Multiregional hypothesis

Multiregionalism or the Multiregional Evolution (MRE) hypothesis is a model of Pleistocene human evolution, which argues the human species emerged in Africa 2 million years ago, and "developed their modern forms in every area of the Old World". It disputes the competing and more widely accepted Recent African Origin (RAO) hypothesis, specifically the idea anatomically modern humans evolved exclusively in Africa and that there was a population replacement of all archaic humans (e.g. Neanderthals) by these African migrants across the Old World, with negligible to no genetic admixture (within the last 120,000 years). An intermediate model between RAO and MRE is the Assimilation model (AM).

The MRE model is often misinterpreted as the polyphyletism, or evolutionary polygenism  of races, when as Multiregionalists explain:

Because of the key role played by genic exchanges in this model, multiregional evolution means that no human species, subspecies, or race can have multiple 'independent origins' in different regions.

Multiregionalism denies a speciation event in the last 2 mya, and sinks H. erectus, archaic and modern humans into a single lineage and species. It describes human evolution spanning the Pleistocene as a "trellis" where all Old World populations are interconnected by gene flow, but where populations at the peripheries (furthest away from Africa) maintained a unique set of morphological traits via a Center-and-Edge effect. As evidence against Recent African Origin (RAO), Multiregionalists argue these trait complexes (called morphological clades) persisted in their regions and were never totally replaced by African migrants at anytime spanning Late Pleistocene 'out of Africa' exit dispersals.

It has been observed that by the "late 1990s most paleoanthropologists were interpreting the fossil and genetic evidence as incompatible with a 'strong' version of the multiregional hypothesis" subsequently Multiregionalists revised their model to allow for a dominant role for Africa during Pleistocene human evolution (Wolpoff et al. 2000).

Origin of modern humans
Multiregionalists such as Milford H. Wolpoff liken the evolution of human modernity to throwing pebbles into a pond:



Each pebble is an advantageous feature, appearing at different times and places. Ripples from the pebbles can spread across the entire pond surface; the interconnections of the populations provide the pathways for the traits to spread.

In other words, Multiregionalists do not think modern humans evolved in one region or anatomical modernity evolved as a single "bio-package", but that modern traits (as a process) appeared individually in separate populations at different times, not limited to Africa, and spread through gene flow.

By 2000-2001, Wolpoff and his colleagues modified MRE from what has been described as MRE 1 (classic or strong Multiregionalism) to MRE 2 (weak Multiregionalism). The revised model now recognizes the vast majority (but not all) of anatomical traits associated with modernity evolved in Africa, which had the largest population size throughout the Pleistocene:

... continuing debate on the nature of modern human origins, with advocates of the Multiregional Evolution (MRE) model arguing against a strictly 'out of Africa' origin for modern humans. However, supporters of the MRE model have more recently accepted the possibility that the majority of the modern human gene pool could have an African origin (Wolpoff et al., 2000, 2001, 2004), and primarily on the argument by Relethford (1999) that until recently more people live in Africa than elsewhere.

Although MRE has always maintained more gene flow came from Africa given its larger population size  MRE 1 did not credit Africa as having a dominant role in human evolution and anatomical modernity. Caspari and Wolpoff (2013) for example now write (consistent with MRE 2) Africa was where most adaptive mutations spread from:

The dominance of African population size for most of the Pleistocene has widely discussed consequences (Eller et al., 2004; Relethford, 2003; Hawks and Wolpoff, 2003). One of these is that with a significant proportion of the world’s population for most of the Pleistocene, Africa must also have been a significant source of adaptive mutations.

Wolpoff cautions in his view "this does not mean that modernity dispersed with Africans". Chris Stringer, regards this dispute to be almost meaningless, i.e. there is little (and insignificant) difference between anatomically modern humans having evolved exclusively in Africa, versus almost entirely in Africa, but Wolpoff & Lee (2012) discuss since there was gene flow connecting modern humans in Africa and Neanderthals in Europe, the evolutionary process of anatomical modernization in Africa had a minor Neanderthal genetic component, meaning "modern human origins are multiregional".

