A fossil of a lower jaw bone found in Ethiopia is about 400,000 years older than other fossils from the earliest known Homo line that ultimately led to modern human.
Scientists who made the discovery, reported in Science, about 400 kilometres outside Addis Ababa believe the individual lived about 2.8 million years ago.
If correct, that dates the individual to the period when ape-like animals were transitioning into the tool-using forbears of humans.
“This is the the first inkling we have of that transition to modern behaviour. We were no longer solving problems with our bodies but with our brains,” Brian Villmoare at the University of Nevada in Las Vegas, told The Guardian.
The fossil has some features in common with an ancient forerunner of modern humans called Australopithecus afarensis, such as the famous three-million-year-old Lucy – but also has more modern traits.
Scientists now believe that about three million years ago, the ape-like Australopithecus afarensis died out and was superseded by two very different human form – Paranthropus, with a small brain, large teeth and strong jaw muscles for chewing its food – and the Homo lineage, which much larger brains.