“Out of Africa” Hypothesis
“Out of Africa” Hypothesis
ALTERNATIVES: THE ROLE OF AFRICA
STATUS OF THE OUT OF AFRICA HYPOTHESIS
The “Out of Africa” hypothesis is an evolutionary theory of modern human origin that posits that modern humans arose in the late Pleistocene, about 100,000–200,000 years ago, in Africa. There are different versions of “Out of Africa,” but its major tenet is that modern humans originated as a discrete population or species that rapidly expanded and replaced archaic humans that were indigenous to other parts of the Old World: Homo erectus (or its descendents) in East and South Asia, and Neanderthals in Europe. In the most common version of Out of Africa, modern humans are considered a new species, with negligible gene-flow (mating) between the migrating African people and the indigenous archaic groups. Therefore, the Out of Africa hypothesis, as it is most generally understood, posits that the African population is the unique Pleistocene ancestor of all living humans. The other groups of archaic humans essentially died out and became evolutionary dead ends.
The Out of Africa hypothesis gained rapid acceptance in the late 1980s, with pioneering analyses of mitochondrial DNA (mtDNA), which revealed very low mean nucleotide variation between the mtDNA of individuals from diverse populations (Cann, Stoneking, and Wilson 1987). This suggested that the species was young, since one interpretation of low levels of variation is that there was a genetic bottleneck in the recent past, such as would occur at speciation, and little time since then for subsequent variation to accrue. Moreover, the fact that more variation occurred in African groups suggested Africa as the source. The Out of Africa, or mitochondrial “Eve Theory” as it is also known, has been promoted as underscoring the close relationship between all living humans, and the theory therefore gained ascendancy for sociopolitical reasons as well as scientific ones (Gould 1988).
DEVELOPMENT OF THE HYPOTHESIS
The Out of Africa hypothesis is sometimes dubbed “Out of Africa 2” because it is not the first migration of Homo out of Africa. It is well accepted that the hominid lineage (the unique human lineage since divergence from the last common ancestor with chimpanzees) evolved in Africa, and for two-thirds of its 6-million-year history was an exclusively African clade. However, at the beginning of the Pleistocene 1.8 million years ago, soon after the emergence of the genus, Homo, hominids are found outside of Africa. These archaic Homo populations expanded from Africa to parts of Eurasia in the early Pleistocene, and subsequent populations were able to adapt to more temperate, colder, and even glacial environments in the Ice Ages. The question that the Out of Africa hypothesis addresses concerns the emergence of modern humans: Do modern human beings represent a new species that arose in Africa recently (in the last 200,000 years) and that replaced the earlier migrants?
Throughout the nineteenth and much of the twentieth centuries, the question of modern human origin was subsumed into the context of the “origin of races” (Howells 1942). Racial thinking dominated the science of human variation; the human species was thought to be composed of a number of discrete types of people (“races”) who had separate origins (Stanton 1960), and after the rise of Darwinian thinking, separate evolutionary histories (Figure 1). These separate evolutionary histories were envisioned and depicted as nonreticulating (nonrecombining) branches of phylogenetic trees (Brace 1981). Workers differed on the number and constituents of different racial groups and when in the past they shared a common ancestor, but the “tree” model of human variation, implicit in the race concept, generally prevailed (Caspari 2003). Many workers believed in a few (three to five) primary races (such as Africans, Europeans, and Asians), and then multiple “secondary” and “tertiary” races, all of which could be represented as branches, or twigs, on an evolutionary tree (Coon, Garn, and Bird-sell 1950).
Tree models are unrealistic representations of the relationships between human populations because they fail to represent gene-flow between human groups (Moore 1994). However, they are a part of the construction of “race” because they represent an easily conceived and visualized mechanism to explain human differences—a process of continual isolation, branching, and separation (Caspari 2003).
