Pan‐Africanism vs. single‐origin of Homo sapiens: Putting the debate in the light of evolutionary biology

Abstract The scenario of Homo sapiens origin/s within Africa has become increasingly complex, with a pan‐African perspective currently challenging the long‐established single‐origin hypothesis. In this paper, we review the lines of evidence employed in support of each model, highlighting inferential limitations and possible terminological misunderstandings. We argue that the metapopulation scenario envisaged by pan‐African proponents well describes a mosaic diversification among late Middle Pleistocene groups. However, this does not rule out a major contribution that emerged from a single population where crucial derived features—notably, a globular braincase—appeared as the result of a punctuated, cladogenetic event. Thus, we suggest that a synthesis is possible and propose a scenario that, in our view, better reconciles with consolidated expectations in evolutionary theory. These indicate cladogenesis in allopatry as an ordinary pattern for the origin of a new species, particularly during phases of marked climatic and environmental instability.

which show that diversity is greater in Africa than in any other region of the world, decreasing with increasing geographic distance from this continent. The fact that small portions of the present genome of H. sapiens are of Eurasian "archaic" origin (i.e., introgressions from Neanderthals, Denisovans and other deeply divergent lineages) 19,20 rejects the strictest versions of RAO-that is, a full replacement scenario-although this does not provide support to the intercontinental and long-standing gene flow claimed by MRE. [21][22][23] Now that research on modern human origins has shifted its focus to what happened within the African continent at the dawn of our species, some scholars suggest that a continent-wide process could have occurred during the second half of the Middle Pleistocene, leading to the hypothesis commonly referred to as "pan-African." 3,6,24,25 This stands in contrast to the idea, implicit in some of the early RAO formulations, of a cladogenetic and punctuated event of speciation (sensu Eldredge & Gould 26,27 ; Lieberman & Eldredge 28 ), with the subsequent dispersal of H. sapiens in and outside Africa.
In this paper, we critically review the two latter positions from the perspective provided by evolutionary biological knowledge of speciation. We suggest that, when a "simple single-origin" (i.e., localized evolution of the entire "package" of modern traits) is excluded, the actual alternative is between the pan-African scenario and an "extended single-African-origin." This is viewed here as the result of both premodern and postmodern phases of mosaic evolution of traits, interposed by the crucial change represented by the appearance of a new architecture of the neurocranium (i.e., globularity), with its underlying ontogenetic mechanisms and determinants.

| Contenders for the cradle of modern humans
Different bodies of evidence have been used to support the view that our species evolved within a single ancestral population, which should be traced back to a localized region in Africa. Based on different tangles of independent lines of evidence, an eastern and a southern birthplace for H. sapiens have both been proposed. 14,[29][30][31][32][33] The East African system of rift valleys, with a complex topographic and ecological structure favouring niche subdivision and therefore promoting diversity, 34 has always been in the spotlight of human evolutionary research, offering a wealth of paleoanthropological and archaeological discoveries, thus becoming the top candidate as "cradle of humankind." 14,35 The patchy sets of environments and the variety of biomes have been shown to house hotspots of endemism in many vertebrate taxa (particularly amphibians, birds and mammals 36 ). Thus, a sort of "East side story" (Coppens 37 ), as proposed for the origin of hominins, has also been suggested for the emergence of our species. [38][39][40] The biological evidence that is usually cited to support an eastern birthplace for H. sapiens is twofold.
First, the earliest accepted fully modern human skulls have been found at Ethiopian sites, in the Kibish Formation of Omo Valley 14 and at Herto, in the Middle Awash, 13 with the generally reported ages of 197 and 160 ka, respectively. Recently, Vidal et al. 16 have proposed a new minimum age for the Omo fossils of 233 ± 22 ka, by dating the proximal deposits of the Shala volcano's eruption. Omo Kibish 1 and Herto 1 specimens are endowed with a modern cranial morphology, which is usually held to consist in a high, rounded and voluminous vault, and a small, gracile face, with evidence of a canine fossa and mental eminence (in Omo 1), 41,42 thus providing East Africa with the strongest case for human phenotypic evolution. These representatives of anatomically modern humans were still more robust than more recent ones, and some specimens show a still strong supraorbital torus, although dived into central and distal parts. 42 In Table 1 an overview of H. sapiens-derived (autapomorphic) features is reported, according to various authors. As we will also detail later (see Figure 1 and Section 4.2), such traits should not be considered equivalent from an evolutionary perspective. We believe that changes in "architectural" features, like cranial shape, bear major evolutionary implications, even when they appear combined with the expression of peculiar discrete traits ("archaic reminiscences," like a strong supraorbital torus, as in Herto 1). From a geographic perspective, although the material evidence of sedimentary basins of East Africa takes advantage of particularly favorable conditions of fossilization, some still argue for a major role of East Africa as a crucial area of endemism for its particular biogeographical context. 43 Second, the above-mentioned datings for Omo and Herto remains sat well with pioneering genetic studies of mitochondrial DNA (mtDNA) of different modern populations worldwide. Studies performed in the late '80s estimated that the most recent matrilineal common ancestor (mt-MRCA)-the so-called "mitochondrial Eve"-dated to 200 ka and T A B L E 1 Derived traits of Homo sapiens as reported in the literature

