Multiregional origin of modern humans

The multiregional hypothesis is a scientific model that provides an explanation for the pattern of human evolution. The hypothesis holds that humans first arose near the beginning of the Pleistocene two million years ago and subsequent human evolution has been within a single, continuous human species. This species encompasses archaic human forms such as Homo erectus and Neanderthals as well as modern forms, and evolved worldwide to the diverse populations of modern Homo sapiens sapiens. The theory contends that humans evolve through a combination of adaptation within various regions of the world and gene flow between those regions. Proponents of multiregional origin point to fossil and genomic data and continuity of archaeological cultures as support for their hypothesis. [1]

A majority of scholars prefer the primary competing hypothesis, which postulates a recent African origin of modern humans, contending that modern humans arose in Africa, most likely around 150-200,000 years ago, moving out of Africa around 50-60,000 years ago to replace archaic human forms.[2]

Contents

Overview

The idea of multiregional evolution goes back to Franz Weidenreich in the early 20th century, who saw similarities between the archaic human Peking man fossils and modern humans in China.[3][4] Weidenreich merged the Asia hypothesis into the multiregional, extending many other regions into his theory from the Old World with gene flow between various populations which influenced many anthropologists of the time.[5][6] Weidenreich did not use the word Multiregional to describe his evolution model, but instead Polycentric.[7]

The term "multiregional hypothesis" was first applied to the theory in the early 1980s by Milford H. Wolpoff and colleagues, who used the theory to explain these and other similarities between archaic humans and modern humans in various regions, in what they called regional continuity.[8][9]

Wolpoff rejected the proposal by Coon of parallel evolution,[9] and proposed that the mechanism of clinal variation allowed for the necessary balance between both local selection and overall evolution as a global species, with Homo erectus, Neanderthals, Homo sapiens and other human forms as subspecies. This species arose in Africa two million years ago as H. erectus and then spread out over the world, developing adaptations to regional conditions. Some populations became isolated for periods of time, developing in different directions, 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. By these mechanisms, surviving local varieties of the species evolved into modern humans, retaining some regional adaptations but with many features common to all regions.[9]

Fossil evidence

Proponents of the multiregional hypothesis see regional continuity of certain morphological traits from archaic humans to modern humans, demonstrating regional genetic continuity, even as changes in other traits occur in parallel over time across all regions, demonstrating lateral genetic exchange.[10] For example, in 2001 Wolpoff and colleagues published an analysis of character traits of the skulls of early modern human fossils in Australia and central Europe. They concluded that the diversity of these recent humans could not "result exclusively from a single late Pleistocene dispersal", and implied dual ancestry from Javan Homo erectus for Australia and from Neanderthals for Central Europe.[11][12]

Southeast Asia

Alan Thorne held that there was regional continuity in the human fossils in southeast Asia. Wolpoff, initially skeptical, became convinced when reconstructing the Sangiran 17 Homo erectus skull from Indonesia, when he was surprised that the skull's face to vault angle matched that of the Australian modern human Kow Swamp 1 skull. Wolpoff had expected the skull to match that of the Homo erectus specimens from China like the Dali skull, but instead, the face to vault angle seemed to be retained regionally over time, even while the fossils in the two regions showed parallel increases in brain case size and parallel reductions in masticatory structures over the intervening approximately 750,000 years.[10]

China

Franz Weidenreich, who oversaw the excavations of numerous "Peking man" Homo erectus fossils at Zhoukoudian in the early 20th century, believed the fossil record demonstrated certain unique features linking prehistoric and modern human populations in China. Many subsequent Chinese paleoanthropologists, such as Wu Xinzhi, were also disposed to favor the multiregional hypothesis for the same reason.[4]

More recent finds provide additional support for regional human continuity in China. The Tianyuan 1 specimen unearthed in 2003 in Tianyuan Cave, Zhoukoudian, and Carbon 14 dated to 42-39 kya exhibits a series of typical modern human features such as a distinct chin. However, the skeleton also has archaic traits such as low anterior to posterior dental proportions indicating relatively large molars and certain leg bone proportions typical of archaic forms such as Neanderthals. Shang et al. (1999) conclude that this combination of modern and archaic traits "implies that a simple spread of modern humans from Africa is unlikely."[13] A jaw bone found in 2008 and dated to 110 kya may also exhibit a mixture of archaic and modern human traits.[14]