Evolutionary trees were also used to explain inequality. Some influential nineteenth-century evolutionary trees (Figure 2) depicted European racial groups with longer branches than other groups, implying that they are “more evolved.” The perceived inferiority, and shorter branches, of other racial groups were considered the consequence of their unique evolutionary histories. Racial thinking and the concomitant branching models were so widely accepted that the alternative view—that of a “network” or “trellis” depicting the evolution of the human species as interconnected groups—was largely ignored, or misinterpreted (Figure 3).
Thus, the question of “the origin of races” focused on whether human races had a recent or an ancient common ancestor. Some models, reminiscent of the polygenism of the “American School” of the early
nineteenth century, postulated a very ancient origin of races. This was epitomized by Carleton Coon’s The Origin of Races (1963), in which the five major races Coon recognized were thought to have very long and separate evolutionary histories, sharing a common ancestor prior to the emergence of Homo sapiens. According to Coon, the five races crossed the threshold between Homo erectus and Homo sapiens at different times, and the length of time that races were in the “sapiens” state was related to their cultural “advancement.” Coon wrote that Europeans and Asians crossed the threshold earlier and Africans and Australians considerably later.
This work, first published in 1963, had clear political implications, and was used as propaganda against school desegregation in the early 1960s (Jackson 2001). While Coon’s overt linkage of polygenism to racial discrimination caused a backlash in the anthropological community, with many anthropologists and other scientists denouncing Coon (Dobzhansky 1963; Hulse 1963; Montagu 1964), the tree premise on which his model was based has been more ingrained. For most, tree versus network thinking as applied to human variation was not a focus of the debate. The tree was the dominant model and the question was whether the root of the tree was recent or ancient. By equating gene trees with population trees, tree metaphors are still inappropriately used to reflect relationships between human populations.
Modern thinking on Out of Africa began in the 1970s with the argument that because fossils phenotypically resembling recent humans are found in Africa earlier than anywhere else, “modern humans” originated there (Protsch 1975). Gunter Bräuer (1978, 1984) subsequently used new evidence to argue that Europeans must be of African descent. However, in arguing for African ancestry, neither Protsch nor Bräuer contended that early humans of modern form in Africa implied unique African origins. The Out of Africa hypothesis— the idea of an African origin for a recent modern human species—owes its genesis to interpretations of mtDNA, which suggested that the ancestors of recent humans first appeared in Africa and replaced other populations because they were a new species that did not interbreed (Cann, Stoneking, and Wilson 1987; Stoneking and Cann 1989). This model of replacement without mixture in the process of recent human origin was accepted by some paleoanthropologists (Stringer and Andrews 1988) and remains an influential model in the early 2000s.
THE GENETIC FOUNDATIONS
The Out of Africa hypothesis, the theory of a recent unique African origin for the modern human species, was supported by early interpretations of the variation of mtDNA (Cann, Stoneking, and Wilson 1987; Stoneking and Cann 1989). Advances in gene sequencing technology in the 1980s provided the techniques to sequence the mitochondrial genome, and Rebecca Cann initially compared mtDNA variants from representatives of several different populations. Mitochondria are organelles in the cytoplasm of cells, which play an important role in cell metabolism. Their DNA consists of a single chromo-some, which is inherited maternally and does not recombine. It reproduces by mitosis, so all variation between mitochondria is a consequence of mutation. Assuming that mtDNA is selectively neutral and assuming constant population size, the amount of variation (number of nucleotide differences) between individuals and populations was interpreted to be a consequence of two factors: mutation rate and time since divergence of the mtDNA
lineages. Therefore, given the same mutation rate, two mitochondria with fewer nucleotide differences would have a more recent common ancestor than two with a larger number of nucleotide differences.
Gene trees, like evolutionary trees in general, are hierarchical structures based on a particular gene or locus (these may be single nucleotides, haplotypes, genes, or the entire genome, which can be practical when the genome is short); variants of a gene that share more mutations are clustered together. The mtDNA gene trees derived from Cann’s work were rooted in Africa, based on the observations that more variation was found in Africa and that all human populations had some African mitochondrial variants. The time of this root, based on the mutation rate derived for mtDNA (assuming neutrality), was estimated to be between 100,000–200,000 years ago. In the creation of the “Eve Theory” (as the Out of Africa hypothesis was frequently called), this gene tree was interpreted as a population tree and the root was thought to represent a population bottleneck, a massive reduction in population size where variation is greatly reduced. Such population bottlenecks often accompany speciations, and hence the mtDNA data were thought to reflect the time and place of the birth of the modern human species.
