Models Of Modern Human Origins Research Articles

The Assimilation Model of modern human origins in light of current genetic and genomic knowledge

The Assimilation Model (AM) was developed to explain the presence of evidence for small, but not insignificant, anatomical contributions of Eurasian archaic humans (including Neandertals) to modern Eurasians within a framework of the documented African origin of the majority of modern Eurasian ancestry. The AM was based entirely on morphological evidence for introgression in Eurasia and was initially not well supported by genetic studies, including ancient DNA. However, beginning with the publication of the first Neandertal draft genome in 2010, genomic data have consistently demonstrated low levels of archaic human gene flow into modern Eurasian populations and recently have shown modern gene flow into a Siberian Neandertal. Current data also indicate the complexity of archaic–modern human population dynamics during the late Pleistocene, another tenet of the AM. We conclude that the AM is strongly supported by current genetic and genomic data, as well as morphological evidence that has been documented elsewhere.

Ancient Human Migration after Out-of-Africa.

The serial founder model of modern human origins predicts that the phylogeny of ancestries exhibits bifurcating, tree-like behavior. Here, we tested this prediction using three methods designed to investigate gene flow in autosome-wide genotype data from 3,528 unrelated individuals from 163 global samples. Specifically, we investigated whether Cushitic ancestry has an East African or Middle Eastern origin. We found evidence for non-tree-like behavior in the form of four migration events. First, we found that Cushitic ancestry is a mixture of ancestries closely related to Arabian ancestry and Nilo-Saharan or Omotic ancestry. We found evidence for additional migration events in the histories of: 1) Indian and Arabian ancestries, 2) Kalash ancestry, and 3) Native American and Northern European ancestries. These findings, based on analysis of ancestry of present-day humans, reveal migration in the distant past and provide new insights into human history.

Right for the Wrong Reasons

The sequencing of the Neanderthal genome answered once and for all the question of whether these hominins played a role in the origins of modern humans—they did, and a majority of humans alive today retain a small portion of Neanderthal genes. This finding rejects the strictest versions of the Recent African Origin model and has been celebrated by supporters of Multiregional Evolution (MRE). However, we argue that MRE can also be rejected and that other, intermediate, models of modern human origins better represent the means by which modern humans became the only extant human species. We argue this because we reject one of the major tenets of MRE: global gene flow that prevents cladogenesis from occurring. First, using reconstructions of Pleistocene hominin census size, we maintain that populations were neither large nor dense enough to result in such high levels of gene flow across the Old World. Second, we use mammalian divergence and hybridization data to show that the emergence of Homo is recent enoug...

The Fate of the Neandertals

The fate of the Neandertals is the oldest debate in paleoanthropology and one of the longest, most contentious in science. Here I present my perspective on the biological distinction of Neandertals and their role in the emergence of modern people in Europe and the circum-Mediterranean. Neandertals were highly adapted to life in the cold, demanding realm of Europe during the later Pleistocene. This adaptation was strongly biological, and it came at a high energetic price—a price that negatively affected Neandertal reproduction and ultimately resulted in their “extinction.” The word “extinction” is in quotes because Neandertals did not go extinct in the classic sense. Rather, both morphological and genetic evidence demonstrate a relatively small, but certainly not insignificant, contribution by them to the earliest modern populations that migrated into their range. From my perspective, the current data best support the assimilation model of modern human origins, first formally presented by two of my graduate students and me almost 25 years ago (Smith et al. 1989), to explain Neandertal–early modern population dynamics. I present an update of that argument here. Also I present my views on why it took modern people so long to establish themselves in Europe and what all this means for the study of biological race in humans.