Europe

Proponents of the multiregional hypothesis argue for regional continuity in Europe on the basis of skeletal anatomy, morphology and genetics of speech, and the archaeology of the middle to upper paleolithic transition, which they believe to be inconsistent with the possibility of complete replacement of the Neanderthals in Europe without interbreeding.[15]

Some detractors of the theory have argued, in contrast, that the morphological differences between Neanderthals and early and modern humans indicate that they are different species, based on skull differences more distinct than between any subspecies pairs examined except for the two subspecies of gorilla, implying limited or no interbreeding.[16][17]

Many of the multiregional claims regarding skeletal morphology in Europe center on forms with both archaic Neanderthal traits and modern traits, to provide evidence of interbreeding rather than replacement. Examples include the Lapedo child found in Portugal[18] and the Oase 1 mandible from Peştera cu Oase, Romania,[19] though the Lapedo child example is disputed by some.[20] In a 2007 paper examining numerous samples from European early modern humans, later European humans from the Gravettian period, and the earlier Neanderthal and east African populations from whom the first two populations could have descended, Erik Trinkaus identified numerous features in the later European samples which were absent from the African sample, but present in the Neanderthal sample. These features included various aspects of skull and mandible shape, tooth shape and size, and shapes and proportions of other bones. Trinkaus concluded that early modern Europeans had predominant African ancestry with a substantial degree of admixture from the Neanderthals then indigenous to Europe.[21][22]

Genetic evidence

Genetic evidence from the late 1980s on the mitochondrial genome indicated that all living women can trace their maternal line of descent to a single female living in Africa about 200,000 years ago, the so-called Mitochondrial Eve. This led to a hypothesis that Homo sapiens evolved in Africa, with a small founder population of humans leaving Africa and eventually replacing all archaic humans then living elsewhere without interbreeding. Recent analyses of DNA taken directly from Neanderthal specimens, however, indicates that they contributed to the genome of all humans outside of Africa. The Homo sapiens who populated the world outside Africa all have Neanderthals among their descendents. Denisova hominins also contributed to the DNA of Melanesians and Australians.

Mitochondrial DNA

A 1987 analysis of mitochondrial DNA from 147 people from around the world indicated that their mitochondrial lineages all coalesced in a common ancestor in Africa about 200,000 years ago. The analysis suggested that this reflected the worldwide expansion of modern humans as a new species, replacing rather than mixing with local archaic humans.[23] Later analysis of mitochondrial DNA from neanderthals and from the denisova hominin indicated that those mitochondrial strains had diverged from the living human mitochondrial line long before 200,000 years ago, consistent with lack of interbreeding between early modern and archaic humans.[24][25]

The original mitochondrial DNA results and the resulting recent African replacement theory posed a serious challenge to the multiregional hypothesis.[26] Mitochondrial DNA alone, however, could not entirely rule out interbreeding between early modern and archaic humans, since archaic human mitochondrial strains from such interbreeding could have been lost due to genetic drift or a selective sweep.[27][28]

Nuclear DNA

Initial analysis of Y chromosome DNA, which like mitochondrial DNA is inherited from only one parent, was consistent with a recent African replacement model. However, the mitochondrial and Y chromosome data could not be explained by the same modern human expansion out of Africa; the Y chromosome expansion would have involved genetic mixing that retained regionally local mitochondrial lines. In addition, the Y chromosome data indicated a later expansion back into Africa from Asia, demonstrating that gene flow between regions was not unidirectional.[29]

