Multiregional origin of modern humans



In paleoanthropology, the multiregional hypothesis is one of two accounts of the origin of anatomically modern humans, Homo sapiens.

The multiregional hypothesis 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.

Besides Milford H. Wolpoff, paleoanthropologists most closely associated with the multiregional hypothesis include  James Calcagno, John Hawks and Erik Trinkaus.

The alternative theory is the recent single-origin hypothesis (or Out-of-Africa model). While Out of Africa has clearly emerged as the mainstream view since the 1990s, advocates of the Multiregional Hypothesis point to some recent genetic data, continuing the debate between the two viewpoints.

Classification of hominid species
Evaluation of the Multiregional theory revolves around the assumption or non-assumption of species barriers between early hominids.

Because of the scarcity of fossils and the discovery of important new finds every few years, researchers disagree about the details and sometimes even basic elements of human evolutionary history. While they have revised this history several times over the last decades, researchers currently agree that the oldest named species of the genus Homo, Homo habilis, evolved in Africa around two million years ago, and that members of the genus migrated out of Africa somewhat later, at least 1.5 million years ago. The descendants of these ancient migrants, which probably included Homo erectus, have become known through fossils uncovered far from Africa, such as those of "Peking man" and "Java man". Homo neanderthalensis is also considered a descendant of early migrants.

Polygenism
An older 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 S. Coon, it is biologically impossible since all populations of a monophyletic taxonomical group, like the human species, must have a single origin with a common ancestor. 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, sexual selection, and historical accidents (genetic drift).

Weidenreich-Coon
The Multiregional Hypothesis has its origin in the work of Franz Weidenreich in the 1930s. At that time, Weidenreich originated the "Weidenreich Theory of Human Evolution" based on his examination of Peking Man. Weidenreich was an anatomist and observed numerous anatomical characteristics that 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 theory proposed by Weidenreich, genes that were generally adaptive (such as those for intelligence) flowed relatively rapidly from one part of the world to the other, while those that were locally adaptive, would not. This is the direct opposite to theories of human evolution that have been popularized in the press with one superior race (e.g. Modern Humans) displacing other races (e.g. Neanderthals). A vocal proponent of the Weidenreich theory was Carleton Coon.

Regional Continuity (Wolpoff)
The term "multiregional hypothesis" was first coined in the early 1980s 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. These scientists explained the apparent regional continuity by claiming 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 and 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.

Hybrid-origin theory
The hybrid-origin hypothesis of human origins argues that all or at least some of the genetic variation between the contemporary human races is attributable to genetic inheritance from at least two widely divergent hominid species, or subspecies, that were geographically dispersed throughout Africa, Southeast Asia and the Indian subcontinent, prior to the evolution of modern Homo sapiens sapiens (according to hybrid-origin theory, approximately 35,000 years ago).

Hominid populations, put forward by the hybrid-origin theory as sources for genetic admixture, include Homo neanderthalensis and Peking Man (a subspecies of Homo erectus), and Cro-Magnon man (who physically and culturally differs significantly from Homo erectus). This theory was first introduced in 1971 by the British psychologist Stan Gooch. Here is a brief summary of Gooch's theory (from Guardians of the Ancient Wisdom 1979):


 * 1) From other human species, Cro-Magnon man evolves in Northern India during millions of years of isolation, develops and practices sun worship and hunting magic; the culture is patriarchal.
 * 2) Elsewhere during the same period, different forms of Neanderthal evolve in Europe and the Middle East, while moon worship and earth magic is developed and practiced; the culture is matriarchal.
 * 3) Around 35,000 years b.p. Cro-Magnon abandons India and heads west through the Middle East into Europe, overrunning Neanderthal.  By 25,000 years ago, the predominant type in Europe is Cro-Magnon.
 * 4) In the Middle East a hybrid population, a cross between the Cro-Magnon and Neanderthal types, emerges.  Pure Neanderthal has largely ceased to exist either here or in Europe.
 * 5) By 15,000 years ago, pure Cro-Magnon man has also ceased to exist, driven out of north and west Europe, into southern Europe, by renewed glaciation, absorbed by the hybrid type (that is, ourselves).

Meanwhile mixed types have also migrated back into Africa (during glaciation in Europe the Sahara had become well-watered, grassy plains) and back into India and then on to China. In these places the mixed type further mingled with the local Neanderthal types.

