Single-origin hypothesis

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In paleoanthropology, the single-origin hypothesis (or Out-of-Africa model) is one of two accounts of the origin of anatomically modern humans, Homo sapiens. (The other theory is the multiregional hypothesis, which includes the Hybrid-origin theory)

Image:Map-of-human-migrations.jpg

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Pre-modern (non-sapiens) hominids

Because of the scarcity of fossils and the discovery of important new finds every few years, researchers disagree about the details and sometimes even basic elements of human evolutionary history. While they have revised this history several times over the last decades, researchers currently agree that the oldest named species of the genus Homo, Homo habilis, evolved in Africa around two million years ago, and that members of the genus migrated out of Africa somewhat later. The descendants of these ancient migrants, which probably included Homo erectus, have become known through fossils uncovered far from Africa, such as those of "Peking man" and "Java man". The Homo neanderthalensis is also considered a descendant of early migrants.

African evidence of "modern" humans

According to the single-origin model, however, every species of the genus Homo but one, Homo sapiens, was driven extinct. This species had evolved in eastern Africa between 100,000 and 200,000 years ago and, some time afterwards, in a relatively recent exodus, began colonizing the rest of the world. According to the single-origin model, these more recent migrants did not interbreed with the scattered descendants of earlier exoduses. For this reason, the model is sometimes called the "replacement scenario". In support of it, advocates have drawn from both fossil and DNA evidence, in particular from mitochondrial and Y-chromosome DNA sequences. Supporting evidence has now been found that systhesises new genetic, archaeological, climatic and linguistic discoveries. Genetic diversity amongst modern human populations is greatest in the African continent. There are 15 surviving mtDNA lineages dating before 80,000 years ago in Africa, and only one for the rest of the world, that descending from L3. This clearly suggests an early radiation of anatomically modern humans within Africa, before they were found in any other continent. Based upon point source mutations on mitochondrial DNA (mtDNA) and the Human Y chromosome, the oldest changes in our genome took place in Africa 150,000 - 190,000 years ago. Archaeologically this would suggest that anatomically modern humans (AMH) evolved from the archaic human lineage similar to that previously called Homo rhodesiensis. The find of 3 skulls near the [Herto] village in Ethiopia was previously thought to date the earliest AMH persons yet found, dated to 160,000 yrs ago. However more recent work on tools and human remains found at [Kibish], at Omo River (Ethiopia), previously thought by Richard Leakey to date to about 130,000 years ago, on the basis of careful re-examination of potassium-argon dating at Omo 1, is now thought to date to 195,000 years ago, making it a good candidate to be a site type for AMH ancestors.

Genetically, ethnically, linguistically and now archaeologically four distinct African populations on the basis of the L0 mutation in mtDNAcan within the human genome can be determined:

  • The Khoisan speakers of Capoid race, L1 = mtDNA, M91 = Y chromosome (chr) Haplogroup A genomes of Southern Africa - led to the evolution of modern, Hazda of Tanzania and the Bushmen (San) and Hottentot (Khoikhoi) speaking populations. It has been suggested that this group preserves many of the characteristics of the ancestral lineage of modern humans not found in other populations. Similarly the click languages of the Khoisan peoples are unique (having over 130 phonemes compared to English 40) and may show features subsequently lost in other human populations. This group may have descended from populations similar to those found at the [Border Cave], Klassies River, and the Lion Cave in Southern Africa, where using a long-lived Middle Paleolithic technology showed dietary diversity characteristic of Homo sapiens, with evidence already of some trading in goods with neighbouring groups. The dominant technology of this region was called Sangoan after the type site at Sango Bay Uganda, or Tumbian. The assemblage of Sangoan technology persisted from 120,000 to 10,000 years ago. Blombos cave, excavated between 1997 and 2004, has been shown to date to before 70,000 years ago, but has a typically Middle Paleolithic cultural assemblage, possibly related to the nearby Stillbay assemblages. Nevertheless a range of bone tools, finely crafted bifacial stone points, ochre pieces engraved with deliberate designs, an engraved bone fragment and evidence for 'modern' subsistence practices including fishing, typical of European Upper Paleolithic cultures from 40,000 years ago. An engraved tally stick of 77,000 years ago is the first recorded evidence of human counting systems anywhere in the world. The Apollo 11 Cave in Namibia, occupied for 70,000 years, contains ochre incisions dated to 100,000 years before the present, the first evidence of human art anywhere in the world.
  • The West African genomes of the Niger Congo and Nilo-Saharan speaking peoples (mtDNA = L2 (59-78,000 yrs ago), Y chr = M60 (began 50-60,000 yrs ago)) - led to the evolution of the Ivory and Slave Coast West African Negro and modern Bantu populations. This population adopted cultural innovations suitable for rainforest and woodland zones. Adaptation to these ecological regions was clearly well established as early as 88,000 with the finding of the Katanda harpoon, the first in the world, which led to the development of the Semliki harpoon culture.
  • The genomes of Mbuti and Bayaka pygmy peoples of the Ituri and other Congo rainforests also shares the M60 Y chr, L2 mtDNA - led to the evolution of the smaller groups surviving in the Congo rainforests. It seems that this group possibly diverged at an early stage from the West African genomes as there are a number of characteristics common to the two groups. Although the pygmies are only a tiny fraction of the African population, L2, found only in Africa, is 1/3rd of the total population of the continent. Unfortunately few fossils have been found in the woodland and rainforest areas, in part due to the acidity of the soil and the poor fossilisation conditions.
  • The Ethiopian mtDNA L3 genomes of the Cushitic speaking peoples - this group is probably the one from which all other groups of human beings on the Earth (predominantly derived from M and N Haplogroups) descended, about 80,000 years ago. Y chr M168 (dated between 30-79,000 yrs ago), which first appeared in this group is also found in Yemen and Southern Arabia, being evidence of the first wave "Out of Africa". L3 may have come from human populations similar to those found at Ngaloba and Laetoli and dated to 120,000 years before the present. This group appears to have had more in common with the Khoisanid populations than with the West African and Mbuti pigmy L2 lineages. By 125 AMH humans were already demonstrating the dietary flexibility for which they are famous, at Yuga Bay, on the Zuli Gulf of Eritrea, shell middens are proof of the importance of shell fish, in what has been called “the world’s first oyster bar”.