High and rounded neurocranium
Lieberman et al. 44 Bruner et al. 45 Stringer 15 Mounier and Lahr. 46 Basicranial flexion Lieberman et al. 44 Bastir et al. 147 Stringer 15 Small and bipartite (or absent) supraorbital torus White et al. 13 Stringer 15 Galway-Witham et al. 48 Small and retrocessive face Stringer 15 Lacruz et al. 49 Full, inverted T-chin Mounier et al. 50 Mounier and Lahr. 46 Absent retromolar space Mounier et al. 50 Prolonged postnatal growth period Kuzawa et al. 51 Hublin et al. 52 Stringer 15 Narrow pelvis Stringer 15 Galway-Witham et al. 48 lived in Sub-Saharan Africa. 12 24,54 ; in the middle), and Skhul 5 (ca. 100-130 ka 55 ); although the facial shape of Irhoud shows some similarities with more recent specimens such as Skhul 5, its elongated cranial shape is clearly plesiomorphic, whereas the latter specimen exhibits a globular braincase and a high, vertical forehead, though combined with some reminiscence of "archaic" discrete traits (e.g., the prominent brow ridges). Conversely, Broken Hill Is definitively more "archaic" in both architectural and discrete features.
ones. Extending Mayr's geographical perspective on speciation, Gould and Eldredge derived a macroevolutionary mechanism for variability in rates of evolution, the "punctuated equilibria" theory, 26,27 arguing that speciation is a rare event that punctuates a system in apparent equilibrium (or "stasis"). According to such view, frequently the onset of new species is a rapid process (geologically speaking), and new species are to be found in narrowly limited regions, geographically distant from (or isolated with respect to) the area of their ancestors.
Inevitably, these ideas exerted-and still do-an indirect but significant impact on paleoanthropological research, [70][71][72]  There is no doubt that the current debate has added new depth and complexity to the narrative of modern human origins, as we shall explore in the following sections. However, theoretical ambiguity, regarding for instance the morphological diagnosability of early members of H. sapiens and the significance of the label "multi- 3 | PAN-AFRICAN VIEW

| Challenges and implications of Jebel Irhoud
There is little doubt that recent discoveries and new dating efforts at Jebel Irhoud (Morocco) have played a major role in promoting the view that our origins may have involved the African continent at a broader scale, and over a longer period of time. 15,24,54 The site was discovered during mining activities in the '60s, and it has since then yielded many human specimens, notably an almost complete skull (Irhoud 1), an adult braincase (Irhoud 2), and an immature mandible (Irhoud 3). The interpretation of the fossils has long been highly controversial due to uncertainties in the geological age and their problematic mixture of archaic and derived (more sapiens-like) morphologies, swinging between different conclusions and implications (see Table 2 for an overview). The findings are sometimes too hastily referred to as "the oldest Homo sapiens fossils" or "modern human fossils" not only by media coverage, 83,84 but also in scholarly publications. 85,86 In fact, as also shown by Hublin and colleagues 24 in their principal component analysis (PCA), the braincase of the Jebel Irhoud specimens is elongated, with an angled occipital, therefore visibly not appearing "sapiens"-like (see Figure 1). On the other hand, the relatively gracile faces and the dentition appear to be closer to modern variability (Bruner &