An early analysis of 15 noncoding sites on the X chromosome found additional inconsistencies with the recent African replacement hypothesis. The analysis found a multimodal distribution of coalescence times to the most recent common ancestor for those sites, contrary to the predictions for recent African replacement; in particular, there were more coalescence times near 2 million years ago (mya) than expected, suggesting an ancient population split around the time humans first emerged from Africa as Homo erectus, rather than more recently as suggested by the mitochondrial data. While most of these X chromosome sites showed greater diversity in Africa, consistent with African origins, a few of the sites showed greater diversity in Asia rather than Africa. For four of the 15 gene sites that did show greater diversity in Africa, the sites' varying diversity by region could not be explained by simple expansion from Africa, as would be required by the recent African replacement hypothesis.[30]

Later analyses of X chromosome and autosomal DNA continued to find sites with deep coalescence times inconsistent with a single origin of modern humans,[31][32][33][34][35][36] diversity patterns inconsistent with a recent expansion from Africa,[37] or both.[38][39] For example, analyses of a region of RRM2P4 (ribonucleotide reductase M2 subunit pseudogene 4) showed a coalescence time of about 2 Mya, with a clear root in Asia,[40][41] while the MAPT locus at 17q21.31 is split into two deep genetic lineages, one of which is common in and largely confined to the present European population, suggesting inheritance from Neanderthals.[42][43][44][45] In the case of the Microcephalin D allele, evidence for rapid recent expansion indicated introgression from an archaic population.[46][47][48][49]

In a 2005 review and analysis of the genetic lineages of 25 chromosomal regions, Alan Templeton found evidence of more than 34 occurrences of gene flow between Africa and Eurasia. Of these occurrences, 19 were associated with continuous restricted gene exchange through at least 1.46 million years ago; only 5 were associated with a recent expansion from Africa to Eurasia. Three were associated with the original expansion of Homo erectus out of Africa around 2 million years ago, 7 with an intermediate expansion out of Africa at a date consistent with the expansion of Acheulean tool technology, and a few others with other gene flows such as an expansion out of Eurasia and back into Africa subsequent to the most recent expansion out of Africa. Templeton rejected a hypothesis of recent African replacement with greater than 99% certainty (p < 10−17).[50]

Ancient DNA

Main article: Archaic Homo sapiens admixture with modern humans

By 2006, extraction of DNA directly from some archaic human samples was becoming possible. The earliest analyses were of neanderthal DNA, and indicated that the Neanderthal contribution to modern human genetic diversity was no more than 20%, with a most likely value of 0%.[51] By 2010, however, detailed DNA sequencing of the Neanderthal specimens from Europe indicated that the contribution was nonzero, with Neanderthals sharing 1-4% more genetic variants with living non-Africans than with living humans in sub-Saharan Africa.[52][53] In late 2010, a recently discovered non-Neanderthal archaic human, the Denisova hominin from southern Siberia, was found to share 4-6% of its genome with living Melanesian humans and with no other living group, supporting lateral gene transfer between two regions outside of Africa.[54][55]

In August 2011, Denisovan and Neandertal archaic HLA types were found to represent more than half the HLA alleles of modern Eurasians.[56]

Recent African replacement

Main article: Recent African origin of modern humans

The primary competing scientific hypothesis is currently recent African origin of modern humans, which proposes that modern humans arose as a new species 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 with little or no interbreeding.[57][58][59][60][61][62] This differs from the multiregional hypothesis in that the multiregional model predicts more interbreeding with preexisting local human populations.[62]

Dobzhansky paleovariabilism

A version of multiregional evolution (also a modification of polygenism that rejects independent origin of identical mutations) suggests that Homo sapiens evolved not through new mutations, but through selective pressure altering the statistical proportion between different alleles. Instead of the migrations suggested by Wolpoff, the Dobzhansky model suggests that all alleles needed to make a Homo sapiens already existed within the individual genetic variation of all Homo erectus populations by the time Homo erectus first migrated from Africa, albeit at a very low (but non-zero) frequency. The total absence of sapiens specimens from that time is explained by the fact that biological modernity is controlled by a multitude of genes (not by one single gene), and that the low frequency of modern alleles at that time means that not a single individual carried sufficient numbers of modern alleles to be considered a modern human[63].

See also

References

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Humans and Proto-humans