The warm interglacial and the green Sahara allowed humans living in Ethiopia and the southern Sudan to travel north, crossing the Sinai Peninsula into modern Israel 115,000 years ago. This forms the basis of the Sahul populations found in this area. A sudden freezing of the world’s climates in about 90,000 years ago, saw the return of severe arid conditions to the Middle East, with the loss of major game, and when the dry spell ended, the cold conditions saw the replacement of anatomically modern humans found at Jebel Qafseh in Israel, with cold adapted Neanderthal populations some of which have been found at Skhul in the Mount Carmel caves.

Research on the X chromosome claims to be the first genetic evidence against this model, suggesting an alternative model incorporating admixture between divergent African branches of the genus Homo.

Single exodus from Africa?

Assuming only relatively recent migrants from Africa gave rise to today's non-African humans, was there more than one migration that left descendants? (for example, one each via the north and south ends of the Red Sea)

There are two possible routes out of Africa.

1. The first and most obvious one is from Egypt, across the Sinai into the Levant. This route is confronted by the major impediment of the arid zone of the Sahara and Sinai deserts, and thus tends to be only passable during the short periods of interglacial optimum when the Sahara is covered by fresh water lakes, rivers and abundant game.

2. The second route, only opened when sea levels fall is across the Bab-el-Mandeb, between Yemen and Djibouti. This route too is confronted by a barrier, this time the Red Sea and its hazardous reefs, and so is usually only opened when there is a major fall in sea levels. Although, humans must have had ocean-going vessels at least 60,000 years ago to reach Australia, which was separated by hundreds of miles of ocean even at the ocean's lowest level, so it is also possible that humans had vessels capable of crossing a gap of ocean at the strait of Aden not long earlier. This area, at the times at which sea levels were low, is also an area of high aridity, probably keeping a beachcombing human population close to the ancient shorlines, now well below sea-level, making the finding of early human fossils here very difficult.

Genetic evidence suggests humans left Africa only once. The mtDNA M* and N* haplogroups derive from haplogroup L3, and suggest a single exit from Africa. The distribution of the M158 Y chromosome haplotype of the "Eurasian Adam" shows a similar history, dating to the period between 30-79,000 years ago. In the period in which it is thought, on genetic evidence, humans left Africa, between 85 and 75,000 years ago, paleoclimatological evidence suggests a vast belt of desert stretched from the West African Atlantic to the Eastern Siberian Pacific. These deserts kept AMH humans confined south of that line, and reduced the food returns to cultures based exclusively on grassland and woodland based hunter gatherer technologies. The M* haplogroups seems to support the existence of this barrier. The drop in sea levels at the time did open up the second route out of Africa, and the growth of a beachcombing lifestyle, confirmed at the perched coastline shell-middens at Zuli Bay, did allow dietary supplementing of hunting of large game with highly nutritious shellfish.

Low glacial sea levels at this period would have been the first time in millennia permitting a dry walk from the Gulf of Aden to the islands East of Java facing Australia. Among Y-chromosomal haplogroups, the M130 and the M174 YAP gene haplogroups in particular confirm this hypothesis as their path traces a great arc along the shorelines of Saudi Arabia, India, South East Asia and Australia. This beachcomber culture moved on through southern China, and Taiwan to Japan and Eastern Siberia. There about 8-10,000 years ago the M130 haplogroup was carried by Na-Dené speaking peoples into the North West Pacific coast of America.

Multiregional hypothesis

The opponents of a single origin argue that interbreeding indeed occurred, and that the characteristics of modern humans, including those that have been and still are perceived by some to distinguish races, could only be the result of genetic contributions from several earlier lineages that evolved semi-independently in different parts of the world. This is the "multiregional hypothesis", including the hybrid-origin theory. While the genetic evidence is making this theory increasingly hard to support, it is politically attractive to some groups who see it as a way of establishing a much more ancient claim of connection of habitation of their area; for example, the idea of descendency of modern Chinese from Peking Man is promoted by the government of the People's Republic of China.

Proponents of the single-origin hypothesis

See also

Further reading

  • Cavalli-Sforza, Luigi Luca and Francesco Cavalli-Sforza, The Great Human Diasporas – The History of Diversity and Evolution (Italian original Chi Siamo: La Storia della Diversit`a Umana), ISBN 0-201-44231-0 (paperback), 1993.
  • Crow, Tim J, Editor The Speciation of Modern Homo Sapiens, ISBN 0-19-726311-9 (paperback) 2002.
  • Foley, Robert, Humans Before Humanity, ISBN 0-631-20528-4 (paperback), 1995.
  • Oppenheimer, Stephen, The Real Eve: Modern Man's Journey Out of Africa, ISBN 0-786-71192-2 (Hardcover), 2003.
  • Stringer, Chris and Robin McKie, African Exodus, ISBN 0-7126-7307-5 (paperback), 1996.
  • Sykes, Brian, The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry (2002)
  • Wells, Spencer, The Journey of Man: A Genetic Odyssey (2003)

References

External links

nl:Single-origin hypothesis zh:单地起源说