According to this theory the resulting hybrid 'Homo sapiens sapiens', was superior to both its ancestors due to what is commonly termed hybrid vigour.

Fossil evidence
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. 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 inter-mixture of diverse human populations. 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." 

In an article appearing in the Proceedings of the National Academy of Sciences 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."

Proponents of the multiregional hypothesis point to a recent Australian study of an ancient Aboriginal skeleton known as Mungo Man. 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. These proponents interpret the study to mean 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.

Genetic evidence
Proponents of the Multiregional or Hybrid-origin hypothesis point to the study of divided deply in time, in scale of 1MYA, genetic linaeges and interpret it as genetic evidence for inter-breeding between 'Humans' and other 'hominids'.


 * Microcephalin.

Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage;

PNAS | November 28, 2006 | vol. 103 | no. 48 | 18178-18183 ;

quote: "... As such, microcephalin shows by far the most compelling evidence of admixture among the human loci examined thus far.

Speculation about the identity of the archaic Homo population from which the microcephalin D allele introgressed into the modern human gene pool points to the Neanderthal lineage as a potential (although by no means only) candidate. Anatomically modern humans and Neanderthals shared a long period of coexistence, from as early as 130,000 years ago in the Middle East (39) to as late as 35,000 years ago in Europe (40), consistent with the estimated introgression time of the microcephalin D allele at or sometime before {approx}37,000 years ago. Furthermore, the worldwide frequency distribution of the D allele, exceptionally high outside of Africa but low in sub-Saharan Africa (29), suggests, but does not necessitate, admixture with an archaic Eurasian population. Finally, our estimate of the separation time between D and non-D alleles (i.e., {approx}1,100,000 years with a lower-bound confidence interval of {approx}530,000 years) is largely consistent with the divergence time between modern humans and Neanderthals based on mitochondrial DNA (mtDNA) sequence difference (320,000–740,000 years; refs. 41 and 42) and with the earliest appearance of Neanderthals in the fossil record {approx}500,000 years ago (43). It would be of great interest to sequence the microcephalin locus in Neanderthals or other archaic Homo lineages, should it become technically feasible to retrieve and analyze nuclear DNA from ancient hominid remains.

Our results not only provide genetic evidence in support of the possibility of admixture between modern humans and an archaic Homo lineage but also support the notion that the biological evolution of modern humans might have benefited from the contribution of adaptive alleles from our archaic relatives. In the case of microcephalin, it is all the more intriguing given the fact that the adaptive allele is associated with an important brain development gene. ..." URL:http://www.pnas.org/cgi/content/full/103/48/18178


 * RRM2P4 region on X chromosome.
 * PDHA1 locus on X chromosome.

Criticism of Multiregionalism
Multiregional evolution contrasts with the mainstream "Recent African Origin" (RAO) theory. According to that 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 RAO, is the most recent example of species replacement.

Aspects of multiregionalism have been criticized as not being based on objective scientific observation. Some critics even argue that multiregionalism may be motivated by ethnocentrism and is meant to instill beliefs of purity of lineage.

Multiregionalists have long claimed that modern Europeans are descended from the Neanderthals. In 1997, DNA testing performed on 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, from the ethnic Siberians to the !Kung people of Africa, are more closely related to each other than to the Neanderthals -- further evidence supporting the Out-of-Africa theory.

Another example is the case of Peking man, a fossil skull of homo erectus found in China dating to possibly 400,000 years. Some Paleoanthropologists in China have asserted that the modern Chinese are descendents of earlier forms of humans such as Peking Man. However, Chinese geneticists performed microsatellite analysis on the Chinese population in 1998 and discovered genetic similarities with Africans, yielding the first evidence that the Chinese descended from Africa. A recent study undertaken by Jin Li showed no inter-breeding between modern human immigrants to East Asia and Homo erectus, contradicting the Peking Man-origin hypothesis and affirming that the Chinese descended from Africans. In 2001, Chinese geneticists analyzed Y chromosomes in Chinese people and concluded that all Chinese samples contained a mutated gene M168G which is a marker believed to have appeared in the last 79,000 years on a number of Africans.

The fossil record also favors a single origin. This is because mainly in Africa is there a sensible progression of fossils over the last 3 million years that shows the various intermediate stages of evolution from the most archaic ancestors to modern man.