The fragmented remains of ancient permanent teeth, unearthed in China and parts of south-east Asia, reveal that the popular "out of Africa" hypothesis – which suggests that our modern human ancestors, known as hominins, migrated from Africa about 60,000 years ago – needs revising.
Elaborate dating tests and investigations of the teeth, including one right upper second molar and one left lower second molar, reveal that our ancestors departed their African homelands as much as 120,000 and perhaps even 130,000 years ago.
The teeth, discovered in a cave, called Lunadong, in China's autonomous region of Guangxi Zhuang, may be as old as 126,000 years. At least one tooth is almost certain to have belonged to a member of modern Homo sapiens, the species of bipedal primates to which modern humans belong.
One of us: A model of the 3.2 million-year-old hominid known as Lucy reveals what our early ancestors looked like. Photo: AP Photo/Pat Sullivan
(Among other things, Homo sapiens have a brain capacity averaging 1400 cubic centimetres and rely on the use of language and relatively complex tools.)
"The Lunadong modern Homo sapien's teeth contribute to growing evidence that modern and/or transitional humans were likely in eastern Asia … [during] a period that some researchers have suggested no hominins were present in the region," the research team, led by anthropologist Christopher Bae of the University of Hawaii, writes in the respected journal Quaternary International.
"The primary point of our paper is that the human evolutionary record, particularly when accounting for increasing finds in eastern Asia, is a lot more complicated than generally believed," Associate Professor Bae says. "There were probably multiple dispersals of modern humans out of Africa and into Eurasia, with some degree of interbreeding occurring."
The finding also gives a clearer idea of the route our ancient forebears took after leaving Africa, he points out.
"Most research currently suggests that modern humans took a southern route once they left Africa and travelled more or less along the Arabian Peninsula before arriving in south-east Asia," Associate Professor Bae says.
"There may have been a second later dispersal into north-west Asia, where those groups eventually moved into Europe and along the northern Asian route eventually arriving in Siberia and then on to the Americas."
Do the team's findings support claims that Australia's Aborigines might have been here for 100,000 years or longer?
"Unfortunately, no," he replies. "Current data indicates that Australia was probably only peopled sometime after 60,000 years ago – although one occasionally comes across an Australian find supposedly dated much earlier than that."
Fellow experts have reacted with interest to the latest research. "The early colonisation of Australia, now documented at between 50,000 and 60,000 years ago, makes more sense if the movement of humans out of Africa was substantially earlier," says Peter Hiscock, the Tom Austen Brown Chair of Australian Archaeology at the University of Sydney.
The latest find, he explains, is part of a broader revision of the chronology of the "out of Africa" dispersion. "Anatomically modern humans found in caves in Israel, and dated to more than 100,000 years ago, have long suggested either multiple migrations out of Africa or else a need to revise the chronology of the dispersion," Professor Hiscock says.
Very roughly 25 million years ago, the family of primates known as the hominoids, or human-like animals, first surfaced in Africa. The particular line to which humans belong diverged from that of the gorillas and chimpanzees between five and seven million years ago – although we still share 98.4 per cent of our make-up with modern chimps.
Roughly 200,000 years ago, modern humans, who are anatomically indistinguishable from us, emerged in Africa. Then, as most textbooks would have it, about 60,000 years ago, humans hunted and gathered their way out of Africa and on to other parts of the world.
The latest discovery of teeth in China means that the textbook version of events almost certainly needs re-writing.
It is thought by 40,000 years ago, anatomically modern humans had replaced more archaic populations, such as Neanderthals, our closest extinct human relatives, to become a globally distributed species, says Simon Armitage from Royal Holloway, a University of London college.
Based on analyses from around the world, it had been suggested that modern humans moved into Arabia and southern Asia sometime after 65,000 years ago.
Why so long?
Assuming the latest finding that modern humans left Africa between 120,000 and 130,000 years ago is correct, why would they have taken more than 100,000 years to reach other regions?
Between about 75,000 and 55,000 years ago, archaeologists believe, a series of remarkable technological and cultural innovations occurred in southern Africa. They included personal ornamentation, such as perforated shells, art in the form of engraved ochre pieces, for instance, and more effective hafted hunting weapons – and perhaps the systematic exploitation of marine fish.
Such innovations, it has been suggested, probably helped modern humans to survive in less familiar environments – and to out-compete pre-existing archaic populations.
An earlier study, led by Professor Hans-Peter Uerpmann of Eberhard Karls University in Tübingen, Germany, and published in the journal Science, describes findings from an eight-year archaeological excavation at Jebel Faya in the United Arab Emirates.
The researchers analysed the Palaeolithic stone tools found there and concluded that they were technologically similar to instruments produced by early modern humans in east Africa. But they were notably different from tools created to the north, in the Levant and the mountains of Iran.
This provides compelling evidence that early modern humans migrated into Arabia directly from Africa and not via the Nile Valley and the Near East, as usually suggested, Dr Armitage says. The direct route from east Africa to Jebel Faya crosses the southern Red Sea and the flat, waterless Nejd Plateau of the southern Arabian interior, both of which present major obstacles to human migration.
Another team member, Professor Adrian Parker of Oxford Brookes University, studied detailed records relating to sea levels and climate change for the region and concluded that the direct migration route may have been passable for brief periods in the past.
Between 140,000 and 130,000 years ago, the Red Sea was about 100 metres lower than today – due to vast quantities of water being stored on land as ice during the second to last Ice Age. "So the seaway that separates east Africa from Arabia – the Bab-el-Mandab Straits – would have shrunk to roughly five to 10 kilometres in width," Dr Armitage explains. "By about 130,000 years ago, southern Arabia was much wetter than it is now."
"Archaeology without ages is like a jigsaw with the interlocking edges removed," Dr Armitage explains. "You have lots of individual pieces of information – but can't fit them together to produce the big picture."
He calculated that stone tools at Jebel Faya were about 125,000 years old using a technique called luminescence dating. This measured the time since sediment surrounding the artefacts was last exposed to light, allowing him to determine when the artefacts were buried.
The dates obtained revealed that modern humans were at Jebel Faya about 125,000 years ago, immediately after the Bab al-Mandab seaway and Nejd Plateau were passable.
The evidence suggests modern humans left Africa, crossed the Bab-el-Mandab Straits and occupied southern Arabia by roughly 125,000 years ago. They left because the Bab-el-Mandab Straits were passable and southern Arabia was wet enough to be habitable. These conditions only occur together at the transition from the penultimate glacial to the present interglacial – that is, about 130,000 years ago.
Once modern humans reached Jebel Faya, they would have needed to cross the Straits of Hormuz to reach Asia.