Human exodus may have reached China 100,000 years ago

Catherine Brahic, New Scientist, 08/08/14

OUR direct ancestors may have found their way out of Africa much earlier than we think. As new fossil remains emerge from China and south-east Asia, the traditional story of how we left Africa is being challenged.

The accepted view is that Homo sapiens evolved in Africa 200,000 years ago and stayed there until 60,000 years ago, when they struck out through the Middle East and spread around the world. Any older hominin bones found outside Africa are deemed dead ends. So although the more primitive Homo erectus made it all the way to Indonesia, and probably gave rise to the Neanderthals and the Denisovans, all of these lines eventually died out. Our own species evolved solely in Africa.

The evidence for this comes partly from dating bones to specific periods, but also from genetics. As you move further away from Africa, across Asia and then the Americas, the genetic diversity of indigenous populations drops. This implies that the source population was in Africa and gradually lost diversity as it expanded. The genetics suggest that human genes went through this bottleneck 60,000 years ago.

The "out of Africa at 60,000 years ago" scenario remains the majority view. But the orthodoxy is slowly being challenged as ancient bones are uncovered in the east. As yet, though, they remain few and far between.

Last month, Christopher Bae of the University of Hawaii at Manoa, Wei Wang of the Guangxi Museum of Nationalities in Nanning, China, and their colleagues described two teeth from the Luna cave in China's Guangxi Zhuang region. Based on the proportions of the teeth, the team argue that at least one of them must have belonged to an early Homo sapiens (Quaternary International, doi.org/tz5).

The teeth are clearly old. Calcite crystals, which formed as water flowed over the teeth and the cave floor, date them to between 70,000 and 125,000 years ago. So Bae and Wang say they are evidence of an early wave of modern humans in eastern Asia.

But it is not clear that the teeth belong to modern humans. "I am not convinced that these teeth are diagnostic," says Erik Trinkaus of Washington University in St Louis, Missouri. He says too little is known about how teeth evolved over the millennia in Asia.

However, other fossils are more convincing. Bones found in Israel, including an upper jaw from Misliya cave, could be 150,000 years old. In China, Trinkaus was involved in the identification of a jawbone and two molars from Zhirendong, a cave in Guizhou province (PNAS, doi.org/fxbjgj). Though the bone is over 100,000 years old, Trinkaus's team says the shape of its chin is suggestive of modern humans.

"There is solid evidence of modern humans at Tam Pa Ling [in Laos] around 50,000 or 60,000 years ago, and the Zhirendong mandible has modern features," says Trinkaus. "So yes, modern humans were present in at least south-east Asia and south China by somewhere in this time range."

Though the Asian fossils are Homo sapiens-like, another species could have evolved these features in parallel, says John Hawks of the University of Wisconsin-Madison. But he also doubts there was a single, rapid exodus 60,000 years ago.

A closer look at the genetics also suggests there was an earlier migration. Recently, Katerina Harvati of the University of Tubingen in Germany and her colleagues tested the classic "out of Africa at 60,000 years ago" story against the earlier-exodus idea. They plugged the genomes of indigenous populations from south-east Asia into a migration model. They found that the genetic data was best explained by an early exodus that left Africa around 130,000 years ago, taking a coastal route along the Arabian peninsula, India and into Australia, followed by a later wave along the classic route (see map) (PNAS, doi.org/tz6).

An early exit from Africa is still very much a minority view, says Chris Stringer at the Natural History Museum in London. But he says Harvati's evidence has left him "open to the possibility".

This article appeared in print under the headline "Humanity's forgotten pioneers"

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