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In paleoanthropology, the recent African origin of modern humans is the mainstream 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.^{[citation needed]} The recent single origin of modern humans in East Africa is the near-consensus position held within the scientific community.^[1]

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.^[2][3] 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.^[4]

Contents 1 History of the theory 2 Early Homo sapiens 3 Genetic reconstruction 3.1 Mitochondrial DNA 3.2 Y-chromosomal DNA 3.3 Genomic analysis 4 Exodus from Africa 5 Subsequent expansion 6 Competing hypotheses 7 See also 8 References 9 Further reading 10 External links

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:^[5]

“

In each great region of the world the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject, for an ape nearly as large as a man, namely the Dryopithecus of Lartet, which was closely allied to the anthropomorphous Hylobates, existed in Europe during the Upper Miocene period; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.[6]

”

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.^[7] 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.^[8] This work was later questioned^[9] and explained by W. James Peacock, leader of the team who sequenced Mungo man's ancient mtdna.^[10]

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 Awash in Ethiopia, lived about 160,000 years ago.^[11] It is the oldest known anatomically modern human and classified as extinct subspecies^{[clarification needed]}. Fossils of modern humans were found in Qafzeh cave in Israel 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.^{[citation needed]} 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 cave dated at 80,000–100,000 Liu interpret as isolated early offshoot that retracted back to Africa.^[12]

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.^[13][14] 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 Okinawa Island, Japan.^[15][16] An alternative date of 111,000 to 139,000 years before present has been offered for Liujiang Man.^[17]

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^{[citation needed]}. Stone tools show regular patterns that are reproduced or duplicated with more precision^{[citation needed]}. Tools made of bone and antler appear for the first time^{[citation needed]}.^[18][19]

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^{[citation needed]} from one woman who lived in Africa about 160,000 years ago;^[20][21] 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.^[22]

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.^[23][24] 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 CEPH Human Genome Diversity Panel samples was published.^[25] Those samples were taken from 1138 unrelated individuals.^[25] 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.^[26] 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.^[26] 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.^[26] 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.^[27]

Exodus from Africa

Some 70 millennia ago, a part of the bearers of mitochondrial haplogroup L3 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.^[28] 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 Sinai. 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.^[29][30] 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 straits the Red Sea 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 Eritrea,^[31] 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 Persia until reaching India, which appears to be the first major settling point. M is found in high frequencies along the southern coastal regions of Pakistan and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred.^[32] Sixty percent of the Indian population belong to Haplogroup M. The indigenous people of the Andaman Islands 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 Thailand and Indonesia all the way to Papua New Guinea. 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.^[33][34][35]

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.^[36]

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.^[37]

See also

• Toba catastrophe theory
• Behavioral modernity
• Early human migrations
• Hofmeyr Skull
• Identical ancestors point
• Mitochondrial Eve
• Most recent common ancestor
• Multiregional hypothesis
• Paleolithic
• Sahara pump theory
• Y-chromosomal Adam

References

• Bowler, Peter J. (2003), Evolution: The History of an Idea (3rd ed.), University of California Press, ISBN 0-520-23693-9 
• Darwin, Charles (1871), The Descent of Man, and Selection in Relation to Sex (1st ed.), London: John Murray, http://darwin-online.org.uk/EditorialIntroductions/Freeman_TheDescentofMan.html, retrieved 2009-09-05 

Further reading

This article's citation style may be unclear. The references used may be made clearer with a different or consistent style of citation, footnoting, or external linking. (September 2009)

• Modern Men Trace Ancestry to African Migrants, Science, 11 May 2001
• Underhill et al. (2001). "The phylogeography of Y chromosome binary haplotypes and the origins of modern human populations" Ann. Hum. Genet. 65: 43-62. PDF. Retrieved 25 July 2007.
• Neanderthals 'mated with modern humans', BBC News, 21 April 1999
• New analysis shows three human migrations out of Africa - Replacement theory 'demolished', Washington University in St. Louis, 2 February 2006
• Harding, Rosalind M.; Eugene Healy, Amanda J. Ray, Nichola S. Ellis, Niamh Flanagan, Carol Todd, Craig Dixon, Antti Sajantila, Ian J. Jackson, Mark A. Birch-Machin, and Jonathan L. Rees (2000). "Evidence for variable selective pressures at MC1R". American Journal of Human Genetics 66: 1351–1361. doi:10.1086/302863. 
• Long and Kittles (2003). "Human genetic variation and the nonexistence of human races" Human Biology, 75: 449-471. PDF. Retrieved 10 January 2007.
• Risch, N., Burchard, E., Ziv, E. and Tang, H. (2002). "Categorization of humans in biomedical research: genes, race and disease". Genome Biology 3 (7): comment2007.2001 - comment2007.2012. doi:10.1186/gb-2002-3-7-comment2007. 
• Tishkoff, S. and Kidd, K. Implications of biogeography of human populations for 'race' and medicine Nature Genetics 36: S21 - S27 (2004) doi:doi:10.1038/ng1438
• 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.
• Olsen, Steve, Mapping Human History: Discovering the past through our genes ISBN 0618352104 2002
• 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) ISBN 0552152188
• Wade, Nicholas, Before the Dawn: Recovering the Lost History of Our Ancestors (2006) ISBN 1594200793
• Wells, Spencer, The Journey of Man: A Genetic Odyssey (2003) ISBN 069111532X
• Wells, Spencer, Deep Ancestry: Inside the Genographic Project (2006) ISBN 0792262158
• "New Research Proves Single Origin Of Humans In Africa," Science Daily, July 19, 2007, retrieved July 19, 2007
• Climate change led mankind out of Africa
• Ancient African Megadroughts May Have Driven Human Evolution Out Of Africa
• DNA studies trace human migration from Africa retrieved February 21, 2008

External links

• An mtDNA view of the peopling of the world by Homo sapiens
• National Geographic: Atlas of the Human Journey
• Bradshaw Foundation: The Journey of Mankind
• Natural History Museum - Video
• "No evidence of Neandertal mtDNA contribution to early modern humans" 2004 article by Svante Pääbo et al.

v • d • e Part of the series on Human evolution Hominini Sahelanthropus tchadensis • Orrorin tugenensis • Ardipithecus • Kenyanthropus platyops Australopithecines Australopithecus: A. anamensis • A. afarensis • A. bahrelghazali • A. africanus • A. garhi Paranthropus: P. aethiopicus • P. boisei • P. robustus Humans and Proto-humans Homo: H. habilis • H. rudolfensis • H. georgicus • H. ergaster • H. erectus (H. e. lantianensis • H. e. palaeojavanicus • H. e. pekinensis • H. e. nankinensis • H. e. wushanensis • H. e. yuanmouensis • H. e. soloensis) • H. cepranensis • H. antecessor • H. heidelbergensis • H. neanderthalensis • H. rhodesiensis • H. floresiensis • Archaic Homo sapiens • Anatomically modern humans (H. s. idaltu • H. s. sapiens) Topics: Timeline of human evolution • List of human evolution fossils • Human evolutionary genetics Models: Recent African origin• Multiregional origin