Fossilised remains belonging to some of humanity's oldest ancestors are far older than scientists had originally thought, new research says.
The fossils, including one belonging to ancient cave woman Mrs Ples, were buried for millennia in South African caves known as the Cradle of Humankind.
Modern testing methods now suggest the group of early humans roamed the earth between 3.4 and 3.7 million years ago.
This new timeline could reshape common understandings of human evolution.
It means there are now more possible ways by which our ancestors could have evolved into early humans.
For years scientists believed the Australopithecus africanus species, whose fossils were discovered in the Sterkfontein caves near Johannesburg, had been less than 2.6 million years old.
The underground complex has yielded more of the early human remains than anywhere else in the world – including the near complete skull discovered in 1947 belonging to an early cave woman nicknamed Mrs Ples.
According to the Smithsonian Museum, the species – which walked on two feet – were much shorter than modern-day humans. Males averaged 4ft 6in (138cm) in height and females averaged 3ft 9in (115cm).
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But new radioactive dating techniques suggest Mrs Ples, and the others discovered around her, are actually one million years older than thought.
Researchers came up with the revised age by testing sediment around the fossils for levels of a rare isotope created when the rocks were exposed to cosmic rays – before they fell into the cave.
Previously the Australopithecus africanus hominids were considered by scientists to be too young to have evolved into homo genus, our ancestors, who were already roaming earth around 2.2 million years ago.
These findings now suggest they had one million additional years to make that evolutionary leap – making it a possibility that Mrs Ples, and the species she was part of, were ancestors of early humans.
As a result the species existed on Earth at the same time as the ape known as Lucy, whose 3.2-million-year-old remains belonging to Africa's Australopithecus afarensis were long considered to be the species that gave rise to the earliest humans.
The updated timeline means the two species could have interacted and bred, scientists say, complicating our picture of where humans came from and suggesting it may not have been such a simple evolutionary line.
It means our family tree is "more like a bush," said French scientist Laurent Bruxelles who was part of the study.
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