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The recent African origin of modern humans is the mainstream paleoanthropological model describing the origin and early dispersal of anatomically modern humans, Homo sapiens sapiens. The theory popularly is called the (Recent) Out-of-Africa model, and academically the recent single-origin hypothesis (RSOH), Replacement Hypothesis, and Recent African Origin (RAO) model. The hypothesis that humans had originated in Africa was published in Charles Darwin's Descent of Man (1871). The concept was speculative until the 1980s, when it was corroborated by a study of present-day mitochondrial DNA, combined with evidence based on physical anthropology of archaic specimens.

According to both genetic and fossil evidence, archaic Homo sapiens evolved to anatomically modern humans solely in Africa, between 200,000 and 100,000 years ago, with members of one branch leaving Africa by 60,000 years ago and over time replacing earlier human populations such as Neanderthals and Homo erectus. According to this theory, around the above time frame, one of the African subpopulations went through a process of speciation, prohibiting gene flow between African and Eurasian Human populations. The recent single origin of modern humans in East Africa is the near-consensus position held within the scientific community.

The competing hypothesis is the multiregional origin of modern humans. Some push back the original "out of Africa" migration—in this case, by Homo erectus, not by Homo sapiens—to two million years ago. A recently discovered fossilized mandible that is putatively a hybrid between Homo sapiens and an earlier hominid, that is likely to be 110,000 years old, may also challenge this hypothesis.

History of the theory

The traditional biblical view was that humans had all descended from Adam and Eve, but with the development of anthropology in the early 19th century there was a vigorous dispute between the monogenism of those like Johann Friedrich Blumenbach and James Cowles Pritchard who held that since the creation, the various human races had developed as different varieties sharing descent, and the polygenism of their opponents such as Louis Agassiz and Josiah C. Nott who held that human races had been created as separate species, or had developed as separate species through transmutation of species from apes, with no common ancestor.

Charles Darwin was one of the first to propose common descent of living organisms, and among the first to suggest that all humans had in common ancestors who lived in Africa. In the Descent of Man he speculated that humans had descended from apes which still had small brains but walked upright, freeing their hands for use which favoured intelligence, and that these apes were African:

The prediction was insightful because at the time, in 1871, there were hardly any human fossils of ancient hominids available. Almost fifty years later Darwin's speculation was supported when anthropologists began finding numerous fossils of ancient small brained hominids in several areas of Africa (list of hominina fossils).

The debate in anthropology had swung in favour of monogenism by the mid 20th century. Isolated proponents of polygenism held forth in the mid-20th century, such as Carleton Coon, who hypothesized as late as 1962 that Homo sapiens arose five separate times from Homo erectus in five separate places. The "Recent African origin" of modern humans means "single origin" - monogenism - and has been used in various contexts as an antonym to polygenism .

With the advent of archaeogenetics in the 1990s, it became possible to date the "out of Africa" migration with some confidence until 2000, when the mtDNA sequence of Mungo Man was published. This work was later questioned and explained by W. James Peacock, leader of the team who sequenced Mungo man's ancient mtdna.

The question of whether there was inheritance of other typological (not de facto) Homo subspecies into the Homo sapiens genetic pool remains under debate.

Early Homo sapiens

Archaic Homo sapiens originated in Africa about 250,000 years ago. The trend in cranial expansion and the acheulean elaboration of stone tool technologies which occurred between 400,000 years ago and the second interglacial period in the Middle Pleistocene (around 250,000 years ago) provide evidence for a transition from H. erectus to H. sapiens. In the RAO scenario, migration within and out of Africa eventually replaced the earlier dispersed H. erectus.

Homo sapiens idaltu, found at site Middle Awashmarker in Ethiopia, lived about 160,000 years ago. It is the oldest known anatomically modern human and classified as extinct subspecies . Fossils of modern humans were found in Qafzehmarker cave in Israelmarker and have been dated to 100,000 years ago. However these humans seem to have either gone extinct or retreated back to Africa 70,000 to 80,000 years ago, possibly replaced by south bound Neanderthals escaping the colder regions of ice age Europe. Hua Liu & al. analyzing autosomal microsatellite markers dates to c. 56,000±5,700 years ago mtDNA evidence. The paleontological fossil of early modern human from Qafzeh cavemarker dated at 80,000–100,000 Liu interpret as isolated early offshoot that retracted back to Africa.