In the twenty years since this early research, genetic analyses have become far more sophisticated; in addition to mtDNA (which, because it does not recombine and is inherited as a single unit, can be considered only one gene), many nuclear genes now contribute to our understanding of human evolution, and the evolutionary models based on genetics have become more complicated. It is now widely understood that many factors, from population size and structure to natural selection, affect genetic variation and that different genes have different histories; in other words, gene trees are not population trees. Because of recombination, autosomal genes within the same individual will have very different evolutionary trees. Different genes reflect different aspects of our ancestry; moreover, if natural selection is acting on a gene, it may give no information about population history at all. The trees of some genes, particularly those on chromosomes that do not recombine, such as mtDNA and part of the Y chromosome, have shallow roots; other loci have roots that are millions of years old.
The Out of Africa theory is based on the loci whose evolutionary trees have shallow roots through the assumption that the recent root of the gene tree represents the recent root of the human species. However, it is now recognized that there are many potential explanations for the shallow roots of these gene trees: The relative effective population size of haploid loci is four times smaller than that of the autosomes, which alone causes the roots of their gene trees to be four times shallower; variations in past population sizes affect the structure and roots of gene trees; and perhaps most importantly, natural selection may have a larger effect on non-recombining loci because of the effects of linkage. For example, selection favoring one locus on the mitochondrial genome affects the entire mitochondrial chromosome because it is inherited together. Given the importance of mitochondria in many functions of cell metabolism, such selection and selective sweeps are highly probable. Any selective sweep affecting mtDNA (or any other locus) will reduce variation and give gene trees an even shallower root. Therefore, Out of Africa (or a population bottleneck at speciation) is only one of many possible explanations for the genetic observations of loci with shallow rooted gene trees (Garrigan and Hammer 2006; Templeton 1998, 2002; Relethford 1998, 2001).
ALTERNATIVES: THE ROLE OF AFRICA
Much of the current genetic evidence is incompatible with the Out of Africa scenario because it does not reflect a bottleneck associated with recent speciation. While there are a number of nuclear loci that do fit the hypothesis (i.e., autosomal loci with roots four times as deep as the mtDNA and loci on the nonrecombining Y), the rate of discovery of loci with deep genealogical histories is rapidly increasing, and some of these have roots outside of Africa (Garrigan and Hammer 2006). The new evidence argues against a recent population bottleneck (speciation) because many genetic loci did not undergo reduction in variation at that time and there is no evidence of the postspeciation population expansions in Africa that would be expected under the Out of Africa model. Moreover, genealogical roots outside of Africa provide evidence of gene-flow between archaic humans in different regions, indicating that they were not separate species. Therefore, recent genetic research suggests that a simple, single origin model for the evolution of modern humans is incorrect, and that the genome of modern humans consists of contributions from multiple archaic populations. However, the many loci that have a recent common ancestor in Africa, as well as the early appearance of many modern skeletal features there, indicate the importance of Africa for the origin of modern humans.
The alternatives to the Out of Africa hypothesis are versions of “Multiregional Evolution,” a model that hypothesizes evolutionary change within the human species with gene flow between “archaic” and “modern” humans rather than evolution due to recent speciation. The multiregional theory does not recognize Pleistocene Africans and archaic groups from Europe and Asia as different species. According to the multiregional model, gene-flow was an integral part of the evolution of modern peoples, dispersing adaptive genes throughout the species, and any one living human is likely to have had Pleistocene ancestors from different parts of the globe. Developed by Franz Weidenreich (1947) as “polycentric theory” in the 1940s, it differed from the prevailing evolutionary models in being network based rather than tree based; it was a reticulating model depicting the evolution of human populations as an intraspecific process, with gene-flow at its core.