Archaic human genomics

For much of the 20th century, the predominant view of human evolutionary history was derived from the fossil record. Homo erectus was seen arising in Africa from an earlier member of the genus and then spreading throughout the Old World and into the Oceania. A regional continuity model of anagenetic change from H. erectus via various intermediate archaic species into the modern humans in each of the regions inhabited by H. erectus was labeled the multiregional model of human evolution (MRE). A contrasting model positing a single origin, in Africa, of anatomically modern H. sapiens with some populations later migrating out of Africa and replacing the local archaic populations throughout the world with complete replacement became known as the recent African origin (RAO) model. Proponents of both models used different interpretations of the fossil record to bolster their views for decades. In the 1980s, molecular genetic techniques began providing evidence from modern human variation that allowed not only the different models of modern human origins to be tested but also the exploration demographic history and the types of selection that different regions of the genome and even specific traits had undergone. The majority of researchers interpreted these data as strongly supporting the RAO model, especially analyses of mitochondrial DNA (mtDNA). Extrapolating backward from modern patterns of variation and using various calibration points and substitution rates, a consensus arose that saw modern humans evolving from an African population around 200,000 years ago. Much later, around 50,000 years ago, a subset of this population migrated out of Africa replacing Neanderthals in Europe and western Asia as well as archaics in eastern Asia and Oceania. mtDNA sequences from more than two-dozen Neanderthals and early modern humans re-enforced this consensus. In 2010, however, the complete draft genomes of Neanderthals and of heretofore unknown hominins from Siberia, called Denisovans, demonstrated gene flow between these archaic human species and modern Eurasians but not sub-Saharan Africans. Although the levels of gene flow may be very limited, this unexpected finding does not fit well with either the RAO model or MRE model. More thorough sampling of modern human diversity, additional fossil discoveries, and the sequencing of additional hominin fossils are necessary to throw light onto our origins and our history.

Local Extinction and Recolonization, Species Effective Population Size, and Modern Human Origins

A primary objection from a population genetics perspective to a multiregional model of modern human origins is that the model posits a large census size, whereas genetic data suggest a small effective population size. The relationship between census size and effective size is complex, but arguments based on an island model of migration show that if the effective population size reflects the number of breeding individuals and the effects of population subdivision, then an effective population size of 10,000 is inconsistent with the census size of 500,000 to 1,000,000 that has been suggested by archeological evidence. However, these models have ignored the effects of population extinction and recolonization, which increase the expected variance among demes and reduce the inbreeding effective population size. Using models developed for population extinction and recolonization, we show that a large census size consistent with the multiregional model can be reconciled with an effective population size of 10,000, but genetic variation among demes must be high, reflecting low interdeme migration rates and a colonization process that involves a small number of colonists or kin-structured colonization. Ethnographic and archeological evidence is insufficient to determine whether such demographic conditions existed among Pleistocene human populations, and further work needs to be done. More realistic models that incorporate isolation by distance and heterogeneity in extinction rates and effective deme sizes also need to be developed. However, if true, a process of population extinction and recolonization has interesting implications for human demographic history.

Into Eurasia: A geometric morphometric re-assessment of the Upper Cave (Zhoukoudian) specimens

Since the discovery of the human remains from the Upper Cave of Zhoukoudian in the 1930s there has been speculation over their affinities. In particular, the degree to which the three adult crania exhibit recent East Asian morphology, as well as their degree of within-group variability, has long been debated. Several early researchers described a resemblance to East Asian populations, but these findings have been for the most part rejected by more recent metric and non-metric analyses. Nevertheless, the Upper Cave specimens have not been classified conclusively into any recent modern human population to which they have been compared, and classification results differ for each cranium. Here, the question of the affinities of Upper Cave 101 and 103, the two better-preserved crania, is examined from the perspective of the Late Pleistocene human fossil record using the methodology of 3-D geometric morphometrics. The degree of morphological variation between the two specimens is also evaluated within the context of recent population variability. Neurocranial and facial morphology are analyzed separately so as to maximize comparative samples. Results show a morphological resemblance of the Upper Cave material to Upper Paleolithic Europeans. It is proposed that the Upper Cave specimens retain important aspects of modern human ancestral morphology, and possibly share a recent common ancestral population with Upper Paleolithic Europeans, in accordance with the Single Origin model of modern human origins.

