By Richard Gray, Science Correspondent
Published: 9:00PM BST 09 May 2009
The entire human race outside Africa owes its existence to the survival of a single tribe of around 200 people who crossed the Red Sea 70,000 years ago, scientists have discovered.
Research by geneticists and archaeologists has allowed them to trace the origins of modern homo sapiens back to a single group of people who managed to cross from the Horn of Africa and into Arabia. From there they went on to colonise the rest of the world.
Genetic analysis of modern day human populations in Europe, Asia, Australia, North America and South America have revealed that they are all descended from these common ancestors.
It is thought that changes in the climate between 90,000 and 70,000 years ago caused sea levels to drop dramatically and allowed the crossing of the Red Sea to take place.
Dr Peter Forster, a senior lecturer in archaeogenetics at Anglia Ruskin University in Cambridge who carried out some of the genetic work, said: "The founder populations cannot have been very big. We are talking about just a few hundred individuals."
Homo sapiens, known casually as "modern humans", are thought to have first evolved around 195,000 years ago in east Africa – the earliest remains from that time were uncovered near the Omo River in Ethiopia.
It is thought that by 150,000 years ago these early modern humans had managed to spread to other parts of Africa and fossilised remains have been found on the Cape of Good Hope in South Africa.
The earliest homo sapien remains found outside of Africa were discovered in Israel and are thought to be around 100,000 years old. They are remains of a group that left Africa through what is now the Sahara desert during a brief period when the climate grew wetter, turning the desert green with vegetation. This excursion, however, failed and the population died out when the climate started to dry out again.
While there are 14 ancestral populations in Africa itself, just one seems to have survived outside of the continent. The latest genetic research has shown that it was not until around 70,000 years ago that humans were able to take advantage of falling sea levels to cross into Arabia at the mouth of the Red Sea, which is now known as the Gate of Grief.
At the time the 18 mile gap between the continents would have dropped to just 8 miles. It is not clear how they might have made such a journey but once a cross, the humans were able to spread along the Arabian coast where fresh water springs helped support them.
It has long been assumed that humans success in spreading around the world was due to their adaptability and hunting skills. The latest research, however, suggests that the very early human pioneers who ventured out of Africa owe far more of their success to luck and favourable changes in climate change than had previously been realised.
Dr Stephen Oppenheimer, a geneticist at the school of anthropology at Oxford University who has also led research on the genetic origins of humans outside Africa, said: "What you can see from the DNA of all non Africans is that they all belong to one tiny African branch that came across the Red Sea.
"If it was easy to get out of Africa we would have seen multiple African lineages in the DNA of non-Africans but that there was only one successful exit suggests it must have been very tough to get out. It was much drier and colder then."
Within around 5,000 years some of these early human pioneers had managed to spread along the edge of the Indian Ocean and down through south east Asia and arriving in Australia around 65,000 years ago.
Others made their way north through the Middle East and Pakistan to reach central Asia.
Around 50,000 years ago they also began spreading into Europe via the Bosporus at the Istanbul Strait. Again low sea levels allowed them to almost walk into Europe.
Once there they will have encountered Neanderthals, who, with bigger bodies were more adapted to the cold weather at the time, had been living in Europe for nearly a quarter of a million years but are thought to have died out due to changes in the climate.
By 25,000 years ago humans had spread into northern Europe and Siberia and then walked across the Bering land bridge into Alaska around 20,000 years ago.
The peak of the last ice age, which was reached around 19,000 years ago, saw human populations pushed south by the extreme cold and it was about 15,000 years ago that South America became the last continent on the planet to be colonised.
Britain and northern Scandinavia is thought to have been recolonised by modern humans after the last ice age between 10,000 and 8,000 years ago.
Dr Alice Roberts, an anatomist at Bristol University said: "There seems to have been a huge amount of luck involved as they were totally at the whim of the climate. The climate changed at just the right time to allow them to expand out of Africa and they were allowed to expand geographically as a result, but when the climate changed they shrank back again."
The idea that all non-African humans are descended from a single group of individuals contradicts previous theories that the different modern races evolved seperately from an earlier human ancestor known as Homo erectus in different parts of the world. Archaeologists in China, for example, believe they have strong evidence that the Chinese evolved directly from a lineage of Homo erectus that arrived in China 2 million years ago and not from African Homo sapiens.