Weaker versions of the Out of Africa hypothesis, such as the “Assimilation Theory” (Smith, Jankovic, and Karavanic 2005), where modern humans are a population, or deme, rather than a species are consistent with the multiregional gene-flow model because they do not involve speciation. Contemporary versions of multiregionalism reflect the importance of Africa in modern human origins. The contemporary multiregional model (Wolpoff, Wu, and Thorne 1984; Wolpoff 1989), in its center-and-edge contention, proposed that Africa was a significant source of new genetic variants during human evolution, because throughout human evolution the predominant direction of gene-flow was from the more densely occupied center (Africa) to the more sparsely occupied edges (Europe, East Asia, Australia). The hallmarks of the Out of Africa hypothesis are also addressed by multiregional evolution: Low genetic diversity among human populations is explained through gene-flow rather than recency of origin, and the greater genetic diversity in Africa is explained by larger population size, greater ecological diversity, and natural selection. The inequality of Pleistocene population sizes and the evolutionary consequences of the dominance of African population size have been widely discussed (Harpending, Batzer, and Gurven 1989; Harpending 1996; Relethford 2001; Hawks and Wolpoff 2003). Because of these factors, Africa has provided the strongest regional contribution to modern humans, which is observable genetically and morphologically, but genetic and morphological data also suggest that gene-flow occurred between African and non-African populations. Therefore, while current evidence suggests that the Out of Africa speciation model is incorrect, Africa played a predominant role in Pleistocene human evolution and the origin of modern humans.
RACIAL IMPLICATIONS
Race is intricately involved in human origin theories because these theories address the origin and nature of human biological variation. For many historical reasons, and perhaps some psychological ones, race impacts our understanding of human variation in a circular way: Folk (or social) understandings of variation (race) influence science, and conversely, science has been used to validate social meanings of race (Wolpoff and Caspari 1997).
The Out of Africa hypothesis is no exception. When it was first proposed, it was used to validate progressive political positions; it gained considerable publicity as underscoring the close relationships of living humans. As Steven J. Gould put it in 1988 (p. 21), “Human unity is no idle political slogan … all modern humans form an entity united by physical bonds of descent from a recent African root.”
The idea that the mtDNA ancestor reflected the root of all human populations meant that we all share common ancestors from less than 200,000 years ago, underscoring the “brotherhood of man,” and this view was thought to undermine the race concept. Conversely, the Out of Africa hypothesis has also been used to emphasize the importance of racial difference. Sarich and Miele (2004), for example, have argued that since the species is young, “race” must be biologically important: With little time for differences to accrue, there must have been isolation, strong selection, and different evolutionary histories. Thus, the same theory can be used to support conflicting political ideologies.
STATUS OF THE OUT OF AFRICA HYPOTHESIS
This entry has explored the relationship between the concept of race and evolutionary theories of the origin of modern humans, in particular the Out of Africa hypothesis. It is ironic that the Out of Africa theory, while recently promoted as proof of the “brotherhood of man,” inadvertently undermines this important concept because the assumptions that underlie the model are dependent on an unrealistic “tree model” of human variation—a view that is a legacy of the race concept. Fossil and genetic data support the hypothesis that there was gene-flow both between modern and archaic populations, and between geographic groups of modern humans after their emergence.
However, while recent evidence no longer supports the Out of Africa hypothesis per se, Africa remains important in all theories of modern human origin. Africa was the center of Pleistocene human evolution: Modern human form appears there first, and Africa made the largest regional contribution to the gene pool of modern humans. Africa is central to both single origin and multiregional models of modern human origin. Therefore, while it seems increasingly likely that some gene-flow occurred between African and non-African populations both before and after the emergence of modern humans, and the “new species” version of the Out of Africa hypothesis appears to be incorrect, the importance of Africa as a central region for the evolution of recent humans is well supported.
SEE ALSO Human and Primate Evolution; Human Genetics.
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Rachel Caspari