Sterkfontein is a deep and complex cave system that holds the remains of four million years of human evolution.

It is part of the set of South African sites known as the 'Cradle of Humanity' and has provided Science with some of the best preserved specimens of

Australopithecus africanus

.

Among them, a specimen known as Mrs. Ples, one of the most complete skulls of a hominid before sapiens, discovered by Robert Broom and John T. Robinson in 1947. Also an almost complete skeleton, known as Little Foot, found in the the 90's.

However, specifying the period and the species to which each fossil belongs in a place inhabited millions of years ago, with the multiple changes in the terrain that have occurred since then, is a complex task.

Now, in a new study published this Monday in the journal

Proceedings of the National Academy of Sciences

, a team of scientists - which includes researchers from South Africa, France and the United States - proposes dating with a new method for the sediments of the caves in which Most of the

Australopithecines were found:

the analysis places them between 3.4 and 3.7 million years ago, that is, almost a million years before the 2-2.5 million that other dates had estimated.

Most of Sterkfontein's

Australopithecus

bones have been found in a particular sector of the cave, which the researchers refer to as 'Member 4' and which has been the main focus of the work described in the new study.

"The dates we propose for Member 4 indicate that the Sterkfontein hominins were contemporaries of other early

Australopithecus

species , such as

A. afarensis

from East Africa," explains Dominic Stratford, a professor at Wits University of South Africa and one of the authors. article main.

It is estimated that Lucy, the most famous representative of

A. afarensis

,

lived in what is now Ethiopia 3.2 million years ago and that the origin of its species dates back to 3.9 million.

The authors explain that in East Africa the volcanoes of the Great Rift Valley deposit layers of ash that can be more easily dated, but that in South Africa, especially inside caves, it is more laborious work.

Fossils of animals found around the bones are often used to estimate their age or a geological phenomenon called lava, visible in the stone walls.

The problem is that the bones can move within the deposits and that sediments from different times can mix.

New method

In this case, the researchers have focused on analyzing the block that covers the fossil, with a new dating based on the radioactive decay of the aluminum-26 and beryllium-10 isotopes in the quartz mineral.

"These radioactive isotopes, known as cosmogenic nuclides, are produced by high-energy cosmic ray reactions near the ground surface, and their radioactive decay allows us to date when the rocks are buried in the cave along with the fossils." , says Professor Darryl Granger, from Purdue University (USA).

Granger and his team use accelerator mass spectrometry (AMS) to measure radioactive nuclides in rocks, which they combine with geological study and sediment dynamics.

To do this, they use a mass spectrometer, a device that allows the composition of different chemical elements and atomic isotopes to be analyzed, separating the atomic nuclei based on their relationship between mass and charge.

In addition to these new dates, the researchers have made detailed maps of the cave deposits and thus have been able to show that fossils of animals from different eras would have been mixed during the excavations of the 1930s and 1940s, which has caused decades of confusion and debate.

"I hope this work helps to convince that this dating method gives reliable results", adds Granger, "thanks to it we can more accurately place ancient humans and their relatives in correct time periods, both in Africa and elsewhere. of the world".

Possible predecessor

The age of the fossils is important because it influences the interpretation of the evolutionary process: how the first hominids related to each other in the ecosystem and who their closest relatives were.

That is why placing the Sterkfontein fossils in their proper context is an important step in putting the pieces of that puzzle together.

"The reassessment of the age of the

Australopithecus

fossils at Sterkfontein has important implications for the role of South Africa in evolution," Stratford notes.

"The youngest hominins [subfamily of hominids characterized by upright posture], including

Paranthropus

and our genus

Homo

, appear between 2.8 and 2 million years ago;

based on the dates suggested above, the South African species of

Australopithecus

were too recent to be their ancestors, so it has been considered more likely that the other species evolved in East Africa."

But the new dating implies that

Australopithecines

were already in Sterkfontein almost a million years before the appearance of

Paranthropus

and Homo, enough time to evolve in the region.

"The age of some of the most

interesting

fossils in the investigation of human evolution goes back a million years", points out the researcher, "such as that of Mrs. Ples, one of the most emblematic of South Africa, who would have lived in the same time as other famous early hominins in East Africa, like Lucy.

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