Multiregional origin of modern humans
The multiregional hypothesis is an alternative 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.
The term "multiregional hypothesis" was coined in the early 1980s by Milford H. Wolpoff and colleagues, who used the theory to explain regional similarities between archaic humans and modern humans in various regions, in what they called regional continuity. Wolpoff 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.
The primary alternative model, which is the predominant position held in the scientific community, is the recent African origin of modern humans, which holds that modern humans arose in Africa around 100,000–200,000 years ago, moving out of Africa around 50,000–60,000 years ago to replace archaic human forms with limited interbreeding: at least once with Neanderthals and once with Denisovans.
The idea of regional human evolution has its roots in Schwalbe's 1904, Gorjanović-Kramberger's 1906 and Hrdlička's 1927 theory that archaic Neanderthals were the direct ancestors of modern humans. This led Reginald Ruggles Gates to theorise in 1929 that Neanderthals were regionally ancestral to modern Europeans. Java Man was discovered in 1891, and Klaatsch in 1908, Dubois in 1922 and Oppenoorth in 1932 proposed that Java man was ancestral regionally to modern Australian aborigines based on what they saw as similar features in the crania.
Franz Weidenreich published a paper in 1939 proposing a sequence from archaic human Peking man fossils such as facial flatness and shovel shaped incisor teeth. In 1940, Weidenreich extended his theory to Neanderthals and modern Europeans, and in 1943 to Java Man and modern Australian Aborigines, seeing a continuation of regional archaic morphological traits in these modern races or populations, along with substantial interregional gene flow. While Weidenreich used the term "polycentric" rather than "multiregional" to describe his evolution model, his work had a strong influence on Wolpoff. Other theories, such as those of Carleton Coon, also adopted modified forms of Weidenreich's polycentric origin of the races.
Wolpoff in his book Race and Human Evolution traces multiregionalism to Franz Weidenreich's evolutionary sequence, first published in 1939, which proposed a continuity from archaic Peking man fossils to Mongoloids in China. This theory was based on evidence for morphological continuity due to similarities evident between the archaic soloensis fossils, found near the village of Ngandong, and the extant anatomically modern Chinese population such as facial flatness and shovel shaped incisors.
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. 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.
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.
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.
Proponents claim that recent finds support 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." A jaw bone found in 2008 and dated to 110 kya may also exhibit a mixture of archaic and modern human traits.
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.
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.
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 and the Oase 1 mandible from Peştera cu Oase, Romania, though the Lapedo child example is disputed by some. 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.
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 originally led to a hypothesis that Homo sapiens evolved in Africa, with a founder population of humans leaving Africa and eventually replacing all archaic humans then living elsewhere (without interbreeding), known as the "Out of Africa" or "Replacement Hypothesis". Recent analyses of DNA taken directly from Neanderthal specimens indicates that they or their ancestors contributed to the genome of all humans outside of Africa, indicating there was some degree of interbreeding with Neanderthals before their replacement. It has also been shown that Denisova hominins contributed to the DNA of Melanesians and Australians through interbreeding.
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. 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.
The original mitochondrial DNA results and the resulting recent African replacement theory posed a serious challenge to the multiregional hypothesis. 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.
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.
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.
Later analyses of X chromosome and autosomal DNA continued to find sites with deep coalescence times inconsistent with a single origin of modern humans, diversity patterns inconsistent with a recent expansion from Africa, or both. 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, 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. In the case of the Microcephalin D allele, evidence for rapid recent expansion indicated introgression from an archaic population.
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).
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%. 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. In late 2010, a recently discovered non-Neanderthal archaic human, the Denisova hominin from southern Siberia, was found to share 4-6% more of its genome with living Melanesian humans than with any other living group, supporting lateral gene transfer between two regions outside of Africa. In August 2011, human leukocyte antigen (HLA) alleles from the archaic Denisovan and Neanderthal genomes were found to show patterns in the modern human population demonstrating origins from these non-African populations; the ancestry from these archaic alleles at the HLA-A site was more than 50% for modern Europeans, 70% for Asians, and 95% for Papua New Guineans. Proponents of the multiregional hypothesis believe the combination of regional continuity inside and outside of Africa and lateral gene transfer between various regions around the world supports the multiregional hypothesis. However, "Out of Africa" Theory proponents also explain this with the fact that genetic changes occur on a regional basis rather than a continental basis, and populations close to each other are likely to share certain specific regional SNPs while sharing most other genes in common. Migration Matrix theory (A=Mt) indicates that dependent upon the potential contribution of Neanderthal ancestry, we would be able to calculate the percentage of Neanderthal mtDNA contribution to the human species. As we do not know the specific migration matrix, we are unable to input the exact data, which would answer these questions irrefutably.
Recent African Origin
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 limited interbreeding. This differs from the multiregional hypothesis in that the multiregional model predicts interbreeding with local human populations in any such migration.
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