Center-and-Edge
Multiregionalism proposes there was a distinct pattern of morphological continuity in the furthest regions from Africa (the "edges") for over 1 million years, when these areas were settled by H. erectus. To explain how these complexes of skeletal traits persisted for so long and unchanged in these Old World peripheral populations, Multiregionalists do not propose parallel evolution (i.e. no gene flow and separate evolutionary lineages). In 1981, Alan Thorne attempted to solve this paradox: "how did populations retain geographical distinctions and yet evolve together?" without resorting to discredited racial polyphyletism. Thorne realized that isolation is not necessary for long-term marked phenotypic differentiation if the peripheral populations (of a species range) are small. This is because genetic drift will reduce variation (in large populations, drift has less effect), and selection will increase because the peripheries of a species range are least ecologically optimal and most harsh. Conversely, in optimal environments where a species originated (usually always at the center of a species geographical distribution) and has adapted the longest - selection is relaxed and more trait variants can flourish. The speciation center also tends to maintain a large number of occupants (with the greatest polymorphism and heterogeneity), while populations at the peripheries, reflect small populations, with founder and peninsula (bottleneck) effects. Reduced variation in peripheral populations can result in monomorphism, meaning intraregional homogeneity in phenotype, where virtually all individuals look very similar.

Thus, the 'centre and edge' hypothesis provides an explanation for how the contrasting pattern of central variability and peripheral intraregional homogeneity combined with interregional heterogeneity was initially established.

According to Wolpoff and colleagues, regional continuity skeletal traits were maintainable at the peripheries with gene flow if there was a stable interaction between genetic drift, gene flow and selection. Strong gene flow through migration swamping would likely have erased most morphological evidence for regional continuity, but Multiregionalists point out that gene flow throughout the Pleistocene was reduced at the "edges" (unlike the center, where Africa received gene flow that was more multidirectional ) to explain how morphological clades persisted at the peripheries of the Old World:

...the levels of gene flow at the peripheries would be such as to allow both differentiation and stabilization of morphological patterns, and prevent speciation events.

More recently, MRE 2 posits migration swamping occurred across the Old World throughout the Pleistocene because of the larger population in Africa: Under a Primary African Origin, the gene flow out of Africa would affect all traits equally, and any biological distance measure that is an average of all available traits will show the closer relationship to earlier African[s]... Because of the random nature of genetic drift, some traits will retain regional continuity in the face of the overall impact of gene flow.

Note that whereas gene flow affects all traits equally, genetic drift has a different expectation for each trait. So a "Primary African Origin" model (MRE 2) still supports (limited) regional continuity.



Morphological clades
The mark of ancient Java is on all of them.

The mark of ancient Java is on none of them.

Multiregionalists argue that the human populations at the three "edges" of the Old World: Europe, North China and Java, Indonesia (from when H. erectus colonized these areas, to the late terminal Pleistocene) were fairly close to being monomorphic, and therefore had an ensemble of skeletal traits that uniquely characterized each geographic region which Thorne & Wolpoff (1981) coined a morphological clade. It is a key tenet of Multiregional evolution that in the fossil record at these geographical peripheries there should be evidence of skeletal continuity "spanning the period before and after the replacement time" (of the competing RAO hypothesis). Each periphery is said to contain a distinguishing co-occurring set of morphological traits, "a consequence of the colonization events that limited peripheral variability, primarily because of drift and bottlenecking".

Regional continuity is not the claim individual traits do not appear in other regions, only combinations of features:

The regional continuity traits essential to MRE are not autapomorphic (unique derived traits via mutation, diagnostic for a reproductively isolated population); Wolpoff explains: "if single traits clearly reflected the evolutionary sequence in different regions, it would be a disproof of Multiregional evolution because it would suggest there was no network of genetic exchanges linking populations". Instead, morphological clades are said to represent a unique combination of skeletal traits established via founder effect, and maintained by genetic drift at the "edges" through small population-size.

Heterogeneity at the species center (i.e. Africa) would mean such unique "edge" morphological combinations would not appear there given the extremely high variation and countless number of variant combinations of features, e.g. a, b, d, f, but not a, c, f, j etc. Although all the traits a - z individually would be present in African fossils, this point is often misunderstood or overlooked (see below).

There are individual traits that show a disparate frequency by region, i.e. while not exclusive, are found in only one peripheral fossil record at high-moderate frequency, very rarely in another:

The horizontal-oval mandibular foramen is virtually unique to European fossils. It is found in almost no other remains... the horizontal-oval foramen has a significant frequency in the subsequent post-Neandertal populations of Europe and only decreases to rarity in recent Europeans.



