Were South African Australopithecines contemporaneous with Lucy and other East African hominin species, thereby expanding the elusive cradle of humanity to a near-continental scale? Fossil StW 573, baptized Little Foot, an almost complete Australopithecus prometheus found at the Sterkfontein site in South Africa, “had already set foot in the door”, recalls geoarchaeologist Laurent Bruxelles (National Institute for Preventive Archaeological Research, CNRS): in 2015, an international team of which he was a member had proposed a new dating, increasing it from 2.2 to 3.7 million years – against 3.2 for Lucy, a young Ethiopian representative of the Australopithecus species afarensis.

The same team continued this patient work of reassessing dating at this site 35 kilometers northwest of Johannesburg. It is among the richest in Australopithecine fossil remains – up to 500 remains for one of the deposits, called “Member 4”, where the STS 5 fossil, Mrs Ples (“Madame Ples”) was found in 1947. , one of the first complete Australopithecine skulls. The researchers come to the same conclusions for representatives of the Australopithecus africanus species, of which Mrs Ples is a member: the textbooks will have to be rewritten to make them go from around 2.5 million years old to 3.4 million years old. ‘seniority. Again, a huge leap into the past! The new analysis is published in the American journal PNAS on June 27.

Cosmogenic method

To understand what motivates these massive date revisions, you have to imagine the sites surveyed by researchers. While the older Little Foot was found in a karst cave, the Member 4 fossils were found on the surface, as erosion had eventually stripped the ceiling of the cave in which they had accumulated. “The fossils are in a kind of cone of sediment and stones that fell into the cave through a hole in its ceiling, like an hourglass,” explains Laurent Bruxelles.

There too, the first datings had taken as reference calcite floors covering this millefeuille. But the geoarchaeologist, an expert in karstic stratigraphy, was able to show that these accumulations of calcite were misleading and that they had occurred much more recently. It was therefore necessary to resort again to an alternative dating technique, the same one that had done justice in 2015 to Little Foot: the so-called “cosmogenic” method, which is based on the measurement of the radioactive decay of certain minerals. .

The principle is as follows: the rocks that outcrop are naturally bombarded by cosmic rays and become charged with radioactive isotopes, in particular aluminum 26 and beryllium 10. When they fall into the cave, sheltered from cosmic rays, they begin to “discharge”. The measurement of the different isotope compositions makes it possible to date this fall. “The cone of pebbles which forms vertically from the opening of the cave has a very particular inclination, which allows us to know that the rocks and the fossils accumulated are very contemporary”, specifies Laurent Bruxelles.

Although an Australian team disputed this approach, as did Lee Berger, arch-rival of Ron Clarke (University of the Witwatersrand, Johannesburg), the discoverer of Little Foot and one of the co-signers of the PNAS article, the Frenchman defends the method: “We don’t question all dating: it’s not a posturing. In the cave of Swartkrans, 1 kilometer from Sterkfontein, he says, the date remains unchanged.

“Two regions for a cradle of humanity”

How to interpret the aging of the Sterkfontein fossils? “We’re turning things upside down, but we’re making them more coherent,” says Laurent Bruxelles. After Little Foot in 2015, this confirms that South African Australopithecines were contemporaneous with those of East Africa. Lucy is even younger than Mrs Ples. Will the squabble over the origin of the human branch be reignited? “This does not necessarily mean that we have two ‘cradles of humanity’ which would have evolved in a similar way, but rather that these two regions are part of a single and same cradle, on the scale of the continent this time. here,” says the researcher.

For Pascal Picq (Collège de France), who did not participate in this work, “dating in South Africa has always been problematic, and we felt that we would have to age them. These new results give them more consistency.” It shows that our evolutionary tree between 4 and 3 million years ago was “incredibly diverse, with five to seven species: that’s still a lot of people”!

This does not remove the mystery of the emergence of the genus Homo, whose oldest fossil to date, a mandible about 2.8 million years old, was found in 2013 in Ethiopia. But the South African australopithecines are now also older than him or later H. habilis and H. rudolfensis. It is confirmed that the group of South African gracile australopithecines was well adapted to a mosaic environment, of forest and savannah, where they could take advantage of their bipedal and arboreal characters, before a drying of the climate. And the 4,000 kilometers separating Lucy the Ethiopian and Mrs Ples the South African no longer appear as a totally impassable barrier.

Diverse groups of contemporary Australopithecines present on the north-south axis, but also east-west? Laurent Bruxelles is convinced of this. He prospected in Namibia, Zimbabwe and Mozambique, and leaves in the fall on a mission to Botswana, in search of ancient land.