Multiregional origin of modern humans

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A graph detailing the evolution to modern humans using the Multiregional theory of human evolution. The horizontal lines represent 'multiregional evolution' gene flow between regional lineages.

The multiregional hypothesis is a theory (held by a minority of paleontologists) of how anatomically modern humans, Homo sapiens sapiens, evolved with a worldwide distribution. 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 human species, evolving worldwide from Homo erectus to modern Homo sapiens.^[1]

A competing theory, the recent African origin of modern humans (also known as "Out of Africa"), has emerged as the near consensus view since the 1990s,^[2][3] proposing that modern humans arose in Africa around 100-200,000 years ago, moving out of Africa around 50-60,000 years ago to replace existing human species such as Homo erectus and the Neanderthals.^[4]

Proponents of the multiregional hypothesis point to fossil and genomic data^^[5] as support for their hypothesis. The gene flow, interbreeding or "admixture" between modern and ancestral human population has not been ruled out,^[6][7] although there is not yet any paleogenetic evidence for a contribution from Neanderthals to modern Europeans.^[8] To differentiate the current theory from earlier hypothesis some researchers prefer the term multiregional evolution.

[edit] History

[edit] Weidenreich-Coon

The multiregional hypothesis has its origin in the work of Franz Weidenreich in the 1930s, based on his examination of Peking Man. Weidenreich was an anatomist and observed numerous anatomical characteristics that he thought Peking Man had in common with modern Asians. The Weidenreich Theory stated that human races have evolved independently in the Old World from Homo erectus to Homo sapiens sapiens, while at the same time there was gene flow between the various populations. According to the Weidenreich Theory, genes that were generally adaptive (such as those for intelligence and communication) would flow relatively rapidly from one part of the world to the other, while those that were locally adaptive, would not. A vocal proponent of the Weidenreich theory was Carleton Coon.^[9]

[edit] Regional continuity

The term "multiregional hypothesis" was first coined in the early 1980s by Milford H. Wolpoff and colleagues as an explanation for the apparent similarities seen in Homo erectus and Homo sapiens fossils from the same region, what they called regional continuity.^[10]

Wolpoff rejected the earlier proposal by Coon of parallel evolution,^[10] and proposed a theory based on clinal variation that would allow for the necessary balance between local selection and a global species. He proposed that Homo erectus, Neanderthals, Homo sapiens and other humans were a single species. This species arose in Africa two million years ago as H. erectus and then spread out over the world, developing adaptations to regional conditions. It was proposed that for periods of time some populations became isolated, developing in a different direction, but through continuous interbreeding, replacement, genetic drift and selection, adaptations that were an advantage anywhere on earth would spread, keeping the development of the species in the same overall direction, while maintaining adaptations to regional factors. Eventually, the more unusual local varieties of the species would have disappeared in favor of modern humans, retaining some regional adaptations, but with many common features.^[10]

[edit] Fossil evidence

Some supporters of the multiregional hypothesis, including Wolpoff, argue that fossil evidence is more reliable than estimates based on genetic evidence and molecular clocks, which they contend are subject to genetic drift, bottlenecks and other complicating factors.

[edit] Neanderthals

Multiregionalists claimed that the discovery of a possible hybrid Homo sapiens X neanderthalensis fossil child at the Abrigo do Lagar Velho rock-shelter site in Portugal in 1999 further supports the multiregional hypothesis, by reflecting the admixture of diverse human populations^[11]. Two other archaeologists dispute this: "the analysis by Duarte et al. of the Lagar Velho child's skeleton is a brave and imaginative interpretation, of which it is unlikely that a majority of paleoanthropologists will consider proven."^[12]

In an article appearing in the Proceedings of the National Academy of Sciences^[13] in 2007, Erik Trinkaus has brought together the available data, which shows that early modern humans did exhibit evidence of Neandertal traits. "When you look at all of the well dated and diagnostic early modern European fossils, there is a persistent presence of anatomical features that were present among the Neandertals but absent from the earlier African modern humans," Trinkaus said. "Early modern Europeans reflect both their predominant African early modern human ancestry and a substantial degree of admixture between those early modern humans and the indigenous Neandertals."^[14]

[edit] Peking man

Shang et al see continuity in skeletal remains of archaic people from east Asia.^[15]

[edit] Early modern humans

Wolpoff and colleagues published an analysis in 2001 of character traits of the skulls of early modern human fossils, which failed to reject a theory of dual ancestry from Javan Homo erectus for Australian early modern humans and Neanderthals for Central European modern humans, and which they said ruled out a replacement model.^[16] A subsequent analysis comparing differences of Neanderthal skulls to those of modern humans using 3D morphometric techniques showed a large difference between the two populations, such that Harvatti & al concluded that "we interpret the evidence presented here as supporting the view that Neanderthals represent an extinct human species and therefore refute the regional continuity model for Europe."^[17] It has been argued that these differences are consistent with an evolving lineage, as ancestors are never identical to their descendants.^[18]

[edit] Genetic evidence

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By analysing haplotype data, Alan Templeton found support for three waves of human migration out of Africa, the first 1.9 million years ago, and concluded that it was impossible that existing Eurasian populations had not interbred with African migrants.^[19]

Studies on past population bottlenecks that can be inferred from molecular data have led multiregionalists to conclude that the recent single-origin hypothesis is untenable because there are no population size bottlenecks affecting all genes that are more recent than 2 million years ago.

