Recent single-origin hypothesis

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Map of early human migrations according to mitochondrial population genetics (numbers are millennia before present); The letters on the arrows represent groups of people belonging to the same mitochondrial haplogroup. Haplogroups are a type of genetic categorization that groups people together base on shared variation in their mitochondrial DNA.
Map of early human migrations according to mitochondrial population genetics (numbers are millennia before present); The letters on the arrows represent groups of people belonging to the same mitochondrial haplogroup. Haplogroups are a type of genetic categorization that groups people together base on shared variation in their mitochondrial DNA.

In paleoanthropology, the recent single-origin hypothesis (RSOH, or Out-of-Africa model, or Replacement Hypothesis) is one of two accounts of the origin of anatomically modern humans, Homo sapiens. According to the RSOH, anatomically modern humans evolved in Africa before 200,000 to 100,000 years ago, with members of one branch leaving Africa about 80,000 years ago. These emigrants spread to the rest of the world, replacing (and not interbreeding with) other Homo species already there, such as Neanderthals and Java man.[1]

The alternative theory, multiregional hypothesis, including the Hybrid-origin theory, was rendered largely obsolete with archaeogenetic evidence collected since the 1990s.

Contents

[edit] History

In 1994 Marta Mirazon Lahr and Robert Foley wrote that, although not all would agree, evidence for a single recent origin was accumulating. They theorized that the pattern of diversity behavioural, linguistic, morphological and genetic could be interpreted as supporting the recent single-origin hypothesis.[2] Soon after, a 1996 study of DNA sequences of Alu elements indicated a virtual lack of sequence polymorphism, further supporting a recent single origin for modern humans, with general replacement of archaic populations.[3] A 2005 study stated that the recent dispersal of modern humans out of Africa was at that time "widely accepted"[4] and in 2006 Hua Liu, Franck Prugnolle, Andrea Manica, and François Balloux wrote that "currently available genetic and archaeological evidence is generally interpreted as supportive of a recent single origin of modern humans in East Africa."[5]

[edit] 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, at least 1.5 million years ago. 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". Homo neanderthalensis is also considered a descendant of early migrants. The earliest recorded hypothesis of the single-origin theory originated in 1789 by Lord Monboddo, who based his reasoning on linguistic evolution[6]; Monboddo, however, did not have sufficient data to deduce Africa as the origin, but rather proposed some location east of Europe.

[edit] 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 exodi. 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; it synthesizes new genetic, archaeological, climatic, and linguistic discoveries. Genetic diversity amongst modern human populations is greatest in the African continent.[7][8][9] There are 15 surviving mtDNA lineages in Africa which date back more than 80,000 years, compared with only one for the rest of the world, that descending from L3.[citation needed] 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, the species classified as Homo sapiens idaltu dated to 154,000 and 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. By 125,000 years ago, AMH were already demonstrating the dietary flexibility for which they are famous, at Yuga Bay[10], on the Zula Gulf of Eritrea, shell middens are proof of the importance of shell fish, in what has been called “the world’s first oyster bar”.

Three distinct African populations can be determined genetically on the basis of the L0 mutation in mtDNA, aligning with ethnic, linguistic and now archaeological evidence.

  • mtDNA L1, Y-chromosome haplogroup A = M91 evolved into the Khoisan speakers - the modern Hadza of Tanzania and the Bushmen (San) and Khoikhoi (once called "Hottentot") 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[citation needed]. This group may have descended from populations similar to those found at the Border Cave, Klasies River Caves[11], 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.
  • mtDNA L2 (85-95,000 yrs ago)[1], Y chr B = M60 (began 50-60,000 yrs ago): 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 years ago with the finding of the Katanda harpoon, the first in the world, which led to the development of the Semliki harpoon culture. 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 West African genomes of the Niger Congo and Nilo-Saharan speaking peoples - led to the evolution of the Ivory and West Africa and modern Bantu populations.
    • Also sharing the Y chr B , mtDNA L2 are the smaller groups surviving in the Congo rainforests, the Mbuti and Bayaka pygmy peoples of the Ituri and other 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.
  • mtDNA L3 may have come from human populations similar to those found at Ngaloba and Laetoli and dated to 59-69,000 years before the present[2]. This group appears to have had more in common with the Khoisanid populations than with the West African and Mbuti pigmy L2 lineages.
    • This group evolved into the 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 mtDNA M, Y chr C/D and mtDNA N, Y chr F*) descended, diverging shortly after it emerged. Y chr M168 first appeared in this group, being evidence of the first wave "Out of Africa". 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[citation needed]. 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 Es Skhul in the Mount Carmel caves.