All other fossils of fully modern humans outside Africa have been dated to more recent times (80,000–100,000 year ago). The oldest well dated fossils found distal from Africa are from Lake Mungo, Australia, and have been dated to about 42,000 years ago. However, there are Tianyuan cave remains in Liujiang region China with a large date range between 38,000 and 42,000 years ago which are most similar in morphology to Minatogawa Man, anatomically modern humans that dated between 17,000 and 19,000 years ago, found on Okinawamarker Island, Japan. An alternative date of 111,000 to 139,000 years before present has been offered for Liujiang Man.

Beginning about 100,000 years ago evidence of more sophisticated technology and artwork begins to emerge and by 50,000 years ago fully modern behaviour becomes more prominent. By this time the ritual burying of the dead is noted . Stone tools show regular patterns that are reproduced or duplicated with more precision . Tools made of bone and antler appear for the first time .

Genetic reconstruction

Two pieces of the human genome are quite useful in deciphering human history: mitochondrial DNA and the Y chromosome. These are the only two parts of the genome that are not shuffled about by the evolutionary mechanisms that generate diversity with each generation: instead, these elements are passed down intact. According to the hypothesis, all 6.7 billion people alive today have inherited the same Mitochondria from one woman who lived in Africa about 160,000 years ago; she has been named Mitochondrial Eve. All men today have inherited their Y chromosomes from a man who lived 60,000 years ago, probably in Africa. He has been named Y-chromosomal Adam. It is now believed that more men participated in the out of Africa exodus of early humans than women based on comparing non-sex-specific chromosomes with sex-specific ones.

Mitochondrial DNA

One model of human migration based on Mitochondrial DNA


The first lineage to branch off from Mitochondrial Eve is L0. This haplogroup is found in high proportions among the San of Southern Africa, the Sandawe of East Africa. It is also found among the Mbuti people. These groups branched off early in human history and have remained relatively genetically isolated since then.Haplogroups L1, L2 and L3 are descendents of L0 and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descend from L3.

Y-chromosomal DNA

The mutations defining macro-haplogroup CT (all Y haplogroups except A and B) predate the "Out of Africa" migration, its descendent macro-group DE being confined to Africa. The mutations that distinguish Haplogroup C from all other descendants of CR have occurred some 60,000 years ago, shortly after the first Out of Africa migration.

Haplogroup F originated some 45,000 years ago, either in North Africa (in which case it would point to a second wave of out-of-Africa migration) or in South Asia. More than 90% of males not native to Africa are descended in direct male line from the first bearer of haplogroup F.

Genomic analysis

Although mitochondrial DNA and Y-chromosomal DNA are particularly useful in deciphering human history, data on the genomes of dozens of population groups have also been studied. In June 2009, an analysis of genome-wide SNP data from the International HapMap Project (Phase II) and CEPHmarker Human Genome Diversity Panel samples was published. Those samples were taken from 1138 unrelated individuals. Before this analysis, population geneticists expected to find dramatic differences among ethnic groups, with derived alleles shared among such groups but uncommon or nonexistent in other groups. Instead the study of 53 populations taken from the HapMap and CEPH data revealed that the population groups studied fell into just three genetic groups: Africans, Eurasians (which includes natives of Europe and the Middle East, and Southwest Asians east to present-day Pakistan), and East Asians, which includes natives of Asia, Japan, Southeast Asia, the Americas, and Oceania. The study determined that most ethnic group differences can be attributed to genetic drift, with modern African populations having greater genetic diversity than the other two genetic groups, and modern Eurasians somewhat more than modern East Asians. The study suggested that natural selection may shape the human genome much more slowly than previously thought, with factors such as migration within and among continents more heavily influencing the distribution of genetic variations.

Exodus from Africa

Some 70 millennia ago, a part of the bearers of mitochondrial haplogroupL3 migrated from East Africa into the Near East.

