Multiregional origin of modern humans

A graph detailing the origin of modern humans using the Polygenism theory of human evolution.

The multiregional hypothesis for the human species holds that the evolution of humanity throughout the Pleistocene has been within a single widespread human species, Homo sapiens, in response to the normal forces of evolution: selection, mutation, genetic drift, and gene flow.

Overview

The multiregional hypothesis was first formulated in the early 1980's by Milford H. Wolpoff and a group of associates as an explanation for the apparent similarities of the remains from the Homo erectus and Homo sapiens inhabiting the same region. This phenomenon was termed regional continuity and baffled the scientists at first. This regional continuity could most easily be explained if Homo erectus and Homo sapiens were the same species and there had been just enough interbreeding to cause an overall global development towards the latter, but without stamping out the regional adaptation that had been developed by the former. Such a delicate balance seemed unlikely puzzled the anthropologists.

Eventually, Milford H. Wolpoff proposed an explanation based on clinal variation that would allow for the necessary balance. This was the multiregional hypothesis. It theorizes that Homo erectus, Neanderthals, Homo sapiens and other humans were a single species. This species arose in Africa two million years ago as Homo erectus and then spread out over the world, developing adaptations to regional conditions.

For periods of time some populations became isolated, developing in a different direction. But through a complicated process involving continuous interbreeding, replacement, genetic drift and other vehicles of evolution, 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 while retaining some regional adaptations, but also with many common features.

Multiregional hypothesis and other theories of human origin

Multiregional evolution contrasts with two theories. According to the "Eve theory," human evolution was a consequence of many cases of species replacement, as newer species replaced older ones across the human range. Modern human origins, according to the "Eve theory," is the most recent example of species replacement. The other theory is Polygenic evolution, a multiple origins theory in which the different human populations or races had independent origins and evolved in isolation from each other. Held by many scholars of the 19th century such as Haeckel and Klaatsch, and even some of the 20th, such as Carleton Coon, it is biologically impossible since all populations of a species must have the same, single origin.

Polygenism is sometimes mistaken for Multiregional evolution, because they are both hypotheses of evolution within a single species. However, Polygenic evolution depends on isolation of populations while Multiregional evolution requires population interactions and interbreeding so that genetic changes can spread throughout the human range, especially when they are promoted by natural selection. According to the Multiregional hypothesis, geographic differences between human populations are the results of climatic variation, isolation by distance, and historical accidents (genetic drift).

Recent evidence

The multiregional hypothesis was originally developed from the fossil evidence, but more recent work has focused on molecular data, in which DNA is sequenced. Because this hypothesis posits that human evolution has been within a single species, and not between species as one species replaced another (Eve Theory), it makes genetic predictions. These predictions have been met with the continued study of non-recombining DNA such as mitochondrial DNA and the Y chromosome, as well as regions of Nuclear DNA where recombination is usual. These genetic studies show an ancient history of population mixture throughout the world, perhaps as far back as 2 or more million years. Since for most of that time the majority of humans lived in Africa, more genes moved out of Africa than into it, but gene movements were always multidirectional. The genetic studies also show the independent evolution of different genes; their evolution does not reflect a recent species replacement, which would have affected all genes the same way because of the associated period of very small population size causing a genetic bottleneck.

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 the one at the beginning of the species, some 2 million years ago. 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 inter-mixture of diverse human populations.

A recent Australian study of an ancient Aboriginal skeleton known as Mungo Man supports the multiregional hypothesis. Genetic tests show the mitochondrial DNA of Mungo Man to be from a mtDNA lineage with no descendants today. Yet Mungo man is an anatomically modern human and has been dated to be at least 40,000 years old. The study suggests that mtDNA does not reflect ancestry or divergence times, and this interpretation is supported by the discovery that the gene is subject to natural selection.

A recent, non-fossilized discovery of one metre-tall, small-brained (350 cc), Homo floresiensis, on the Indonesian island of Flores, might imply populations of Homo erectus survived very late, and gave rise to even later, physically dwarfed isolated "erectus" groups. However, this possibility does not address the Multiregional hypothesis, which is only about the human species, and the evidence is marred by the possibility that the single dwarf cranium found on Flores might have been pathological.

Proponents of multiregionalism

Besides Milford H. Wolpoff, paleoanthropologists most closely associated with the multiregional hypothesis include James Ahern, James Calcagno[1], Rachel Caspari, David Frayer, Mica Glanz, John Hawks[2], Andrew Kramer, Sang-Hee Lee, Alan Mann, Janet Monge, Jakov Radovcic, Karen Rosenberg, Mary Russell, Lynne Schepartz, Fred Smith, Alan Thorne, Adam Van Arsdale, Bernard Vandermeersch.

See also

• Genetics
• Human evolution
• Mitochondrial Eve
• Y-chromosomal Adam
• Polygenism

External links

• [3] - 'Genomics refutes an exclusively African origin of humans' (pdf) Vinayak Eswaran, Henry Harpending, Alan R. Rogers, Journal of Human Evolution (2005)
• [4] - 'Templeton tree'
• [5] - 'The Hybrid Child from Portugal'
• ActionBioscience.org - 'Origins of Modern Humans: Multiregional or Out of Africa?' (discoverer of 'Lucy' argues against multi-regionalism), Donald Johanson, American Institute of Biological Sciences (May, 2001)
• 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
• OxfordJournals.org - '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)
• PLoS Genetics - 'Possible ancestral structure in human populations', Vincent Plagnol, Jeff D. Wall, PLoS Genetics, (2006) (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, there is not yet a clear source population candidate in West Africa.)
• 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)