Multiregional origin of modern humans
The multiregional hypothesis, multiregional evolution, or polycentric hypothesis, is a scientific model that provides an alternative explanation to the more widely accepted "Out of Africa" model of monogenesis for the pattern of human evolution.
Multiregional evolution holds that the human species first arose around two million years ago and subsequent human evolution has been within a single, continuous human species. This species encompasses all archaic human forms such as Homo erectus, Denisovans, and Neanderthals as well as modern forms, and evolved worldwide to the diverse populations of anatomically modern humans.
The hypothesis contends that the mechanism of clinal variation through a model of "centre and edge" allowed for the necessary balance between genetic drift, gene flow, and selection throughout the Pleistocene, as well as overall evolution as a global species, but while retaining regional differences in certain morphological features. Proponents of multiregionalism point to fossil and genomic data and continuity of archaeological cultures as support for their hypothesis.
The multiregional hypothesis was first proposed in 1984, and then revised in 2003. In its revised form, it is similar to the assimilation model, which holds that modern humans originated in Africa and today share a predominant recent African origin, but have also absorbed small, geographically variable, degrees of admixture from other regional hominin species.
The multiregional hypothesis is not currently the most accepted theory of modern human origin among scientists. "The African replacement model has gained the widest acceptance owing mainly to genetic data from existing populations. This model is consistent with the realization that modern humans cannot be classified into subspecies or races, and it recognizes that all populations of present-day humans share the same potential." The African replacement model is also known as the "out of Africa" theory, which is currently the most widely accepted model. It proposes that Homo sapiens evolved in Africa before migrating across the world." And: "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 without interbreeding. This differs from the multiregional hypothesis in that the multiregional model predicts interbreeding with preexisting local human populations in any such migration."
History
Overview
The Multiregional hypothesis was proposed in 1984 by Milford H. Wolpoff, Alan Thorne and Xinzhi Wu. Wolpoff credits Franz Weidenreich's "Polycentric" hypothesis of human origins as a major influence, but cautions that this should not be confused with polygenism, or Carleton Coon's model that minimized gene flow. According to Wolpoff, multiregionalism was misinterpreted by William W. Howells, who confused Weidenreich's hypothesis with a polygenic "candelabra model" in his publications spanning five decades:Through the influence of Howells, many other anthropologists and biologists have confused multiregionalism with polygenism i.e. separate or multiple origins for different populations. Alan Templeton for example notes that this confusion has led to the error that gene flow between different populations was added to the Multiregional hypothesis as a "special pleading in response to recent difficulties", despite the fact: "parallel evolution was never part of the multiregional model, much less its core, whereas gene flow was not a recent addition, but rather was present in the model from the very beginning". Despite this, multiregionalism is still confused with polygenism, or Coon's model of racial origins, from which Wolpoff and his colleagues have distanced themselves. Wolpoff has also defended Wiedenreich's Polycentric hypothesis from being labeled polyphyletic. Weidenreich himself in 1949 wrote: "I may run the risk of being misunderstood, namely that I believe in polyphyletic evolution of man".
In 1998, Wu founded a China-specific Multiregional model called "Continuity with Hybridization". Wu's variant only applies the Multiregional hypothesis to the East Asian fossil record, and is popular among Chinese scientists. However, James Leibold, a political historian of modern China, has argued the support for Wu's model is largely rooted in Chinese nationalism. Outside of China, the Multiregional hypothesis has limited support, held only by a small number of paleoanthropologists.
"Classic" vs "weak" multiregionalism
, a leading proponent of the more mainstream recent African origin theory, debated Multiregionalists such as Wolpoff and Thorne in a series of publications throughout the late 1980s and 1990s. Stringer describes how he considers the original Multiregional hypothesis to have been modified over time into a weaker variant that now allows a much greater role for Africa in human evolution, including anatomical modernity.Stringer distinguishes the original or "classic" Multiregional model as having existed from 1984 until 2003, to a "weak" post-2003 variant that has "shifted close to that of the Assimilation Model".
Genetic studies
The finding that "Mitochondrial Eve" was relatively recent and African seemed to give the upper hand to the proponents of the Out of Africa hypothesis. But in 2002, Alan Templeton published a genetic analysis involving other loci in the genome as well, and this showed that some variants that are present in modern populations existed already in Asia hundreds of thousands of years ago. This meant that even if our male line and our female line came out of Africa in the last 100,000 years or so, we have inherited other genes from populations that were already outside of Africa. Since this study other studies have been done using much more data.Fossil evidence
Morphological clades
Proponents of the multiregional hypothesis see regional continuity of certain morphological traits spanning the Pleistocene in different regions across the globe as evidence against a single replacement model from Africa. In general, three major regions are recognized: Europe, China, and Indonesia. Wolpoff cautions that the continuity in certain skeletal features in these regions should not be seen in a racial context, instead calling them morphological clades; defined as sets of traits that "uniquely characterise a geographic region". According to Wolpoff and Thorne : "We do not regard a morphological clade as a unique lineage, nor do we believe it necessary to imply a particulartaxonomic status for it". Critics of multiregionalism have pointed out that no single human trait is unique to a geographical region but Wolpoff et al. note that regional continuity only recognizes combinations of features, not traits if individually accessed, a point they elsewhere compare to the forensic identification of a human skeleton:
Combinations of features are "unique" in the sense of being found in only one region, or more weakly limited to one region at high frequency. Wolpoff stresses that regional continuity works in conjunction with genetic exchanges between populations. Long-term regional continuity in certain morphological traits is explained by Alan Thorne's "centre and edge" population genetics model which resolves Weidenreich's paradox of "how did populations retain geographical distinctions and yet evolve together?". 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 for each region, involving interbreeding with Africans.
Indonesia, Australia
held that there was regional continuity in Indonesia and Australia for a morphological clade. This sequence is said to consist of the earliest fossils from Sangiran, Java, that can be traced through Ngandong and found in prehistoric and recent Aboriginal Australians. In 1991, Andrew Kramer tested 17 proposed morphological clade features. He found that: "a plurality of the seventeen non-metric features link Sangiran to modern Australians" and that these "are suggestive of morphological continuity, which implies the presence of a genetic continuum in Australasia dating back at least one million years" but Colin Groves has criticized Kramer's methodology, pointing out that the polarity of characters was not tested and that the study is actually inconclusive. Phillip Habgood discovered that the characters said to be unique to the Australasian region by Thorne are plesiomorphic:Yet, regardless of these criticisms Habgood allows for limited regional continuity in Indonesia and Australia, recognizing four plesiomorphic features which do not appear in such a unique combination on fossils in any other region: a sagittally flat frontal bone, with a posterior position of minimum frontal breadth, great facial prognathism, and zygomaxillary tuberosities. This combination, Habgood says, has a "certain Australianness about it".
Wolpoff, initially skeptical of Thorne's claims, 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 in excessive prognathism. Durband in contrast states that "features cited as showing continuity between Sangiran 17 and the Kow Swamp sample disappeared in the new, more orthognathic reconstruction of that fossil that was recently completed". Baba et al. who newly restored the face of Sangiran 17 concluded: "regional continuity in Australasia is far less evident than Thorne and Wolpoff argued".