A paper on X chromosome claims to have produced evidence suggesting an alternative model incorporating admixture between divergent African branches of the genus Homo. In this paper the existence of a 1 million year old, non-recombined, 17.5 kilobase sequence of DNA from the X chromosomes of two African people is used in support of a human origin resulting from admixture between archaic and modern humans. [3]

[edit] 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/Eritrea. 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 a minimum of 80 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 shorelines, now well below sea-level, making the finding of early human fossils here very difficult.

The environmental mechanism by which these migrations were accomplished seems to involve some variation of the "Sahara Pump Theory", based upon an association of African pluvials and interpluvial periods, associated with accompanying changes in sea level.

Genetic evidence suggests Homo sapiens left Africa in prehistoric times 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 30,000 - 79,000 years ago. In the period in which it is thought, on genetic evidence, humans left Africa, between 75,000 and 85,000 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.(citation needed) 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.

[edit] Multiregional Theory

The Multiregional theory is based largely on archaeological and fossil evidence. Propenents of the Multiregional theory argue that physical similarities between modern and archaic humans, such as the brow ridges and comparatively robust skeletons of some modern Europeans (as compared to Neanderthals), and the shovel-shaped incisors and other distinct craniofacial features noted in modern Chinese (as compared with Chinese homo erectus), could only be the result of genetic contributions from earlier lineages that evolved semi-independently. The "multiregional hypothesis" often includes the hybrid-origin theory. Opponents, however, cite the lack of DNA evidence supporting these theories.

In 1997, DNA testing performed on a Neanderthal skeleton showed modern humans and Neanderthals last shared a common ancestor between 500,000 and 800,000 years ago, and furthermore that all modern humans, from the ethnic Siberians to the !Kung people of Africa, are more closely related to each other than to the Neanderthals---further evidence supporting the Out-of-Africa theory.

[edit] Proponents of the single-origin hypothesis

[edit] See also

[edit] Line note references

  1. ^ Origins of Modern Humans: Multiregional or Out of Africa? By Donald Johanson
  2. ^ Multiple dispersals and modern human origins Marta Mirazon Lahr and Robert Foley 1994
  3. ^ DNA sequences of Alu elements indicate a recent replacement of the human autosomal genetic complement
  4. ^ Single, Rapid Coastal Settlement of Asia Revealed by Analysis of Complete Mitochondrial Genomes Science 13 May 2005: Vol. 308. no. 5724, pp. 1034 - 1036
  5. ^ A Geographically Explicit Genetic Model of Worldwide Human-Settlement History Hua Liu, Franck Prugnolle, Andrea Manica, and François Balloux The American Journal of Human Genetics, volume 79 (2006), pages 230–237
  6. ^ William Knight, Lord Monboddo and some of his contemporaries, Letter of Lord Monboddo to William Jones dated June 20, 1789, Thoemmes Press, Bristol, England (1900) ISBN 1-85506-207-0
  7. ^ Risch et al. 2002 state A clearer picture of human evolution has emerged from numerous studies over the past decade using a variety of genetic markers and involving indigenous populations from around the world. In summary, populations outside Africa derive from one or more migration events out of Africa within the last 100,000 years. The greatest genetic variation occurs within Africans, with variation outside Africa representing either a subset of African diversity or newly arisen variants.
  8. ^ Long and Kittles, 2003 state that This later value implies that the Sokoto [in Nigeria] possesses more gene diversity than that harbored by all of the non-African populations combined together.
  9. ^ Tishkoff and Kidd, 2004 state Most studies of genetic variation in autosomes, the X chromosome and mtDNA, using many types of markers, show higher levels of genetic variation in African populations than in non-African populations....Additionally, studies of autosomal1 and X-chromosomal haplotype variation, as well as mtDNA variation, indicate that Africans have the largest number of population-specific alleles and that non-African populations carry only a fraction of the genetic diversity that is present in Africa. This would be expected if there were a genetic bottleneck at the time of migration of modern humans out of Africa.
  10. ^ Early Human Occupation on the Red Sea Coast of Eritrea www.nature.com/nature/journal/v405/n6782/full/405065a0.html
  11. ^ Fleming Patrick J., "Human Biological and Cultural Origins" (Revised version of the paper presented at the Eight Annual Meeting of the Language Origin Society, Selwyn College, University of Cambridge, UK, 7-10 September 1992)

[edit] 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 0-7867-1192-2 (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)

[edit] References

[edit] External links

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