These exceptional singular traits such as the H-O mandibular foramen in Europe (which appears in only a single Pleistocene fossil in Africa ) Wolpoff maintains are strong evidence for regional continuity, but because they are such few in number, MRE literature focuses on combinations. The latter also rule out homoplasies: "the double evolution of a whole set of features is improbable to the extreme". For morphological combinations to demonstrate regional continuity, they must show strong heritability (rather than environmental plasticity), and each feature in the set must be independent (not interdependent) of one another. Traits intercorrelated by masticatory functional constraints, or manifestations of the same morphological variable give undue weight - this means regional continuity traits should co-occur with other traits by random chance, and not be mutually dependent.

Groves (1989) tested 16 of Wolpoff's regional continuity traits for North China and 9 for Java, Indonesia. He found none to be exclusive to either these regions, with high overlap of frequency occurrence, concluding: "the Regional Continuity model [MRE] lacks much real substance". Lahr (1992, 1994) and Lieberman (1995) published similar findings. These studies have been criticized by Wolpoff (1996) who notes they only tested traits individually, not combinations (i.e. continuity is "marked by differing combinations of features, and not particularly by differences between their individual frequencies" ).

In two studies by Habgood (1989, 1992) it was shown a limited number of traits combined might lend support to regional continuity: a unique set of 5 traits in North China associated with facial flatness (1. a non-depressed nasal root, 2. nonprojecting, perpendicularly orientated nasal bones, 3. inferior projection or orientation of the zygomatics, 4. a horizontal course of the nasomaxillary and frontomaxillary sutures and 5. an angular junction of the zygomatic bones and zygomatic process of the maxilla), and a unique set of 4 in Indonesia (1. a long and sagittally flat frontal bone with a posteriorly placed minimum frontal breadth,  2. marked facial prognathism, 3. zygomaxillary tuberosity and 4. eversion of the lower border of the zygomatic). Evidence for this small (yet consequential and not trivial) skeletal continuity better fits the Assimilation model (AM) than MRE: "AM differs from MRE by positing that the archaic contribution to modern populations was always relatively small, and thus continuity would only be found in limited details of anatomy".

More recent critics of regional continuity point out that the combinations of morphological traits thought to be unique (and circumscribed) to the 3 peripheries by Multiregionalists, are in fact common in the African fossil record, or occasionally in other regions, e.g. "every one of the regional features claimed to link the Ngandong [Java] and WLH 50 fossils can be found-in the Omo 2 calvaria from Ethiopia".

Habgood (2003) still sees evidence for minor regional skeletal continuity, but proponents of the RAO hypothesis continue to deny any morphological continuity at the Old World peripheries between archaic humans and early moderns (before and after the replacement time by African migrants).

A 2013 re-analysis of regional continuity traits listed by MRE proponents for Indonesia (and the surrounding Australian region) found:

Based on this evidence, it would seem that the hypothesis of an Indonesian component to the ancestry of the first Australians cannot be supported. There are no specific regional characteristics that demonstrate an unequivocal link between the Sangiran or Ngandong fossils and early Australians.

Part of this dismissal of regional continuity was the result of a new reconstruction of the Sangiran 17 skull from Java, which removed the marked facial prognathism and zygomaxillary tuberosity of Wolpoff's reconstruction. Claims of regional continuity in this region have been further criticized by Durband (2004, 2007, 2009). Research by Brown (1981) has highlighted the possibility the flat frontal bones in terminal Pleistocene Australian crania, are artificial deformations. This is problematic for Multiregionalists who cite a flat frontal bone as an indicator of morphological continuity from archaic Indonesians.

Current consensus
Phenotype is an expression of genes, and so it has been calculated the sizable number of MRE continuity features in each periphery - yield estimates of archaic genetic admixture (such as Neanderthal), ranging from 21.5% - 79.9%. Wolpoff et al. (2004) for example argue a Neanderthal contribution to early modern Europeans could be up to 50%. These high percentages have not been met by ancient DNA studies of Upper Paleolithic skeletons, and so MRE 1 has been falsified. A weaker version of Multiregionalism (MRE 2), closer to the Assimilation model (AM) is still though a viable model.

Races


The three morphological clades of the Pleistocene have been called races or allotaxa by Wolpoff, defined as "morphologically diagnosable yet not reproductively isolated populations". According to Wolpoff these races did not continue into the Holocene because the unique morphological combinations at the "edges" disappeared or were sizably reduced in frequency (erasing the Center-and-Edge effect). This is said to have occurred through population size changes at the peripheries (populations were no longer small), increased gene flow, and Neolithic demic-replacements:

Wu (2005) also informs the: "Holocene should have rendered a lot of morphological changes in human skulls and made some of the common features which have existed in Pleistocene China, that no longer exist in recent Chinese populations".