Out of Eden, essay by Stephen Oppenheimer
<i><b>Stephen Oppenheimer of Oxford University is a leading proponent of the view that all non-Africans are descended from just one single group who journeyed out of Africa about 80,000 years ago to spread and inhabit every corner of the world. Out of Eden is based on his work which maps and plots that incredible journey in time and space â across land, river and ocean â through a combination of genetics, archaeology and climate study. </b></i>
We have all wondered why people around the world look so different and what possible ancestral relationships exist between them. Do all the human âracesâ derive from several geographic sources or just one? And if we are all related, how did we get to be where we are today?
The dominant view is that the ancestors of all modern humans expanded recently from Africa to replace all archaic human types throughout the world. The Out-of-Africa theory was originally based on study of variation in anatomical features, but recent genetic study has provided a solid scientific foundation for the hypothesis.
<b>Gene markers</b>
In recent years, geneticists have succeeded in building large âfamily treesâ of individual genes and their variants, using technology that sequences our DNA. This research is helped enormously by the fact that two tiny elements of our DNA pass intact without mixing from generation to generation â the Y Chromosome through the male line from our fathers, and âmitochondrial DNAâ (mtDNA) through the female line by our mothers. The first major breakthrough in using these two gene systems to trace our genetic prehistory was made 15 years ago by Hawai'ian geneticist Rebecca Cann and colleagues. They published the first mtDNA tree showing that all modern humans traced back to a single African female ancestor, often labelled âMitochondrial Eve. The mtDNA finding was subsequently mirrored for the Y Chromosome or the Adam chromosome.
Since mutations occur at a constant â though random â rate, it was then possible to date not only the branches, but the base of the tree. The age of the modern human tree was less than 200,000 years. This confirmed the âGarden of Edenâ theory that modern humans, arising in Africa, had only recently replaced all earlier human species throughout the world.
<b>One Exodus </b>
The Adam and Eve trees also have the power to answer more detailed geographic questions of how we came out of Africa; which exit route did we take, how many exits were there and where did we go next? The most important advance in the past five years was the fine resolution of the mtDNA tree by English geneticist Dr Martin Richards with colleagues from England, New Zealand and Germany. This showed only one of the multiple African mtDNA branches peopling the whole of the rest of the world. Geneticists have confirmed a similar finding for several other gene trees including the Y Chromosome. Logically this single branch pattern makes it extremely unlikely there was more than one Exodus. In other words, only one band successfully made it out to colonise the rest of the world.
The implications of only one Exodus are enormous. First, it means all non-African peoples including Europeans, Indians, Chinese, Australians and Native Americans are related, and are recent descendants of that one small family band. Secondly we can now begin to get an idea of the genetic trail from Africa, which could tell us not only when they left and where they went, but how each modern regional group is related to the others.
It is not quite as easy as that, however. A mitochondrial genetic tree is, in reality, more like several strands of creeping ivy branching over the Earth. Multiple genetic lines migrated in parallel, thus eroding, even further, out-moded concepts of âraceâ. One region will share strands of different genetic branches with neighbouring regions; but each region has its own unique new growth. It is the new twigs and leaves that tell us where people migrated to and where they went after that. Genetic branches, however, only have approximate dates; so the reconstruction of these ancient migrations has to be matched with archaeology, the dramatic effects of changing climate and natural geographic corridors and barriers.
<b>Two routes out</b>
I have taken this reconstruction right back to Africaâs doorstep. There are only two routes out of sub-Saharan Africa to Asia, one up the Nile corridor, through Egypt and the Suez to the Levant, and the other in the South across the mouth of the Red Sea and along the Arabian coast to the Yemen and Oman. <b>For most of the past 100,000 years, the Syrian and Arabian deserts separated South Asia from the Levant and Europe. So, taking the northern route meant that emigrants could only go further north to Europe and the Caucasus. Taking the southern route meant continuing along the coast of the Indian Ocean to India, the Far East and Australia. </b>
<b>Archaeology and climate both favour the southern route. Australia was colonised at least 20,000 years before Europe.</b> If there was only one Exodus through the northern route, Europe should have been colonised earlier. <b>My argument on the reason for the late colonisation of Europe is that the ancestors of West Eurasians had to wait somewhere in South Asia, such as the Arabian Gulf, until the climatic amelioration after 50,000 years ago allowed them to make their way north to the East coast of the Mediterranean. The genetic evidence is consistent with this delay.</b>
There are other elements to the climatic argument. Between 80,000-50,000 years ago sea levels were lower and the short crossing of the mouth of the Red Sea directly from sub-Saharan Africa would have been less hazardous than crossing the Sahara desert to North Africa. Recent evidence of systematic beachcombing near the mouth of the Red Sea on the west coast, stretching back as far as 125,000 years ago, supports this.
