Replacement Hypothesis

Recent African origin of modern humans

In paleoanthropology, the recent African origin of modern humans is one of two hypotheses of the origin 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. According to the theory, anatomically modern humans evolved solely in Africa, between 200,000 and 100,000 years ago, with members of one branch leaving Africa by 60,000 years ago and replacing all earlier human populations such as Neanderthals and Homo erectus. The hypothesis originated in the 1980s based on a study of present day mitochondrial DNA, and its proponents have since presented evidence based on physical anthropology of archaic specimens.

The recent single origin of modern humans in East Africa is currently the near mainstream position held within the scientific community.

Alternative scenarios claim a multiregional origin of modern humans, including claims of interbreeding of Cro-Magnon and Neanderthals or other earlier hominids (e.g. Homo Erectus, see Hybrid-origin). Some of these claims push back the original "out of Africa" migration to 2 million years ago.

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.

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. Such views were 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.

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

The question whether there may have been admixture (hybridization) or rather inheritance of other typolgical (not de facto) Homo sub-/species into 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. It is the oldest known anatomically modern human and classified as extinct subspecies. 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.

All other fossils of fully modern humans outside of Africa have been dated to more recent times. 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.

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. These new changes are suggestive of more advanced behaviour and scientists attribute these changes to the development of language.

Genetic reconstruction

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 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.

Mitochondrial DNA

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

Y-chromosomal 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.

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. 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 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. 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 indicating the diet of early humans was sea food obtained by beachcombing.

Subsequent expansion

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 Iran 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. 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. 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 Saudi 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 has confounded scientists attempting to trace migratory routes.. However, it is well known that such distributions can be explained by way of founder's effect and its accompanying bottlenecking, which result in irregular distributions in genetic patterning due to what amounts to random sampling from a relatively small initial population (i.e. those who migrated out of Africa).

Competing hypotheses

The multi-regional (hybrid-origin) hypothesis proposes admixture of archaic Homo sapiens subspecies resulting in hybrids that gave rise to the world's races. This means that proponents reject the assumption of a species barrier between Homo erectus and Homo sapiens and date the first "out of Africa" migration of Homo sapiens subspecies to 2 million years ago. The recent migration out of Africa 60,000 years ago would then have brought previously isolated subspecies into renewed contact, resulting in a hybrid Homo sapiens sapiens, who was superior to both its ancestor subspecies due to what is commonly termed hybrid vigour. Proponents argue that very strong genetic similarities among all humans do not prove recent common ancestry, but rather reflect the interconnectedness of human populations around the world, resulting in relatively constant gene flow (Thorne and Wolpoff 1992).

Erik Trinkaus is a proponent of hybridization of Cro-Magnon H. sapiens with Homo neanderthalensis around 30,000 years ago.

These theories are based largely on archaeological and fossil evidence. They are not widely recognized, opponents citing the lack of DNA evidence.

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.

While the hybrid-origin scenario at present cannot be ruled out with certainty, more extreme proposals, known as polygenism, were a topic of academic debate in later 19th century, but are obsolete today as historical examples of scientific racism.

Homo heidelbergensis

According to the "Recent Out of Africa" theory, similar "Archaic Homo sapiens" but few 100,000 year more recent and found in Africa (ie. Homo sapiens idaltu), existing in Africa as a part of the operation of the Saharan pump, and not the European forms of Homo heidelbergensis, are thought to be direct ancestors of modern Homo sapiens. Homo antecessor is likely a direct ancestor living 750,000 years ago evolving into Homo heidelbergensis appearing in the fossil record living roughly 600,000 to 250,000 years ago through various areas of Europe.

At comparable time depth is coarsely dated African specimen Gawis cranium(500-200 kya). Gawis has smaller more archaic cranium when comparing to even older Homo heidelbergensis .

Implications

In a recent article, Leonard Lieberman and Fatimah Jackson have suggested that any new support for a biological concept of race will likely come from another source, namely, the study of human evolution. They therefore ask what, if any, implications current models of human evolution may have for any biological conception of race.

Today, all humans are classified as belonging to the species Homo sapiens and sub-species Homo sapiens sapiens. However, this is not the first species of hominids: the first species of genus Homo, Homo habilis, evolved in East Africa at least 2 million years ago, and members of this species populated different parts of Africa in a relatively short time. Homo erectus evolved more than 1.8 million years ago, and by 1.5 million years ago had spread throughout the Old World. Virtually all physical anthropologists agree that Homo sapiens evolved out of Homo erectus. Anthropologists have been divided as to whether Homo sapiens evolved as one interconnected species from H. erectus (called the Multiregional Model, or the Regional Continuity Model), or evolved only in East Africa, and then migrated out of Africa and replaced H. erectus populations throughout the Old World (called the Out of Africa Model or the Complete Replacement Model). Anthropologists continue to debate both possibilities, and the evidence is technically ambiguous as to which model is correct, although most anthropologists currently favor the Out of Africa model.

According to the Out of Africa Model, developed by Christopher Stringer and Peter Andrews, modern Homo sapiens evolved in Africa 200,000 years ago. H. sapiens began migrating from Africa around 50,000 years ago and eventually replaced existing hominid species in Europe and Asia. This model has gained support by recent research using mitochondrial DNA (mtDNA). After analysing genealogy trees constructed using 133 types of mtDNA, they concluded that all were descended from a woman from Africa, dubbed Mitochondrial Eve. Lieberman and Jackson have related this theory to race with the following comment:

There are three major implications of this model for the race concept. First, the shallow time dimension minimizes the degree to which racial differences could have evolved [...]. Second, the mitochondrial DNA model presents a view that is very much different from Carleton Coon's (1962) concerning the time at which Africans passed the threshold from archaic to modern, thereby minimizing race differences and avoiding racist implications. However, the model, as interpreted by Wainscoat et al. (1989:34), does describe "a major division of human populations into an African and a Eurasian group." This conclusion could best be used to emphasize the degree of biological differences, and thereby provide support for the race concept. Third, the replacement of preexisting members of genus Homo (with little gene flow) implies several possible causes from disease epidemics to extermination. If the latter, then from a contemporary viewpoint, xenophobia or racism may have been practiced

See also

References

Further reading

  • 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.
  • 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.
  • Tishkoff, S. and Kidd, K. Implications of biogeography of human populations for 'race' and medicine Nature Genetics 36: S21 - S27 (2004)
  • 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

Search another word or see Replacement Hypothesison Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature