A lower jaw bone and five teeth discovered on a hillside in Ethiopia are the oldest remains ever found that belong to the genus Homo, the lineage that ultimately led to modern humans.
Fossil hunters spotted the jaw poking out of a rocky slope in the dry and dusty Afar region of the country about 250 miles from Addis Ababa.
The US-led research team believes the individual lived about 2.8m years ago, when the now parched landscape was open grassland and shrubs nourished by tree-lined rivers and wetlands.
The remains are about 400,000 years older than fossils which had previously held the record as the earliest known specimens on the Homo lineage.
The discovery sheds light on a profoundly important but poorly understood period in human evolution that played out between two and three million years ago, when humans began the crucial transformation from ape-like animals into forms that used tools and eventually began to resemble modern 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,” said Brian Villmoare at the University of Nevada in Las Vegas.
The new fossil, found at a site called Ledi-Geraru, has a handful of primitive features in common with an ancient forerunner of modern humans called Australopithecus afarensis. The most well-known specimen, the 3m-year-old Lucy, was unearthed in 1974 in Hadar, only 40 miles from the Ledi-Geraru site. But the latest fossil has more modern traits too. Some are seen only on the Homo lineage, such as a shallower chin bone.
The picture that emerges from the fossil record is that 3m years ago, the ape-like Australopithecus afarensis died out and was superseded by two very different human forms. One, called Paranthropus, had a small brain, large teeth and strong jaw muscles for chewing its food. The other was the Homo lineage, which found itself with much larger brains, a solution that turned out to be more successful.
“By finding this jaw bone we’ve figured out where that trajectory started,” said Villamoare. “This is the first Homo. It marks in all likelihood a major adaptive transition.”
What drove Australopithethus to extinction and led to the rise of Homo is a mystery, but researchers suspect a dramatic change in the environment transformed the landscape of eastern Africa. “It could be that there was some sort of ecological shift and humans had to evolve or go extinct,” said Villmoare.
Other fossils recovered nearby the new human remains suggest that the region was much wetter than Hadar where Lucy was found. Remnants of antelopes, prehistoric elephants, primitive hippos, crocodiles and fish were all recovered from the Ledi-Geraru site, researchers said. Details of the discoveries are reported in two papers published in Science.
The human jaw was discovered in January 2013 by Chalachew Seyoum, an Ethiopian national on the team, and a student at Arizona State University. He was part of a group that had set off from camp that morning to look for fossils on a hill that was later found to be brimming with ancient bones.
Villamoare, who was on the expedition, recalled the moment of discovery. “I heard people yelling Brian! Brian! And I went round the corner and there was Chalachew. He recognised it, and said: ‘We’ve got a human.’ It had eroded out of the stratigraphy. It was in two pieces and was missing some of the teeth, but it was clearly of the genus Homo.”
The fossil bones are too fragmentary to give them a human species name. The jawbone could belong to Homo habilis, known as “handy man”, the earliest known species on the Homo lineage. But Villamoare is not convinced. It could be a new species that lived before Homo habilis.
Other researchers agree. In a separate paper published in Nature, Fred Spoor at University College, London, reports a virtual reconstruction of a Homo habilis skull. “By digitally exploring what Homo habilis really looked like, we could infer the nature of its ancestor, but no such fossils were known,” said Spoor. “Now the Ledi-Geraru jaw has turned up as if on request, suggesting a plausible evolutionary link between Australopithecus afarensis and Homo habilis.”
But until more remains are found, the mystery will remain. The US-led team has been back to the site this January to look for more fossils, but Villamoare said he cannot yet talk about what they did or did not find.