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{{Expert-subject-multiple|Evolutionary biology|Anthropology|Human Genetic History|date=June 2009}}
[[Image:Migration map4.png|thumb|350px|right|Map of [[early human migrations]] according to [[Mitochondrial DNA|mitochondrial]] [[population genetics]] ]]
[[Image:Migration map4.png|thumb|350px|right|Map of [[early human migrations]] according to [[Mitochondrial DNA|mitochondrial]] [[population genetics]] ]]


Revision as of 18:27, 29 June 2009

Template:Expert-subject-multiple

File:Migration map4.png
Map of early human migrations according to mitochondrial population genetics

In paleoanthropology, the recent African origin of modern humans is a theory model describing the putative[1] origin and early dispersal of anatomically modern humans, Homo sapiens sapiens. The theory is known popularly as the (Recent) Out-of-Africa model, and academically also as the recent single-origin hypothesis (RSOH), Replacement Hypothesis or Recent African Origin (RAO) model. The hypothesis originated in the 19th century, with Darwin's Descent of Man, but remained speculative until the 1980s when it was corroborated based on a study of present-day mitochondrial DNA, combined with evidence based on physical anthropology of archaic specimens.

According to both genetic and fossil evidence[citation needed], 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 currently the near consensus position held within the scientific community.[2]

The competing hypothesis is the multiregional evolution which push back the original "out of Africa" migration - in this case, by Homo erectus, not by Homo sapiens - to two million years ago.[3][4]

History of the theory

Charles Darwin was one of the first to suggest that all humans had a common ancestor who lived in Africa. In the Descent of Man he writes:

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.[5]

The prediction was highly insightful because at the time, in 1871, there were hardly any human fossils of ancient hominids available. Almost fifty years later Darwin was vindicated, as anthropologists began finding numerous fossils of ancient hominids all over Africa (list of hominina fossils).

In 19th century anthropology, "monogenism" was opposed by "polygenism", the idea that the various human races had evolved independently out of archaic hominids. Polygenism was largely obsolete by the mid 20th century, although there were isolated proponents in the later 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.[6] 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 mtDNA sequence of Mungo Man was published[7]. This work was later questioned [8] and explained by W. James Peacock leader of team who sequenced mungo man ancient mtdna.[9]

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

Early Homo sapiens

Main articles: Anatomically modern humans and Archaic Homo sapiens

Archaic Homo sapiens originated in Africa[citation needed] about 250,000 years[citation needed] 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[10]. 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 56,063±5,678 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.[11]

All other fossils of fully modern humans outside of Africa have been dated to more recent times(80,000–100,000 year ago). The next oldest fossil of modern humans outside of Africa are those of Mungo Man found in Australia and have been dated to about 42,000 years ago.[12]

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].[13][14]

Genetic reconstruction

Further information: Archaeogenetics and Early human migrations

Two pieces of the human genome are particularly useful in deciphering human history. One is the mitochondrial DNA and the other is the Y chromosome. These are the only two parts of the genome that are not shuffled about by the evolutionary mechanisms which generate diversity with each generation. Hence the Mitochondrial DNA and the Y chromosome are passed down generation to generation 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[15][16]; 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-gender specific chromosomes with gender specific ones.[17]

Mitochondrial DNA

Further information: Human mitochondrial DNA haplogroup
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.[18][19] 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

Main articles: Human Y-chromosome DNA haplogroups and Haplogroup F (Y-DNA)

The mutations defining macro-haplogroup CR (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.[20] Those samples were taken from 1138 unrelated individuals.[20] 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. [21] 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.[21] 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.[21] 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.[22]

Exodus from Africa

Red Sea crossing

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[23]. 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.[24][25] 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[citation needed] indicating the diet of early humans included seafood obtained by beachcombing.

Subsequent expansion

Main article: Early human migrations

From the Near East, these populations spread east to South Asia by 50 millennia ago, and on to Australia by 40 millennia ago, Homo sapiens for the first time colonizing territory never reached by Homo erectus. Europe was reached by Cro-Magnon some 40 millennia ago. East Asia (Korea, Japan) was reached by 30 millennia ago. It is disputed whether subsequent migration to North America took place around 30 millennia ago, or only considerably later, around 14 millennia 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.[26] 60% 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.[27][28][29].

