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Ancient humans were living in the highlands of what is now Ethiopia as early as 2 million years ago. A reanalysis of a fossilised jawbone from the region confirms that it belonged to a Homo erectus, and represents the earliest evidence of hominins living in such high-altitude areas.

The highlands represent “a third pole for human evolution in Africa”, says of the Italo-Spanish Archaeological Mission at Melka Kunture and Balchit, based in Rome. Hominins have been found in large numbers in eastern and southern Africa, but not to date in upland areas.

Mussi and her colleagues re-examined the lower jawbone of an infant, which was discovered in 1981 at a site called Garba IV in the Ethiopian highlands. Garba IV is one of a cluster of sites known collectively as Melka Kunture. Mussi has nicknamed the jawbone “Little Garba”.

The jawbone had previously been identified as an early member of the genus Homo, which includes our own species Homo sapiens and several now-extinct groups. However, it wasn’t possible to confidently identify the species.

Mussi and her team used synchrotron imaging to study Little Garba’s teeth, which hadn’t yet erupted from the jawbone. They compared the shape of the teeth to those of multiple hominin species. “The teeth are a very good marker, so we can say for sure this is indeed an early Homo erectus,” says Mussi.

In , Mussi’s team also re-dated the Garba IV site. It consists of layers of sediment laid down over time. In the sediments, the researchers found traces of past shifts in Earth’s magnetic field, which could be matched to similar records elsewhere. Based on this, they conclude that Little Garba is 2 million years old. This makes it one of the oldest H. erectus ever found.

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It may even be that the species evolved in the highlands. “We don’t know if Homo erectus evolved at lower altitudes and came up, or if it evolved locally,” says Mussi.

Furthermore, the researchers re-examined the stone tools found in the sediments at Garba IV. They say there is a transition from older and simpler Oldowan tools to more sophisticated Acheulean tools between 2 and 1.95 million years ago. The Acheulean tools were harder to make because they required careful planning, but they opened up a wider range of foods.

Putting these lines of evidence together, Mussi argues that the H. erectus population had to adapt to conditions in the highlands, and developed new styles of stone tools to do so.

The identification of Little Garba as H. erectus looks solid, says at the University of Bordeaux in France. He is less convinced by the transition from Oldowan to Acheulean, because there aren’t many Oldowan tools in the older layers.

For Zanolli, “the most exciting aspect” of the study is the highland location. “It’s the oldest [hominin] we know to have reached the high plateau of Ethiopia,” he says. While it is possible that future excavations might find even older hominins in the area, “for now it’s the earliest”.

Garba IV is about 2000 metres above sea level, which isn’t so high that low oxygen levels would be a major challenge, says Zanolli. It would have been colder than lowland areas, but the lowlands were also desert, so the highlands may have been more hospitable. “In this high-altitude environment you have more trees, more bushes, probably more animals,” he says. “So it’s very likely easier to find food and to survive there.”

Millennia later, H. erectus became the first known hominin to expand its population beyond Africa, reaching Dmanisi in what is now Georgia by 1.8 million years ago, and Java in what became Indonesia by perhaps 1.3 million years ago. Living in the Ethiopian highlands may have been good preparation, says Mussi. If Homo erectus was able to adapt to this environment, it could also live in other cooler regions away from the equator, she says.