<b>The clincher is the genetic evidence. Reasonably we might suppose that our emigrant band from Africa would leave a trace of their early genetic branches at the start of their trail. There is no evidence for this in the northern route through North Africa, to the Levant or Europe. Furthermore one of the two earliest mtDNA branches outside Africa, the Asian âMâ branch is virtually absent in these places. By contrast in India, the first major dispersal point along the Southern route, we find all the earliest genetic branches outside Africa and at great diversity and antiquity. </b>
Dating the Exodus is another matter. Increasing genetic evidence from East Asia and the Antipodes suggests the Exodus could be as old as 80,000 years, but the genetic dating method lacks precision. Here the archaeology may help me in my synthesis. A picture is growing of rapid spread of beachcombers round the Indian Ocean over land-bridges through Indonesia to Bali. 70,000 years ago a severe glaciation briefly locked up enough water to lower the sea level by 80 metres, taking the coast of Timor to within 100 miles of Australia. Archaeological evidence for the earliest occupation of Australia by modern humans suggests that this is the only time they could have got across.
Other dates further back on the trail support this 80,000 year Exodus. An anatomically modern human skull from Liujiang in South China was recently re-dated to over 70,000 years ago. There is increasing consensus that certain pebble tools only appear in South East Asia with the arrival of modern humans over 70,000 years ago. One group of these tools was found in Kota Tampan in the Malay Peninsula, encased in volcanic ash from the great Toba eruption, now accurately dated to 74,000 years, indicating the presence of modern humans already halfway to Australia.
Later events â such as the peopling of the Americas and the drama of the last Ice Age â are also illuminated by the new genetic tools. Many archaeological preconceptions, such as the size of Neolithic expansions resulting from animal domestications and agriculture, can now be held up to the scrutiny of the tree that tracks people. The study of archaeology is changing forever.
An unabridged version of this article can be found in the July 2002 issue
of Geographical Magazine (www.geographical.co.uk).
<i><b>Stephen Oppenheimer of Oxford University is a leading proponent of the view that all non-Africans are descended from just one single group who journeyed out of Africa about 80,000 years ago to spread and inhabit every corner of the world. Out of Eden is based on his work which maps and plots that incredible journey in time and space â across land, river and ocean â through a combination of genetics, archaeology and climate study. </b></i>
We have all wondered why people around the world look so different and what possible ancestral relationships exist between them. Do all the human âracesâ derive from several geographic sources or just one? And if we are all related, how did we get to be where we are today?
The dominant view is that the ancestors of all modern humans expanded recently from Africa to replace all archaic human types throughout the world. The Out-of-Africa theory was originally based on study of variation in anatomical features, but recent genetic study has provided a solid scientific foundation for the hypothesis.
<b>Gene markers</b>
In recent years, geneticists have succeeded in building large âfamily treesâ of individual genes and their variants, using technology that sequences our DNA. This research is helped enormously by the fact that two tiny elements of our DNA pass intact without mixing from generation to generation â the Y Chromosome through the male line from our fathers, and âmitochondrial DNAâ (mtDNA) through the female line by our mothers. The first major breakthrough in using these two gene systems to trace our genetic prehistory was made 15 years ago by Hawai'ian geneticist Rebecca Cann and colleagues. They published the first mtDNA tree showing that all modern humans traced back to a single African female ancestor, often labelled âMitochondrial Eve. The mtDNA finding was subsequently mirrored for the Y Chromosome or the Adam chromosome.
Since mutations occur at a constant â though random â rate, it was then possible to date not only the branches, but the base of the tree. The age of the modern human tree was less than 200,000 years. This confirmed the âGarden of Edenâ theory that modern humans, arising in Africa, had only recently replaced all earlier human species throughout the world.
<b>One Exodus </b>
The Adam and Eve trees also have the power to answer more detailed geographic questions of how we came out of Africa; which exit route did we take, how many exits were there and where did we go next? The most important advance in the past five years was the fine resolution of the mtDNA tree by English geneticist Dr Martin Richards with colleagues from England, New Zealand and Germany. This showed only one of the multiple African mtDNA branches peopling the whole of the rest of the world. Geneticists have confirmed a similar finding for several other gene trees including the Y Chromosome. Logically this single branch pattern makes it extremely unlikely there was more than one Exodus. In other words, only one band successfully made it out to colonise the rest of the world.