Local Extinction and Recolonization, Species Effective Population Size, and Modern Human Origins

A primary objection from a population genetics perspective to a multiregional model of modern human origins is that the model posits a large census size, whereas genetic data suggest a small effective population size. The relationship between census size and effective size is complex, but arguments based on an island model of migration show that if the effective population size reflects the number of breeding individuals and the effects of population subdivision, then an effective population size of 10,000 is inconsistent with the census size of 500,000 to 1,000,000 that has been suggested by archeological evidence. However, these models have ignored the effects of population extinction and recolonization, which increase the expected variance among demes and reduce the inbreeding effective population size. Using models developed for population extinction and recolonization, we show that a large census size consistent with the multiregional model can be reconciled with an effective population size of 10,000, but genetic variation among demes must be high, reflecting low interdeme migration rates and a colonization process that involves a small number of colonists or kin-structured colonization. Ethnographic and archeological evidence is insufficient to determine whether such demographic conditions existed among Pleistocene human populations, and further work needs to be done. More realistic models that incorporate isolation by distance and heterogeneity in extinction rates and effective deme sizes also need to be developed. However, if true, a process of population extinction and recolonization has interesting implications for human demographic history.

Early modern human diversity suggests subdivided population structure and a complex out-of-Africa scenario

The interpretation of genetic evidence regarding modern human origins depends, among other things, on assessments of the structure and the variation of ancient populations. Because we lack genetic data from the time when the first anatomically modern humans appeared, between 200,000 and 60,000 years ago, instead we exploit the phenotype of neurocranial geometry to compare the variation in early modern human fossils with that in other groups of fossil Homo and recent modern humans. Variation is assessed as the mean-squared Procrustes distance from the group average shape in a representation based on several hundred neurocranial landmarks and semilandmarks. We find that the early modern group has more shape variation than any other group in our sample, which covers 1.8 million years, and that they are morphologically similar to recent modern humans of diverse geographically dispersed populations but not to archaic groups. Of the currently competing models of modern human origins, some are inconsistent with these findings. Rather than a single out-of-Africa dispersal scenario, we suggest that early modern humans were already divided into different populations in Pleistocene Africa, after which there followed a complex migration pattern. Our conclusions bear implications for the inference of ancient human demography from genetic models and emphasize the importance of focusing research on those early modern humans, in particular, in Africa.

Artificial cranial deformation in Kow Swamp 1 and 5: A response to

Recently, Curnoe (2007) tested the predictions of competing models of modern human origins using three crania from Australia: Kow Swamp 1 and 5 and Keilor. The Kow Swamp specimens have long been suspected of having been altered through artificial deformation of the skull. Though Curnoe (2007) provided assurances that no evidence of deformation is present in those specimens, the current study retests the hypothesis that these Australian specimens are artificially deformed. The Australian crania are compared to known deformed individuals from New Britian through canonical variates analysis, and the resulting Mahalanobis distances are examined for statistical significance with random expectation statistics. The results show that Kow Swamp 1 and 5 have strong shape similarities to known deformed individuals, and both crania are very different in shape from Keilor. Keilor is statistically significantly different in shape from both Kow Swamp specimens and all of the known deformed specimens. These findings cast doubt on Curnoe's (2007) conclusions of a shared Australian cranial morphology as well as the retention of an archaic suite of morphologies in the Australians.

The question of cranial robusticity

The existence of a morphologically robust Pleistocene Australian population has been controversial. This is largely due to the pivotal role this group has played in the multiregional model of modern human origins. Some researchers have argued that the robust morphology results at least in part from pseudopathology (artificial deformation) or pathology (haemoglobinopathy) rather than representing normal anatomical form. The present contribution puts these alternative explanations to the test. From a package of cranial deformation diagnostics, only one feature (frontal curvature index) suggests possible pseudopathology in one individual (Kow Swamp 5). Thus, the evidence for deformation even in this individual is at best weak. Further, there seems to be no evidence for pathological hyperostosis among Pleistocene Australians including robust crania. On the contrary, the robust morphology continues to be part of the variability characterising living Aborigines. Recently published luminescence dates for Kow Swa...

Pairwise difference analysis in modern human origins research.

