In the early 20th century, our perception of ourselves received a massive shake-up when we looked eye-to-eye with the ancient hominins who marked the evolutionary separation between modern humans and the great apes like gorillas and chimps.
Discovering these "missing links" between humans and prehistoric primates not only confirmed the Darwinian thesis that humans evolved from apes but began to paint a picture of how we came to be.
A major milestone in the writing of our human story was the 1936 discovery of the first adult specimen of the genus Australopithecus (southern ape).
The fossil of the individual, dubbed TM 1511, was found in the Sterkfontein Caves in South Africa.
Since then, the caves have revealed hundreds of australopithecine fossils including several significant finds, like Australopithecus africanus individual "Mrs Ples", and Australopithecus prometheus specimen "Little Foot". As such, the Sterkfontein Caves are referred to as the "Cradle of Humankind".
Sterkfontein is a deep and complex cave system. In its caverns is preserved a long history of hominin activity in the region.
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Most of Sterkfontein's Australopithecus fossils have been excavated from an ancient cave infill called Member 4, which holds the highest density of Australopithecus fossils in the world.
Estimates for the age of Member 4 range from about three million years to as young as about two million years ago - younger than the appearance of our genus Homo.
But new research using novel dating techniques suggests that the sediment in which the Australopithecus fossils have been found might be over one million years older, at nearly four million years old.
This would place them further back in time than the world's most well-known Australopithecus specimen, "Dinkinesh", also known as "Lucy", of the species afarensis.
The research, published in Proceedings of the National Academy of Sciences (PNAS), was directed by Professor Dominic Stratford from South Africa's University of Witwatersrand.
"The new ages range from 3.4-3.6 million years for Member 4, indicating that the Sterkfontein hominins were contemporaries of other early Australopithecus species, like Australopithecus afarensis, in East Africa," says Stratford who co-authored the PNAS paper.
Coming up with accurate dates for fossils formed over millions of years is tricky.
In East Africa, where many hominin fossils have been found, the volcanic ash in which fossils are encased can be dated. Cave sediments, however, are especially hard to date as rock and bone tumble to the cave floor.
In caves, palaeontologists usually base their dates on other animal fossils found nearby, or flowstone formed by slow-flowing water trickling down the cave walls.
Previous dating of Member 4 was based on calcite flowstone deposits, but recent observations show that the flowstone is actually younger than the cave fill. Thus, previous ages were underestimates.
"Sterkfontein has more Australopithecus fossils than anywhere else in the world," says lead author Darryl Granger, professor at Purdue University in the US.
"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."
Using a novel technique involving radioactive decay of the rare isotopes aluminum-26 and beryllium-10 in mineral quartz, the research team obtained the new age range for the fossils.
"These radioactive isotopes, known as cosmogenic nuclides, are produced by high-energy cosmic ray reactions near the ground surface, and their radioactive decay dates when the rocks were buried in the cave when they fell in the entrance together with the fossils," says Granger.
Aluminum-26 is formed when a rock is exposed to cosmic rays at the surface, but not after it has been deeply buried in a cave.
Measuring levels of aluminum-26 in tandem with beryllium-10 allows researchers to date the sediment. This method is more accurate as it involves dating the concrete-like breccia in which the fossils are embedded.
Placed in a mass spectrometer, the amount of each radioactive nuclide in the rocks was determined.
This, coupled with geological mapping and a thorough examination of how sediments accumulate in caves, gave Granger and Stratford's team their age range.
Because the australopithecines found in Sterkfontein are so old, it may force scientists to rewrite the early chapters of human evolution.
"This reassessment of the age of Sterkfontein Member 4 Australopithecus fossils has important implications for the role of South Africa on the hominin evolution stage.
"Younger hominins, including Paranthropus and our genus Homo, appear between about 2.8 and 2 million years ago.
"Based on previously suggested dates, the South African Australopithecus species were too young to be their ancestors, so it has been considered more likely that Homo and Paranthropus evolved in East Africa," says Stratford.
But the new dates place the "Cradle of Humankind" in South Africa front and centre as the likely location of the evolution of early humans.
"This important new dating work pushes the age of some of the most interesting fossils in human evolution research, and one of South Africa's most iconic fossils, Mrs Ples, back a million years to a time when, in East Africa, we find other iconic early hominins like Lucy," says Stratford.
"The redating of the Australopithecus-bearing infills at the Sterkfontein Caves will undoubtably re-ignite the debate over the diverse characteristics of Australopithecus at Sterkfontein, and whether there could have been South African ancestors to later hominins," adds Granger.
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