• Microcephalin.^[20][21][22]
• RRM2P4^[23] (ribonucleotide reductase M2 subunit pseudogene 4) ^[5][24]
• PDHA1 (pyruvate dehydrogenase) locus on X chromosome.^[25][26][27]
• MAPT locus 17q21.3 split into deep genetic lineages H1 and H2 . H2 lineage in European population sugest inheritance from Neanderthals ^{[28][29][30][31][32]}.

Proponents of the multiregional hypothesis show genetic sequences of several loci in the human genome with million year old genealogy^{[33][34][35][36][37][38]}. Those data of deep genetic lineages are explained in the multiregional theory framework as a result of heredity from structured ancestral population^[39]. The data are not interpreted in light of the RAO hypothesis postulating recent replacement where separated million years ago genetic lineages are at best unpredicted. ^[40][41]

[edit] Researchers

The most prominent current proponents of the multiregional hypothesis are Milford H. Wolpoff, Wu Xinzhi, Alan G. Thorne, James Calcagno^[42], John Hawks^[43], Alan Templeton, and Erik Trinkaus.

[edit] Criticism of the multiregional hypothesis

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Multiregional evolution contrasts with the "Recent African Origin" (RAO) theory. According to the latter theory, human evolution was a consequence of many cases of species replacement, as newer species replaced older ones across the human range^{[citation needed]}. Modern human origins, according to the RAO, is the most recent example of species replacement.

In 1997, testing performed on mitochondrial DNA extracted from 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 mtDNA are more closely related to each other than to the Neanderthals mtDNA.^{[citation needed]}

[edit] See also

• Recent single-origin hypothesis
• Punctuated equilibrium
• Interbreeding of Cro-Magnon and Neanderthals
• Mitochondrial Eve
• Y-chromosomal Adam
• Polygenism

[edit] References

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[edit] Reviews

• Templeton, AR (2002). "Out of Africa again and again". Nature 416: 45–51. 
• Pearson, Osbjorn M (2004). "Has the Combination of Genetic and Fossil Evidence Solved the Riddle of Modern Human Origins?". Evolutionary Anthropology 13: 145-159. 
• Adams, J (2008). "Human Evolutionary Tree". Nature Education (Macmillian) 1 (1). http://www.nature.com/scitable/topicpage/Human-Evolutionary-Tree-417. 
• Johanson, Donald C (May 2001). "Origins of Modern Humans: Multiregional or Out of Africa?". ActionBioscience. http://www.actionbioscience.org/evolution/johanson.html. Retrieved on 2009-05-30. 

[edit] External links

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• [12] - 'Genomics refutes an exclusively African origin of humans' (pdf) Vinayak Eswaran, Henry Harpending, Alan R. Rogers, Journal of Human Evolution (2005)
• [13] - 'Templeton tree'
• [14] - 'The Hybrid Child from Portugal'
• Biochem. Soc. Trans (2005) 33, 582-585 - J. Hardy and others - Molecular Mechanisms of Neurodegeneration (Evidence suggesting that Homo neanderthalensis contributed the H2 MAPT haplotype to Homo sapiens)
• Kent Holsinger's web site - 'Drift and migration' (only 1 migrant per generation between populations of reasonable big sizes can prevent divergence in allelic frequencies)
• Genetics - 'Deep Haplotype Divergence and Long-Range Linkage Disequilibrium at Xp21.1 Provide Evidence That Humans Descend From a Structured Ancestral Population' (first genetic evidence that statistically rejects the null hypothesis that our species descends from a single, historically panmictic population), Daniel Garrigan, Zahra Mobasher, Sarah B. Kingan, Jason A. Wilder, and Michael F. Hammer, University of Arizona, Tucson, Genetics, Vol. 170, 1849-1856, August 2005
• Linfield.edu - 'The Origin of Modern Humans: Multiregional and Replacement Theories', Michael Roberts, Linfield College
• [15] - 'Evidence for Archaic Asian Ancestry on the Human X Chromosome' (suggests ancient RRM2P4 lineage is remnant of introgressive hybrid of anatomically modern humans from Africa and archaic populations in Eurasia), Daniel Garrigan, Zahra Mobasher, Tesa Severson, Jason A. Wilder, Michael F. Hammer, University of Arizona, Tucson, Molecular Biology and Evolution, vol 22, no 2, p 189–192 (2005)
• PNAS.org - 'Mitochondrial DNA sequences in ancient Australians: Implications for modern human origins', Gregory J. Adcock, Elizabeth S. Dennis, Simon Easteal, Gavin A. Huttley, Lars S. Jermiin, W. James Peacock, Alan Thorne, Australian National University, Proceedings of the National Academy of Sciences, vol 98, no 2, p 537-542 (January 16, 2001)
• StephenJayGould.org - 'Out of Africa vs. Multiregionalism', Tod Billings (December 7, 1999)
• TalkOrigins.org - 'The evolution of modern humans: where are we now?' Christopher B. Stringer, General Anthropology, vol 7, no 2, p 1–5 (2001)
• Selection, nuclear genetic variation, and mtDNA