Some scientists believe that only a few people left Africa in a single migration that went on to populate the rest of the world. It has been estimated that from a population of 2,000 to 5,000 in Africa, only a small group of possibly 150 people crossed the Red Sea. This is because, of all the lineages present in Africa, only the daughters of one lineage, L3, are found outside Africa. Had there been several migrations one would expect more than one African lineage outside Africa. L3's daughters, the M and N lineages, are found in very low frequencies in Africa (although haplogroup M1 is very ancient and diversified in North and Northeast Africa) and appear to be recent arrivals. A possible explanation is that these mutations occurred in East Africa shortly before the exodus and by the founder effect became the dominant haplogroups after the exodus from Africa. Alternatively, the mutations may have arisen shortly after the exodus from Africa.

Other scientists have proposed a Multiple Dispersal Model, in which there were two migrations out of Africa, one across the Red Sea travelling along the coastal regions to India (the Coastal Route), which would be represented by Haplogroup M. Another group of migrants with Haplogroup N followed the Nile from East Africa, heading northwards and crossing into Asia through the Sinaimarker. This group then branched in several directions, some moving into Europe and others heading east into Asia. This hypothesis attempts to explain why Haplogroup N is predominant in Europe and why Haplogroup M is absent in Europe. Evidence of the coastal migration is hypothesized to have been destroyed by the rise in sea levels during the Holocene epoch. Alternatively, a small European founder population that initially expressed both Haplogroup M and N could have lost Haplogroup M through random genetic drift resulting from a bottleneck (i.e. a founder effect).

Today at the Bab-el-Mandeb straitsmarker the Red Seamarker is about 12 miles (20 kilometres) wide, but 50,000 years ago it was much narrower and sea levels were 70 meters lower. Though the straits were never completely closed, there may have been islands in between which could be reached using simple rafts. Shell middens 125,000 years old have been found in Eritreamarker, indicating the diet of early humans included seafood obtained by beachcombing.

Subsequent expansion

From the Near East, these populations spread east to South Asia by 50,000 years ago, and on to Australia by 40,000 years ago, Homo sapiens for the first time colonizing territory never reached by Homo erectus. Europe was reached by Cro-Magnon some 40,000 years ago. East Asia (Korea, Japan) was reached by 30,000 years ago. It is disputed whether subsequent migration to North America took place around 30,000 years ago, or only considerably later, around 14,000 years ago.

The group that crossed the Red Sea travelled along the coastal route around the coast of Arabia and Persiamarker until reaching India, which appears to be the first major settling point. M is found in high frequencies along the southern coastal regions of Pakistanmarker and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred. Sixty percent of the Indian population belong to Haplogroup M. The indigenous people of the Andaman Islandsmarker also belong to the M lineage. The Andamanese are thought to be offshoots of some of the earliest inhabitants in Asia because of their long isolation from mainland Asia. They are evidence of the coastal route of early settlers that extends from India along the coasts of Thailandmarker and Indonesia all the way to Papua New Guineamarker. Since M is found in high frequencies in highlanders from New Guinea as well, and both the Andamanese and New Guineans have dark skin and Afro-textured hair, some scientists believe they are all part of the same wave of migrants who departed across the Red Sea ~60,000 years ago in the Great Coastal Migration. Notably, the findings of Harding et al. (2000, p. 1355) show that, at least with regard to dark skin color, the haplotype background of Papua New Guineans at MC1R (one of a number of genes involved in melanin production) is identical to that of Africans (barring a single silent mutation). Thus, although these groups are distinct from Africans at other loci (due to drift, bottlenecks, etc), it is evident that selection for the dark skin color trait likely continued (at least at MC1R) following the exodus. This would support the hypothesis that suggests that the original migrants from Africa resembled pre-exodus Africans (at least in skin color), and that the present day remnants of this ancient phenotype can be seen among contemporary Africans, Andamanese and New Guineans. Others suggest that their physical resemblance to Africans could be the result of convergent evolution.

From Arabia to India the proportion of haplogroup M increases eastwards: in eastern India, M outnumbers N by a ratio of 3:1. However, crossing over into East Asia, Haplogroup N reappears as the dominant lineage. M is predominant in South East Asia but amongst Indigenous Australians N reemerges as the more common lineage. This discontinuous distribution of Haplogroup N from Europe to Australia can be explained by founder effects and population bottlenecks.

Competing hypotheses

The multiregional hypothesis, initially proposed by Milford Wolpoff, holds that the evolution of humans from H. erectus at the beginning of the Pleistocene 1.8 million years BP to the present day has been within a single, continuous worldwide population. Proponents of multiregional origin reject the assumption of a infertility barrier between ancient Eurasian and African populations of Homo. Multiregional proponents point to the fossil record and genetic evidence in chromosomal DNA. One study suggested that at least 5% of the human modern gene pool can be attributed to ancient admixture, which in Europe would be from the Neanderthals.

See also



References

  1. Hua Liu, et al. A Geographically Explicit Genetic Model of Worldwide Human-Settlement History. The American Journal of Human Genetics, volume 79 (2006), pages 230–237, quote: Currently available genetic and archaeological evidence is generally interpreted as supportive of a recent single origin of modern humans in East Africa. However, this is where 'the near consensus on human settlement history ends, and considerable uncertainty clouds any more detailed aspect of human colonization history.
  2. Donald Johanson, "Origins of Modern Humans: Multiregional or Out of Africa?", Action Bioscience.org
  3. Joseph D'Agnese, "Not Out of Africa, Alan Thorne's challenging ideas about human evolution", Discover, Aug 2002
  4. Phil McKenna, "Chinese challenge to 'out of Africa' theory", New Scientist, Nov 2009
  5. Jackson Jr., John P. (2001). "In Ways Unacademical": The Reception of Carleton S. Coon's The Origin of Races, University of Colorado
  6. Mitochondrial DNA sequences in ancient Australians: Implications for modern human origins. PNAS 10/30/2000: Gregory J. Adcock, Elizabeth S. Dennis, Simon Easteal, Gavin A. Huttley, Lars S. Jermiin, W. James Peacock, Alan Thorne; url:http://www.pnas.org/content/98/2/537.full.pdf
  7. Cooper, A., Rambaut, A., Macaulay, V., Willerslev, E., Hansen, A. & Stringer, C. 2001. Human origins and ancient human DNA. Science 292: 1655-1656
  8. http://www.anthro.utah.edu/PDFs/courses/oconnell/3152/cooper2001.pdf O'Connell, University of Utah
  9. Hua Liu, et al. A Geographically Explicit Genetic Model of Worldwide Human-Settlement History f.a. The American Journal of Human Genetics, volume 79 (2006), pages 230–237,
  10. Ancestral tools
  11. Middle to upper paleolithic transition
  12. Cann, R. L., Stoneking, M., Wilson, A. C. (1987) Mitochondrial DNA and human evolution. Nature. Vol. 325. Pp. 31-36.
  13. Vigilant, L., Stoneking, M., Harpending, H., Hawkes, K., Wilson, A. C. (1991) African Populations and the Evolution of Human Mitochondrial DNA. Science. Vol. 253. Pp 1503-1507
  14. Ancient African Exodus Mostly Involved Men, Geneticists Find. Nature Genetics. Harvard Medical School.
  15. Whole-mtDNA Genome Sequence Analysis of Ancient African Lineages
  16. mtDNA Variation in the South African Kung and Khwe
  17. Both Australian Aborigines and Europeans Rooted in Africa
  18. A single origin, several dispersal hypothesis
  19. Searching for traces of the Southern Dispersal, by Dr. Marta Mirazón Lahr, et al.
  20. Most of the extant mtDNA boundaries in South and Southwest Asia were likely shaped during the initial settlement of Eurasia by anatomically modern humans
  21. Evolution of Human Languages
  22. genetic origins of the Andaman Islanders
  23. Genetic affinities of the Andaman Islanders
  24. Mitochondrial Genome Variation and Evolutionary History of Australian and New Guinean Aborigines
  25. Plagnol V, Wall JD: Possible ancestral structure in human populations. PLoS Genet 2006, 2:e105.[1]; quote: ..strong evidence for ancient admixture in both a European and a West African population (p ≈ 10−7), with contributions to the modern gene pool of at least 5%. While Neanderthals form an obvious archaic source population candidate in Europe..


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