| "African multiregionalism" and archaic metapopulations
Scerri and colleagues 25  According to the authors, a "quadruple radiation" involved lineages leading to Khoe-San hunter-gatherers, Central African huntergatherers, East and West Africans, and a "ghost modern" population.
Different approaches are currently present in the literature and might partly reflect different aspects of the divergence process, 99   We favor the latter alternative, which should be referred to as an extended single-African-origin, as to distinguish it from older oversimplified narratives.
We also note that these scenarios resonate well with the models recently proposed in a review by Bergström and colleagues ( Figure 2 in their paper), 3 namely the model of the "long-standing pan-African connectivity" and that of the "expansion pulses." Their review fruitfully distinguishes three major phases in recent human evolution: In what follows, we will approach the debate in terms of a speciation process arising from the hominin variability in Africa during the late Middle Pleistocene and will consider the role of climatic context in shaping biogeography, selective conditions, and connectivity among different demes. To do so, it is necessary to spell out what is meant by "speciation" and "species" in this context, and the significance of cranial globularity as a modern morphological trait.

| Species and speciation
Evolutionary theory indicates (following Mayr 68

| Globularization
There is an extensive consensus among researchers that, when cranial anatomy is considered, the morphology of H. sapiens is Although globularity is surely not the only derived trait in H.
sapiens (Table 1), we suspect that changes in such architectural traits are revelatory of significant evolutionary transitions-a step-change, that is, a speciation process-as major skull-brain reassessments and a whole new developmental program are required. In fact, it has been demonstrated that the morphological changes underlying the globularity of our neurocranium occur early in ontogeny (see Gunz et al. 117 ). As concerns the endocast (brain and meningeal membranes), changes involve a "neomorphic hypertrophy of the F I G U R E 3 The modern cranial architecture (i.e., the cranial shape of Homo sapiens) is clearly distinguishable from more archaic morphologies, as it is demonstrated by a PCA based on geometric morphometric data (a); moreover, this is the result of a peculiar developmental process leading to its globular appearance (b). This picture combines Figure 2 in Mounier and Lahr 46 and Figures 1 and 2 in Gunz et al. 115 : see references for detailed legends. PCA, principal component analysis.

| Extended single-African-origin: A renewed scenario
The remains that should be considered in an extended perspective of the chronology and geography of the emergence of modern humans are those characterizing the phenetic diversity that is recorded across What can discriminate between the two abovementioned evolutionary outcomes-pan-Africanism versus a major localized contribution to our evolution or an extended single-African-origin

| CONCLUDING REMARKS
In this paper, we critically reviewed the evidence regarding two alternative scenarios for the origin (i.e., the speciation) of H. sapiens, both within the general paradigm of a Recent African Origin or RAO 6,10 : the single-origin hypothesis 13,14,29 and the pan-African model. 25 We argue that the former hypothesis represents a sort of "evolutionary ordinariness," being more parsimonious with respect to a continent-wide speciation for H. sapiens and more compatible with present background knowledge in evolutionary biology, as it would most likely be predicted for other vertebrate or mammalian species. 76,107,109,145 By contrast, the latter scenario, in assuming a polycentric appearance for the suite of modern human autapomorphies, appears more appropriate for a microevolutionary process of diversification, leading to subspecific taxonomic ranks.
When viewed from a macroevolutionary perspective a similar scenario, extended also to Eurasia, might describe the evolutionary history of the entire group-that is, the "pan group"-from which our species ultimately originated. In this case, it should therefore be referred to the putatively ancestral, geographically widespread and phenetically diversified (as well as taxonomically controversial 130,146 ).
Homo heidelbergensis, 101 including the diverging Neanderthal and Denisovan lineages, viewed as part of the crown group to which H.
sapiens belongs too.
Conversely, we suggest that the available evidence is compatible with a major event of speciation for the origin of H. sapiens, which was more probably punctuated within the wide African scenario, in view of the crucial and allopatric appearance of a globular braincase. 6 share a suite of morphological traits with modern populations-that is, a more gracile face or a modern-like dentition, it is not sufficient to envisage these samples as part of the same crown node. Instead, they may better represent the occurrence of a stem group emerging from the same basal node.
What is informative in our view is that such novelties coalesce Autapomorphy: derived character state that is restricted to a single lineage.
Cladogenesis: diversification of evolutionary lineages through branching, whereby an ancestral lineage splits into two or more descendant lineages (from the Greek clados, "branch"). Cladogenesis is the fundamental basis of biodiversity, with speciation as its core mechanism.
Globularization: it refers to an early phase in the ontogenetic trajectory of our species in which the endocranial shape changes to a more globular (round) form.

DATA AVAILABILITY STATEMENT
Data sharing not applicable to this article as no new data set was generated or analysed during the current study.