posted on 06/28/2022 06:00
(credit: Jason Heaton and Ronald Clarke/Disclosure)
A discovery in the cave complex known as the Cradle of Humankind alters the position of the branches of the tree of life, anticipating the presence of man’s ancestors in South Africa by 1 million years. This UNESCO World Heritage site is famous for housing some of the most famous hominins ever found: Mrs. Ples and Little Foot.
Hominins refer to the group formed by man and his ancestors Australopithecus (in turn, hominid is the term that also includes the other great primates). These very ancient relatives were bipedal, but they also climbed trees and, as far as is known, are directly linked to the genus Homo. For decades, the study of fossils has helped to understand human evolution, and now, thanks to a dating method developed at Purdue University, in the United States, scientists have discovered that South African specimens are much older than previously thought.
Studying Sterkfontein Cave, in the Cradle of Humankind, geologist Darryl Granger, professor of atmospheric and planetary science, found that the fossils found there are not 2 million to 2.5 million years old, as was believed. Instead, they date from 3.4 million to 3.7 million years ago, which makes them even older than the famous Lucy, 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 even carried out similar work in Brazil (Read the interview). As a doctoral project, he developed a method for determining the age of sediments in caves buried by time, which is now used by geologists around the world. In 2015, also at Sterkfontein, the scientist dated the Little Foot skeleton to 3.7 million years, but the 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 the Toulouse Jean Jaurès University in France, found that not only Little Foot, but all Sterkfontein Australopithecus are at least 1 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 of the area. Understanding the dates of the fossils at the site, he says, can be tricky because, over time, both rocks and bones have ended up at the bottom of a deep hole in the ground, and there are few ways to date such sediments.
In East Africa, where many hominin fossils have been found, the existence of nearby volcanoes in the Great Rift Valley makes the task easier, as deposits of millenary ash layers can be used for dating. But in South Africa—especially in caves—scientists don’t have such a resource. To estimate the age of skeletons, researchers often use other fossils found in the locality, a method subject to many inaccuracies.
A more precise approach is to date the rocks where the bones were found, studying the concrete matrix that houses the fossil, a technology used by Granger. “Sterkfontein has more Australopithecus fossils than anywhere else in the world,” says the scientist. “But it’s hard to get a good match with them. Traditional 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 accelerator mass spectrometry to measure radioactive nuclides in the rocks. They also carried out geological mapping and an in-depth study of how cave sediments accumulate, to determine the age of sediments containing Australopithecus at Sterkfontein. The scientist explains that the focus of the study is the so-called cosmogenic nuclides, which can reveal information about the history of fossils, in addition to the geological characteristics of rocks. These are extremely rare isotopes produced by cosmic rays — high-energy particles that constantly bombard the Earth.
According to Granger, these cosmic rays have enough energy to cause nuclear reactions inside rocks, at the surface of the ground, creating radioactive isotopes in mineral crystals. “One example is aluminum-26: aluminum that lacks a neutron and slowly decays into magnesium over a period of millions of years,” she says. Because aluminum-26 is formed when a rock is exposed at the surface, but not after it has been buried deep in a cave, researchers can date the sediments of these shelters (and the fossils within them) by measuring the isotope levels together. with another cosmogenic nuclide, beryllium-10.
In addition to the new dates, the scientists mapped the deposits of the cave complex and showed how fossils of animals of different ages would have been mixed together during local excavations carried out in the 1930s and 1940s, leading to decades of confusion. “What I hope is that this convinces people that this method gives reliable results,” Granger said. “Using the 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 age of the Sterkfontein fossils has important implications for the role of South Africa in the stage of hominin evolution,” said Dominic Stratford, director of research at the studied caves and one of the study’s authors, in a note. “Younger hominins, including our genus Homo, appear between about 2.8 and 2 million years ago. Based on previously suggested dates, the Australopithecus species of South Africa were too young to be their ancestors, so consider It is more likely that Homo evolved in East Africa,” he said.
two questions for
credit: Purdue University photo/Lena Kovalenko
Darryl Granger, geologist
How does the discovery change the understanding of human history?
The article in Pnas changes the way the Australopithecus branch is organized in the human family tree. What it does show is that the South African species represented at the Sterkfontein site are similar in age to other species that lived in East Africa, including Lucy’s species, Australopithecus afarensis. Many people think that South African Australopithecus species, such as Australopithecus africanus, evolved from A.afarensis. As we now know that they are practically the same age, this cannot be true. Instead, there must be an older common ancestor. The South African species dates back to a time when hominids were diversifying and spreading across different environments across Africa.
Can the dating method be used to find out the age of other fossils?
The dating method we used does not date the fossils directly, but the rocks around them, which were in the cave at the same time as them. The method requires the mineral quartz to be exposed on the surface of the ground and then deposited in a 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) Ivo Karmann at the University of São Paulo and with his students on caves in Brazil, but not for fossil dating. We only work on the evolution of caves, not animals. (DUST)