The freshly dated fossils also belong to the genus Australopithecus, an ancient hominid originally thought to have lived 2 to 2.6 million years ago. Researchers have used a new technique to date sediments from the Sterkfontein Caves, which are part of the Cradle of Humankind UNESCO World Heritage Site, about 50 kilometers northwest of Johannesburg.
The caves included in this network have revealed details of human and environmental evolution that span approximately 4 million years.
The site is home to a treasure trove of fossils that help tell the story of human evolution – a story that seems to change with each discovery.
The significance of the Sterkfontein Caves became apparent in 1936, when the discovery of the first adult Australopithecus fossil was made by paleontologist Dr Robert Broom.
“Sterkfontein has more Australopithecine fossils than anywhere else in the world,” said study lead author Darryl Granger, professor of earth, atmospheric and planetary sciences at the College of Science in Purdue University, in a statement.
“But it’s hard to get a good date on them. People have looked at animal fossils found near them and compared the ages of cave features like lava flows and got a range of different dates. This What our data does is resolve those controversies. It shows that these fossils are old – much older than we originally thought.”
A new look at ancient ancestors
Researchers have determined that all cave sediments, including Australopithecine fossils, date from 3.4 to 3.6 million years ago, placing them around the start of the Australopithecine era, rather than around the end. They now predate other hominids from nearby sites by over a million years.
Many Sterkfontein fossils have been found in Member 4, the richest deposit of Australopithecine fossils found anywhere in the world. Previous research had suggested the deposit could be as young as 2 million years ago – younger than when the genus Homo, to which we (Homo sapiens) belong, first appeared about 3 million years ago. millions of years.
These dates suggested that Australopithecines overlapped with members of the genus Homo, as well as a large-toothed hominid called Paranthropus. Based on this idea, researchers have so far accepted that South African Australopithecines are descended from East African species, such as Lucy and other members of Australopithecus afarensis.
“What our age shows is that it can’t be true, because they’re practically the same age,” Granger said. “There must be an older common ancestor. This also gives the South African species much more time to evolve and reopens the discussion of the role of the South African species in later hominids such as Paranthropus.”
The reassessment of the age of Australopithecines from Sterkfontein has important implications for how South Africa played a role in the diversification and expansion of our earliest human ancestors, says the study’s co-author , Dominic Stratford, director of cave research and professor at the University of the Witwatersrand in Johannesburg. .
Look into the past
To date cave sediments, Granger applied a method he first developed in the mid-1990s and is now used by many researchers in the field.
Granger works with cosmogenic nuclides, “very rare radioactive particles that are produced inside mineral grains by cosmic rays from outer space,” he said. Aluminum-26 and beryllium-10 are two examples of cosmogenic nuclides, both found in the mineral quartz. Aluminum-26 forms when a rock is exposed to cosmic rays while it rests on the Earth’s surface. But that can’t happen once he’s inside a cave.
“Their radioactive decay dates from when the rocks were buried in the cave when they fell at the entrance with the fossils,” Granger said.
Previously, he used his method to date the Little Foot fossil. But the age of the other Australopith fossils in the Sterkfontein Caves has been debated, mainly because the deep and complex cave system has a long history of occupation by hominids who lived in the area at the time.
East Africa is another place rich in ancient hominin fossils, and many have been found in the Great Rift Valley, where volcanoes have created layers of ash that are easier to date.
The same is not true for the caves of South Africa, where researchers must rely on the use of animal fossils to help determine the age of other bones they are close to, or of calcite. As water flows over the walls or floors of caves, it can deposit layers of calcite or other sheet-like carbonate minerals.
Dating cave sediments is tricky – and it gets even harder as rocks and bones move and fall from different layers in the cave. And younger flows can sometimes be found mixed with older sediments.
Rather than just peeling back the layers, it’s more like sorting through a mix.
Granger made a breakthrough in 2014 while working at the Purdue Rare Isotope Measurement Laboratory, and researchers found they could very accurately measure small amounts of aluminum-26, opening the doors to sites that were previously much toughest to date, he said.
Granger and his team studied the breccia, the concrete-like substance in which the fossils are embedded, and used his method to determine new dates for the fossils. Researchers also mapped cave deposits and showed how some of them mixed together during excavations that took place in the 1930s and 1940s.
“Part of the misconception about its age came from the mixing of fossils from different layers in the early excavations,” Granger said. “Using this method, we can more accurately place ancient humans and their relatives in the correct time periods, in Africa and elsewhere in the world.”