Published on 06/28/2022 06:00
(Credit: Jason Heaton and Ronald Clark/Disclosure)
A discovery in the cave complex known as the Cradle of Humanity changes the location of the branches of the Tree of Life, predicting the presence of human ancestors in southern Africa by a million years. This UNESCO World Heritage site is famous for harboring some of the most famous hominins ever found: Mrs. Bliss and Littlefoot.
Hominins refer to the group formed by humans and their ancestors Australopithecus (in turn, human is the term that also includes other great primates). These ancient relatives were bipedal, but they also climbed trees and, as far as is known, are directly related to the human race. For decades, the study of fossils has helped understand human evolution, and now, thanks to a dating method developed at Purdue University in the United States, scientists have discovered that samples from South Africa are much older than previously thought.
While studying the Sterkfontein Cave, in the cradle of the human race, geologist Daryl Granger, professor of atmospheric and planetary sciences, found that the fossils found are not 2 million to 2.5 million years old, as was thought. Instead, they date back 3.4 million to 3.7 million years ago, making them older than the famous Lucy, who has been dubbed the “Mother of Humanity.” This fossil, excavated in Ethiopia, is 3.2 million years old.
Granger is an expert in dating geological deposits and has done similar work in Brazil (read the interview). As a PhD project, he developed a method for determining the age of sediments from buried caves over time, which is currently used by geologists around the world. In 2015, also at Sterkfontein, the scientist dated the little foot skeleton to 3.7 million years old, but other fossils at the site were of unknown age.
In the study published yesterday in the journal Pnas of the American Academy of Sciences, Granger and a team of scientists, including researchers from the University of the Witwatersrand in Johannesburg, South Africa, and Toulouse-Jean Jaurès University in France, found that this is not just Little Foot, but all sterkfontein plants. Australopithecus is at least a million years older than previously thought.
According to the scientist, Sterkfontein is a deep and complex cave system that preserves a long history of hominin occupation in the area. He says that understanding the dates of excavations at the site can be difficult because over time both rocks and bones have ended up at the bottom of a hole deep in the ground, and there are few ways to date these deposits.
In East Africa, where many hominin fossils have been found, the presence of nearby volcanoes in the Great Rift Valley makes the task easier, as deposits of millennial ash layers can be used for dating. But in South Africa – and especially in caves – scientists do not have such a resource. To estimate the age of skeletons, researchers often use other fossils found in the area, a method subject to many inaccuracies.
A more accurate approach is to date the rocks where the bones were found, and to study the concrete matrix comprising the fossil, a technique Granger uses. “Strekfontein contains more Australopithecus fossils than anywhere else in the world,” says the scientist. “But it’s hard to get a good match with them. Conventional methods have gotten a number of different dates. What our data does is resolve these controversies and show that these fossils are ancient – much older than we originally thought.”
Granger and the team used an accelerator mass spectrometer to measure the radionuclides in the rocks. They also conducted geological maps and an in-depth study of how cave deposits accumulate, to determine the age of the deposits containing Australopithecus in Sterkfontein. The scientist explains that the focus of the study is the so-called cosmic nuclides, which can reveal information about the history of fossils, as well as the geological characteristics of rocks. These are extremely rare isotopes produced by cosmic rays – high-energy particles that bombard the Earth constantly.
According to Granger, these cosmic rays have enough energy to cause nuclear reactions inside rocks, at the Earth’s surface, resulting in radioactive isotopes in mineral crystals. “One example is aluminium-26: aluminum that lacks a neutron and slowly decays into magnesium over millions of years,” he says. Because aluminum-26 forms when a rock is exposed to the surface, but not after it is buried deep in a cave, researchers can date the deposits of these shelters (and the fossils inside) by measuring the isotope levels together. Another cosmic nuclide, beryllium-10.
In addition to the new dates, the scientists mapped the sediments of the cave complex and showed how the fossils of animals of different ages mixed together during local excavations conducted in the 1930s and 1940s, leading to decades of confusion. “I hope this will convince people that this method gives reliable results,” Granger said. “Using this approach, we can more accurately place ancient humans and their relatives across time periods, whether in Africa or elsewhere in the world.”
“This reassessment of the Styrkfontein fossil age has important implications for the role of South Africa in hominin evolution,” Dominic Stratford, director of research at Studied Caves and one of the study’s authors, said in a note. “Younger hominins, including our own species, appear about 2.8 to 2 million years ago. Based on previously suggested dates, Australopithecus species in South Africa were too young for their ancestors, so keep in mind that it is likely that humans evolved in East Africa”.
two questions for
Credit: Bordeaux University photo/Lena Kovalenko
Daryl Granger, geologist
How does this discovery change the understanding of human history?
The article in Pnas changes the way the Australopithecus branch of the human family tree is organized. What it shows is that the South African species represented at the Sterkfontein site are similar in age to other species that once lived in East Africa, including that of Lucy, Australopithecus afarensis. Many people believe that the Australopithecus species of South Africa, such as Australopithecus africanus, evolved from A.farensis. Since we now know that they are practically the same age, this cannot be true. Instead, there must be an older common ancestor. The history of the species in South Africa dates back to a time when humans diversified and spread across different environments across Africa.
Can the dating method be used to know the age of other fossils?
The dating method we used does not directly date the fossils, but the surrounding rocks, which were present in the cave at the same time as it. The method requires exposing the mineral quartz to the surface of the earth and then depositing it in the cave. Because the method works well in caves, we can date any fossils found in quartz-containing sediments within these environments. I worked with (geologist) Evo Karman at the University of São Paulo and with his students in caves in Brazil, but not for fossil dating. We’re only working on the evolution of caves, not animals. (soil)
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