Australopithecus fossils older than thought

Mrs Ples

Skull of the Australopithecus woman “Mrs. Ples”. © Jason L. Heaton/ Birmingham-Southern College

More Australopithecus fossils have been found in Sterkfontain Cave in South Africa than anywhere else. Now new dating using rare isotopes reveals that the strata and the Australopithecus fossils preserved within them are around a million years older than previously thought. According to the new dating, they are around 3.4 million years old instead of only around two to 2.6 million years as previously assumed. This means that the pre-humans from Sterkfontain are as old as the representatives of Australopithecus from East Africa and even older than the famous “Lucy”. This also sheds new light on the evolution of this pre-human species and their successors.

Sterkfontain Cave is part of the UNESCO World Heritage Site of the ‘Cradle of Mankind’ in South Africa, a collection of several caves where unique fossils of pre- and early humans have been found. The first skeleton of an adult Australopithecus was discovered in Sterkfontain in 1936. Since then, scientists have found hundreds of other Australopithecus specimens there, including such famous specimens as “Mrs. Ples” or “Little Foot”. The latter is the most complete Australopithecus skeleton known to date and was found in one of the lower chambers of Sterkfontain Cave. While the majority of the fossils are from Australopithecus africanus and were found in the surface Member 4 chamber, Little Foot is from a deeper part of the cave.

Re-dating using “cosmogenic nuclides”

Although the importance of the Sterkfontain Cave and its fossil finds is undisputed, the dating of the finds has so far been less clear. “More Australopithecus fossils are found in Sterkfontain than anywhere else in the world,” says lead author Darryl Granger of Purdue University in the US. “But it’s difficult to date them correctly.” Previous dating attempts were based on dating animal bones found in close proximity to the fossils. They revealed widely differing age data between one and four million years. A uranium-lead dating of limestone deposits and stalactites from the most fossil-rich “Member 4” chamber, however, resulted in an age of only two to 2.65 million years. However, this would mean that these Australopithecus representatives lived in South Africa at the same time as the first representatives of the genus Homo. Granger and his team therefore suspected that both animal bones and the calcareous deposits did not reflect the age of the fossils, but only later accumulated in the cave chamber.

For their study, the scientists have made a new dating based on a different method. They analyzed the stony attachments of the fossils for their content of certain rare isotopes. These so-called cosmogenic nuclides aluminum-26 and beryllium-10 are formed in the rock-forming mineral quartz as long as it is exposed to high-energy cosmic radiation on the earth’s surface. “As these rocks fell into the cave with the fossils, they began to radioactively decay,” Granger explains. The content of these isotopes therefore reveals how long the rocks surrounding the fossils have been covered by overlying deposits. “With this method, we can more precisely assign primeval people and their relatives to a correct time period,” says the researcher. Based on this method, the team had already redated the famous skeleton of “Little Foot” in 2015 – to an age of 3.7 million years.

Older than “Lucy”

For the “Member 4” chamber of Sterkfointain and the Australopithecus fossils found in it, the new dating revealed: The cave and its contents are around 3.41 million years old – around a million years older than previously thought. The pre-humans of Sterkfontain were therefore not latecomers of their genus, but rather early representatives of the australopithecines. “The new dating suggests that the Sterkfontain hominins were contemporaneous with other early Australopithecus species, including Australopithecus afarensis in East Africa,” says co-author Dominic Stratford of the University of the Witwatersrand in Johannesburg. “Thus, the dating also pushes the age of one of South Africa’s best-known fossils, Mrs. Ples, a million years into the past.” 2 million year old “Lucy” found in East Africa and belonging to the species Australopithecus afarensis.

According to the research team, the re-dating of the Sterkfontain fossils also influences the idea of ​​the evolution of human ancestors in Africa. Because in addition to East Africa, South Africa was also one of the centers of development early on. “The reassessment of the age of the Australopithecus fossils from Member 4 at Sterkfontain has significant implications for South Africa’s role in hominin evolution,” says Stratford. Until now, the Sterkfontain Australopithecines had been considered too young to be the direct ancestors of the Paranthropus fossils found nearby and the first representatives of the genus Homo. “It was therefore thought that Homo and Paranthropus must have evolved in East Africa,” the researcher continues. But now it is clear that these genera have had almost a million years to evolve in South Africa from the locally existing Australopithecus species. “The re-dating will therefore undoubtedly reignite the debate as to whether the Australopithecus representatives from Sterkfontain could have been the ancestors of later hominins in South Africa,” says Granger.

Source: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2123516119

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