Pairwise difference analysis is a phenetic method that groups taxa on the basis of the number of differences they exhibit. Recently, pairwise difference analysis has been used to investigate the phylogenetic relationships of hominid fossils at the centre of the modern human origins debate. It has been argued that the results of these analyses disprove the African replacement model of modern human origins, and support instead its competitor, the multiregional evolution model. However, this inference is problematic because the ability of pairwise difference analysis to recover phylogenetic information from morphological data has not been demonstrated. With this in mind, we conducted pairwise difference analyses of craniodental and soft tissue evidence from a group of extant primates for which a reliable molecular phylogeny is available, the hominoids. We found that the phylogenies yielded by the pairwise difference analyses were incompatible with the molecular phylogeny for the group. Given the robustness of the molecular phylogeny, these results suggest that pairwise difference analysis cannot be relied on to generate reliable estimates of primate phylogeny from morphological data. The corollary of this is that the results of published pairwise difference analyses of hominid fossils are not informative regarding the origin of modern humans.

Absence of regional affinities of Neandertal DNA with living humans does not reject multiregional evolution

The recent extraction of mitochondrial DNA sequences from three European Neandertal fossils has led many to the conclusion that ancient DNA analysis supports the African replacement model of modern human origins and rejects models of multiregional evolution that propose some Neandertal ancestry in living humans. This conclusion is based, in part, on the lack of regional affinity of Neandertal DNA to that from living Europeans. Consideration of migration matrix models shows that this conclusion is premature, since under a model of interregional gene flow we expect to see similar levels of Neandertal ancestry in all contemporary regions, and living Europeans should not necessarily show closer affinity. The absence of regional affinity in Neandertal DNA does not distinguish between replacement and multiregional models.

Activity, Climate, and Postcranial Robusticity

Postcranial robusticity—the massiveness of the skeleton—figures prominently in the debate over the origin of modern humans. Anthropologists use postcranial robusticity to infer the activity levels of prehistoric populations, and changes in robusticity are often used to support scenarios of adaptive change. These scenarios explain differences in morphology as the result of a change in lifestyle (habitual activity). One common scenario posits that early modern humans were more gracile than Neandertals because the modern humans’ complex culture required less physical exertion. However, lifestyle is only one of many influences on morphology. Climate has clear correlations with physique and skeletal proportions. Analysis of recent humans that differ in terms of lifestyle and climatic adaptations reveals that limb bone robusticity varies with climate as much as or more than with lifestyle. Many of the differences in robusticity between Neandertals and early modern humans appear to be related to climatic adaptations. The results support the single‐recent‐origin model of modern human origins. The differences in robusticity between Neandertals and early modern humans suggest that population replacement rather than local evolution best explains the emergence of modern humans in Europe. Both climatic adaptations (primarily body proportions) and lifestyle should be considered in analyses of robusticity.

Evolution at the Crossroads: Modern Human Emergence in Western Asia

There is long-standing disagreement regarding Upper Pleistocene human evolution in Western Asia, particularly the Levant. Some argue that there were two dilierent populations, perhaps different species, of Upper Pleistocene Levantine hominids. The first, from the Israeli silcs of Qafzeh and Skhul. is anatomically modern. The second, from sites such as Amud. Kcbara. and Tabun, is archaic, or "Neandertal" in morphology. Others argue that ihis is a false dichotomy and that all of lliese hominids belong to a single, highly variable population. In this paper I attempt to resolve this issue by examining posteranial measures reflective of body shape. Results indicate that the Qafzeh-Skhul hominids have African-like, or tropically adapted, proportions, while tliosc from Amud, Kebara. Tabun. and Shanidar (Iraq) have more European-like, or cold-adapled. proportions. This suggests that iherc were in fact two distinct Western Asian populations and that the Qaf/ch-Skhul hominids were likely African in origin—i result consistent with the "Replacement"' model of modern human origins, [modern human origins, NeunJertals, Qafzeh-Skhul hominids, body shape]

Activity, Climate, and Postcranial Robusticity: Implications for Modern Human Origins and Scenarios of Adaptive Change

Postcranial robusticity--the massiveness of the skeleton--figures prominently in the debate over the origin of modern humans. Anthropologists use postcranial robusticity to infer the activity levels of prehistoric populations, and changes in robusticity are often used to support scenarios of adaptive change. These scenarios explain differences in morphology as the result of a change in lifestyle (habitual activity). One common scenario posits that early modern humans were more gracile than Neandertals because the modern humans' complex culture required less physical exertion. However, lifestyle is only one of many influences on morphology. Climate has clear correlations with physique and skeletal proportions. Analysis of recent humans that differ in terms of lifestyle and climatic adaptations reveals that limb bone robusticity varies with climate as much as or more than with lifestyle. Many of the differences in robusticity between Neandertals and early modern humans appear to be related to climatic adaptations. The results support the single-recent origin model of modern human origins. The differences in robusticity between Neandertals and early modern humans suggest that population replacement rather than local evolution best explains the emergence of modern humans in Europe. Both climatic adaptations (primarily body proportions) and lifestyle should be considered in analyses of robusticity.

