of dating methods for Cradle fossils made it difficult for scientists to understand the relationship between East and South African hominin species. Moreover, the South African record has often been considered undatable compared to East Africa, where volcanic ash layers allow for high resolution dating.

Study co-author Professor Andy Herries, from the La Trobe University in Australia, noted 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, meant 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 said.

Pickering began dating the Cradle caves in 2005 as part of her PhD research. This new publication is the result of 13 years of work and brings together a team of 10 scientists from South Africa, Australia and the United States.

Leading palaeoanthropologist Professor Bernard Wood, of the Center for the Advanced Study of Human Paleobiology at the George Washington University in the US, hailed Pickering and her team for making a major contribution “ to our understanding of human evolution”.

“Exemplary” study

“This is the most important advance to be made since the fossils themselves were discovered,” said Wood, who was not involved in the Pickering study.

He explained that dates matter “a lot”.

“The value of the southern African evidence has been increased manyfold by this exemplary study of its temporal and depositional context,” Wood said.