Madrid
Updated:
Save
More than 40 kilometers north of the city of Johannesburg, in South Africa, is the so-called ‘Cradle of Humankind’, a complex of caves where 40 percent of all fossils of human ancestors have been found to date. Among these galleries are those of Sterkfontein, fundamental to understanding the early origins of our evolution. There was discovered ‘Little foot’, the almost complete skeleton of an australopithecus child from 3.7 million years ago. However, the age of other hominins unearthed at the same site has been the subject of much debate.
Now, an international team of researchers has established a clear time frame for all fossils from the Sterkfontein caves in the journal Proceedings of the National Academy of Sciences (PNAS).
As they explain, many of them are contemporaries of ‘Little Foot’ and, therefore, a million years older than previously believed. This would make them even older than ‘Lucy’, the world’s most famous autralopithecus fossil, at 3.5 to 3.2 million years.
Darryl Granger, a professor in the Purdue University School of Science, and his team found that not just the Little Foot, but all cave sediments containing australopithecines date to approximately 3.4 to 3.7 million years ago. of years, instead of 2 to 2.5 million years as scientists previously theorized. That age places the fossils towards the beginning of the australopithecine era, rather than towards the end.
complicated task
Understanding the dates of the Sterkfontein fossils can be tricky, as the rocks and bones fell to the bottom of a deep hole in the ground, and there are few ways to date the cave sediments.
In eastern Africa, where many hominin fossils have been found, volcanoes in the Great Rift Valley deposit layers of ash that can be dated. Researchers turn to those layers to estimate the age of a fossil. In South Africa, especially in a cave, scientists don’t have that possibility. They usually use other animal fossils found around the bones to estimate their age or the calcite wash deposited in the cave. But bones can move and young lavage can be deposited in old sediments, making those methods potentially wrong. The researchers employed a more precise method, dating the actual rocks in which the fossils are embedded.
“Sterkfontein has more australopithecus fossils than anywhere else in the world,” says Granger, “but it’s hard to get a good date… What our data do is resolve these controversies. It shows that these fossils are much older than we originally thought.”
Granger and the team used accelerator mass spectrometry (AMS) to measure so-called cosmogenic nuclides, extremely rare isotopes produced by cosmic rays, high-energy particles that constantly bombard Earth.
Cosmics rays
These incoming cosmic rays have enough energy to cause nuclear reactions within rocks at the ground surface, creating new radioactive isotopes within mineral crystals.
In addition, the team carefully mapped the cave deposits and showed how animal fossils of different ages would have mixed together during excavations in the 1930s and 1940s, leading to decades of confusion with earlier ages. “Using this method, we can more accurately place ancient humans and their relatives in the correct time periods, in Africa and in other parts of the world.” Granger says.
The age of the fossils is important because it influences scientists’ understanding of the living landscape of the time. How and where humans evolved, how they fit into the ecosystem, and who their closest relatives are and were are pressing and complex questions. Putting the Sterkfontein fossils in their proper context is one step in solving the whole puzzle.
The new dates show that australopithecines existed in Sterkfontein almost a million years before the appearance of Paranthropus (an extinct human ancestor) and the genus Homo (of our lineage), which gave them more time to evolve there, in the Cradle of Humankind, and places the hominids of this site at the forefront of the story of early human evolution.
See them
comments