The early Upper Paleolithic human skeleton from the Abrigo do Lagar Velho (Portugal) and modern human emergence in Iberia.

The discovery of an early Upper Paleolithic human burial at the Abrigo do Lagar Velho, Portugal, has provided evidence of early modern humans from southern Iberia. The remains, the largely complete skeleton of a approximately 4-year-old child buried with pierced shell and red ochre, is dated to ca. 24,500 years B.P. The cranium, mandible, dentition, and postcrania present a mosaic of European early modern human and Neandertal features. The temporal bone has an intermediate-sized juxtamastoid eminence. The mandibular mentum osseum and the dental size and proportions, supported by mandibular ramal features, radial tuberosity orientation, and diaphyseal curvature, as well as the pubic proportions align the skeleton with early modern humans. Body proportions, reflected in femorotibial lengths and diaphyseal robusticity plus tibial condylar displacement, as well as mandibular symphyseal retreat and thoracohumeral muscle insertions, align the skeleton with the Neandertals. This morphological mosaic indicates admixture between regional Neandertals and early modern humans dispersing into southern Iberia. It establishes the complexities of the Late Pleistocene emergence of modern humans and refutes strict replacement models of modern human origins.

GENETICS OF MODERN HUMAN ORIGINS AND DIVERSITY

▪ Abstract A major and continuing debate in anthropology concerns the question of whether modern Homo sapiens emerged as a separate species roughly 200,000 years ago in Africa (recent African origin model) or as the consequence of evolution within a polytypic species spread across several regions of the Old World (multiregional model). Genetic data have been used to address this debate, focusing on the analysis of gene trees, genetic diversity within populations, and genetic differences between populations. Although the genetic data do provide support for the recent African origin model, they also are compatible with the multiregional model. The genetic evidence provides little direct inference regarding phylogeny, but it can tell us a great deal about ancient demography. Currently, neither model of modern human origins is unequivocally supported to the exclusion of the other.

Genetic evidence for a Paleolithic human population expansion in Africa.

Human populations have undergone dramatic expansions in size, but other than the growth associated with agriculture, the dates and magnitudes of those expansions have never been resolved. Here, we introduce two new statistical tests for population expansion, which use variation at a number of unlinked genetic markers to study the demographic histories of natural populations. By analyzing genetic variation in various aboriginal populations from throughout the world, we show highly significant evidence for a major human population expansion in Africa, but no evidence of expansion outside of Africa. The inferred African expansion is estimated to have occurred between 49,000 and 640,000 years ago, certainly before the Neolithic expansions, and probably before the splitting of African and non-African populations. In showing a significant difference between African and non-African populations, our analysis supports the unique role of Africa in human evolutionary history, as has been suggested by most other genetic work. In addition, the missing signal in non-African populations may be the result of a population bottleneck associated with the emergence of these populations from Africa, as postulated in the "Out of Africa" model of modern human origins.

Genetic traces of ancient demography.

Patterns of gene differences among humans contain information about the demographic history of our species. Haploid loci like mitochondrial DNA and the nonrecombining part of the Y chromosome show a pattern indicating expansion from a population of only several thousand during the late middle or early upper Pleistocene. Nuclear short tandem repeat loci also show evidence of this expansion. Both mitochondrial DNA and the Y chromosome coalesce within the last several hundred thousand years, and they cannot provide information about the population before their coalescence. Several nuclear loci are informative about our ancestral population size during nearly the whole Pleistocene. They indicate a small effective size, on the order of 10,000 breeding individuals, throughout this time period. This genetic evidence denies any version of the multiregional model of modern human origins. It implies instead that our ancestors were effectively a separate species for most of the Pleistocene.