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.[30]

Competing hypotheses

Main article: Multiregional origin of modern humans

The multiregional hypothesis holds that the evolution of humanity from the beginning of the Pleistocene 1.8 million years BP to the present day has been within a single, continuous population evolving worldwide till today. This means that proponents of multiregional origin reject the assumption of a infertility barrier between Eurasian and African human populations. Multiregional proponents point to fossil record and milion years old genetic lineages in chromosomal human DNA. On the other hand very strong genetic similarities among all humans reflect the interconnectedness of human populations around the world, resulting in relatively constant gene flow (Thorne and Wolpoff 1992)

Genetic statistical calculation (2006 results) suggested that at least 5% of human modern gene pool can be attributed to ancient admixture, European contribution is the Neanderthal.[31]

See also

References

  1. ^ Schaffner, Sf (2004). "The X chromosome in population genetics" (PDF). Nature reviews. Genetics. 5 (1): 43–51. doi:10.1038/nrg1247. ISSN 1471-0056. PMID 14708015. {{cite journal}}: Unknown parameter |month= ignored (help); Unknown parameter |qoute= ignored (help)
  2. ^ 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.
  3. ^ http://www.actionbioscience.org/evolution/johanson.html Origins of Modern Humans: Multiregional or Out of Africa? By Donald Johanson
  4. ^ http://discovermagazine.com/2002/aug/featafrica Discover: Not Out of Africa, Alan Thorne's challenging ideas about human evolution
  5. ^ The descent of man Chapter 6 - On the Affinities and Genealogy of Man
  6. ^ Jackson Jr., John P. (2001). "InWays Unacademical": The Reception of Carleton S. Coon's The Origin of Races
  7. ^ 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
  8. ^ Cooper, A., Rambaut, A., Macaulay, V., Willerslev, E., Hansen, A. & Stringer, C. 2001. Human origins and ancient human DNA. Science 292: 1655-1656
  9. ^ http://www.anthro.utah.edu/PDFs/courses/oconnell/3152/cooper2001.pdf
  10. ^ White, Tim D., Asfaw, B., DeGusta, D., Gilbert, H., Richards, G.D., Suwa, G. and Howell, F.C. (2003). "Pleistocene Homo sapiens from Middle Awash, Ethiopia". Nature. 423 (6491): 742–747. doi:10.1038/nature01669.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ 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,
  12. ^ human origins by the Museum of natural history
  13. ^ Ancestral tools
  14. ^ Middle to upper paleolithic transition
  15. ^ Cann, R. L., Stoneking, M., Wilson, A. C. (1987) Mitochondrial DNA and human evolution. Nature. Vol. 325. Pp. 31-36.
  16. ^ 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
  17. ^ Ancient African Exodus Mostly Involved Men, Geneticists Find. Nature Genetics. Harvard Medical School.
  18. ^ Whole-mtDNA Genome Sequence Analysis of Ancient African Lineages
  19. ^ mtDNA Variation in the South African Kung and Khwe
  20. ^ a b Coop G, Pickrell JK, Novembre J, Kudaravalli S, Li J; et al. (2009). "The Role of Geography in Human Adaptation". PLoS Genet. 5 (6): 1–16. doi:10.1371/journal.pgen.1000500. {{cite journal}}: Explicit use of et al. in: |author= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)
  21. ^ a b c "Among Many Peoples, Little Genomic Variety". The Washington Post. June 22, 2009. Retrieved 2009-06-25.
  22. ^ "Geography And History Shape Genetic Differences In Humans". Science Daily. June 7, 2009. Retrieved 2009-06-25.
  23. ^ Both Australian Aborigines and Europeans Rooted in Africa
  24. ^ A single origin, several dispersal hypothesis
  25. ^ Searching for traces of the Southern Dispersal, by Dr. Marta Mirazón Lahr, et al.
  26. ^ Most of the extant mtDNA boundaries in South and Southwest Asia were likely shaped during the initial settlement of Eurasia by anatomically modern humans
  27. ^ Evolution of Human Languages
  28. ^ genetic origins of the Andaman Islanders
  29. ^ Genetic affinities of the Andaman Islanders
  30. ^ Mitochondrial Genome Variation and Evolutionary History of Australian and New Guinean Aborigines
  31. ^ 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..

Further reading

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