But recent genetic work at Fudan University in Shanghai tested the Y chromosomes of more than 12,000 men currently living in different parts of China and found that they all descended from the original African humans.
Professor Li Jin, a geneticist at Fudan University in Shanghai whose laboratory carried out the research, said: “We did not find a single individual that could be considered the decendants of homo erectus in China.
“I think we should all be happy with that, as afterall, it means that people from all over the world are not all that different from each other.”
DNA shows Aborigines descended from Africans
Wednesday, May 9, 2007
Source: University of Cambridge
New research confirms theory that all modern humans are descended from the same small group of people
Researchers have produced new DNA evidence that almost certainly confirms the theory that all modern humans have a common ancestry. The genetic survey, produced by a collaborative team led by scholars at Cambridge and Anglia Ruskin Universities, shows that Australia's aboriginal population sprang from the same tiny group of colonists, along with their New Guinean neighbours.
The research confirms the "Out Of Africa" hypothesis that all modern humans stem from a single group of Homo sapiens who emigrated from Africa 2,000 generations ago and spread throughout Eurasia over thousands of years. These settlers replaced other early humans (such as Neanderthals), rather than interbreeding with them.
Academics analysed the mitochondrial DNA (mtDNA) and Y chromosome DNA of Aboriginal Australians and Melanesians from New Guinea. This data was compared with the various DNA patterns associated with early humans. The research was an international effort, with researchers from Tartu in Estonia, Oxford, and Stanford in California all contributing key data and expertise.
The results showed that both the Aborigines and Melanesians share the genetic features that have been linked to the exodus of modern humans from Africa 50,000 years ago. Until now, one of the main reasons for doubting the "Out Of Africa" theory was the existence of inconsistent evidence in Australia. The skeletal and tool remains that have been found there are strikingly different from those elsewhere on the "coastal expressway" – the route through South Asia taken by the early settlers.
Some scholars argue that these discrepancies exist either because the early colonists interbred with the local Homo erectus population, or because there was a subsequent, secondary migration from Africa. Both explanations would undermine the theory of a single, common origin for modern-day humans. But in the latest research there was no evidence of a genetic inheritance from Homo erectus, indicating that the settlers did not mix and that these people therefore share the same direct ancestry as the other Eurasian peoples.
Geneticist Dr Peter Forster, who led the research, said: "Although it has been speculated that the populations of Australia and New Guinea came from the same ancestors, the fossil record differs so significantly it has been difficult to prove. For the first time, this evidence gives us a genetic link showing that the Australian Aboriginal and New Guinean populations are descended directly from the same specific group of people who emerged from the African migration."
At the time of the migration, 50,000 years ago, Australia and New Guinea were joined by a land bridge and the region was also only separated from the main Eurasian land mass by narrow straits such as Wallace's Line in Indonesia. The land bridge was submerged about 8,000 years ago.
The new study also explains why the fossil and archaeological record in Australia is so different to that found elsewhere even though the genetic record shows no evidence of interbreeding with Homo erectus, and indicates a single Palaeolithic colonisation event. The DNA patterns of the Australian and Melanesian populations show that the population evolved in relative isolation. The two groups also share certain genetic characteristics that are not found beyond Melanesia. This would suggest that there was very little gene flow into Australia after the original migration.
Dr Toomas Kivisild, from the Cambridge University Department of Biological Anthropology, who co-authored the report, said: "The evidence points to relative isolation after the initial arrival, which would mean any significant developments in skeletal form and tool use were not influenced by outside sources.
"There was probably a minor secondary gene flow into Australia while the land bridge from New Guinea was still open, but once it was submerged the population was apparently isolated for thousands of years. The differences in the archaeological record are probably the result of this, rather than any secondary migration or interbreeding."
The study is reported in the new issue of Proceedings of the National Academy of Sciences.
Notes :
1. Homo sapiens originated in Africa 150,000 years ago and began to migrate 55,000 to 60,000 years ago. It is thought he arrived in Australia around 45,000 years before present (BP). Australia was, at the time, already colonised by homo erectus. The eastern migration route towards Australia is referred to as the "coastal express" route, due to the comparatively rapid progress made by those who used it. This dispersal, from Africa to Australia through Arabia, Asia and the Malay peninsula, could have occurred at a rate of 1km per year.
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