The implications of only one Exodus are enormous. First, it means all non-African peoples including Europeans, Indians, Chinese, Australians and Native Americans are related, and are recent descendants of that one small family band. Secondly we can now begin to get an idea of the genetic trail from Africa, which could tell us not only when they left and where they went, but how each modern regional group is related to the others.
It is not quite as easy as that, however. A mitochondrial genetic tree is, in reality, more like several strands of creeping ivy branching over the Earth. Multiple genetic lines migrated in parallel, thus eroding, even further, out-moded concepts of âraceâ. One region will share strands of different genetic branches with neighbouring regions; but each region has its own unique new growth. It is the new twigs and leaves that tell us where people migrated to and where they went after that. Genetic branches, however, only have approximate dates; so the reconstruction of these ancient migrations has to be matched with archaeology, the dramatic effects of changing climate and natural geographic corridors and barriers.
<b>Two routes out</b>
I have taken this reconstruction right back to Africaâs doorstep. There are only two routes out of sub-Saharan Africa to Asia, one up the Nile corridor, through Egypt and the Suez to the Levant, and the other in the South across the mouth of the Red Sea and along the Arabian coast to the Yemen and Oman. <b>For most of the past 100,000 years, the Syrian and Arabian deserts separated South Asia from the Levant and Europe. So, taking the northern route meant that emigrants could only go further north to Europe and the Caucasus. Taking the southern route meant continuing along the coast of the Indian Ocean to India, the Far East and Australia. </b>
<b>Archaeology and climate both favour the southern route. Australia was colonised at least 20,000 years before Europe.</b> If there was only one Exodus through the northern route, Europe should have been colonised earlier. <b>My argument on the reason for the late colonisation of Europe is that the ancestors of West Eurasians had to wait somewhere in South Asia, such as the Arabian Gulf, until the climatic amelioration after 50,000 years ago allowed them to make their way north to the East coast of the Mediterranean. The genetic evidence is consistent with this delay.</b>
There are other elements to the climatic argument. Between 80,000-50,000 years ago sea levels were lower and the short crossing of the mouth of the Red Sea directly from sub-Saharan Africa would have been less hazardous than crossing the Sahara desert to North Africa. Recent evidence of systematic beachcombing near the mouth of the Red Sea on the west coast, stretching back as far as 125,000 years ago, supports this.
<b>The clincher is the genetic evidence. Reasonably we might suppose that our emigrant band from Africa would leave a trace of their early genetic branches at the start of their trail. There is no evidence for this in the northern route through North Africa, to the Levant or Europe. Furthermore one of the two earliest mtDNA branches outside Africa, the Asian âMâ branch is virtually absent in these places. By contrast in India, the first major dispersal point along the Southern route, we find all the earliest genetic branches outside Africa and at great diversity and antiquity. </b>
Dating the Exodus is another matter. Increasing genetic evidence from East Asia and the Antipodes suggests the Exodus could be as old as 80,000 years, but the genetic dating method lacks precision. Here the archaeology may help me in my synthesis. A picture is growing of rapid spread of beachcombers round the Indian Ocean over land-bridges through Indonesia to Bali. 70,000 years ago a severe glaciation briefly locked up enough water to lower the sea level by 80 metres, taking the coast of Timor to within 100 miles of Australia. Archaeological evidence for the earliest occupation of Australia by modern humans suggests that this is the only time they could have got across.
Other dates further back on the trail support this 80,000 year Exodus. An anatomically modern human skull from Liujiang in South China was recently re-dated to over 70,000 years ago. There is increasing consensus that certain pebble tools only appear in South East Asia with the arrival of modern humans over 70,000 years ago. One group of these tools was found in Kota Tampan in the Malay Peninsula, encased in volcanic ash from the great Toba eruption, now accurately dated to 74,000 years, indicating the presence of modern humans already halfway to Australia.
Later events â such as the peopling of the Americas and the drama of the last Ice Age â are also illuminated by the new genetic tools. Many archaeological preconceptions, such as the size of Neolithic expansions resulting from animal domestications and agriculture, can now be held up to the scrutiny of the tree that tracks people. The study of archaeology is changing forever.
An unabridged version of this article can be found in the July 2002 issue
of Geographical Magazine (www.geographical.co.uk).