quote:Brenna M. Henn,
Genetic and paleoanthropological evidence is in accord that today’s human population is the result of a great demic (demographic and geographic) expansion that began approximately 45,000 to 60,000 y ago in Africa and rapidly resulted in human occupation of almost all of the Earth’s habitable regions. Genomic data from contemporary humans suggest that this expansion was accompanied by a continuous loss of genetic diversity, a result of what is called the “serial founder effect.” In addition to genomic data, the serial founder effect model is now supported by the genetics of human parasites, morphology, and linguistics. This particular population history gave rise to the two defining features of genetic variation in humans: genomes from the substructured populations of Africa retain an exceptional number of unique variants, and there is a dramatic reduction in genetic diversity within populations living outside of Africa. These two patterns are relevant for medical genetic studies mapping genotypes to phenotypes and for inferring the power of natural selection in human history. It should be appreciated that the initial expansion and subsequent serial founder effect were determined by demographic and sociocultural factors associated with hunter-gatherer populations. How do we reconcile this major demic expansion with the population stability that followed for thousands years until the inventions of agriculture? We review advances in understanding the genetic diversity within Africa and the great human expansion out of Africa and offer hypotheses that can help to establish a more synthetic view of modern human evolution.
- human population growth
- hunter-gatherer demography
- molecular evolution
Genetic data indicate that, approximately 45 to 60 kya, a very rapid population expansion occurred outside of Africa, and spread in all directions across the Eurasian continents, eventually populating the entire world. We dub this event the Great Expansion (Fig. 1). The precise location of the exit from Africa, and the relative timing of the southern coastal migration vs. the expansion into northern Eurasia, are still under intense debate (1⇓–3). However, as we discuss here, many parameters of the Great Expansion are now well understood, including the general timing of the exit, the magnitude of the associated bottleneck, and the mode of subsequent expansion. We review the history of the Out of Africa (OOA) expansion, with an emphasis on recent genomic data, and highlight future avenues of research.
Fig. 1.
Ancient dispersal patterns of modern humans during the past 100,000 y. This map highlights demic events that began with a source population in southern Africa 60 to 100 kya and conclude with the settlement of South America approximately 12 to 14 kya. Wide arrows indicate major founder events during the demographic expansion into different continental regions. Colored arcs indicate the putative source for each of these founder events. Thin arrows indicate potential migration paths. Many additional migrations occurred during the Holocene (11).
It is important first to distinguish between the presence of early near-modern humans in the Near East and the very distinct OOA exit associated with the Great Expansion. It is clear that anatomically near-modern humans occupied the Levant (4) during a warm interglacial period 130 to 80 kya, when this region was ecologically similar to northeastern Africa (5). However, current evidence indicates that this near-modern population did not persist in the Near East and was subsequently replaced by Neanderthals during the following glacial period, with little evidence of temporal overlap (5, 6). It is not until at least 50,000 y ago that evidence of behaviorally modern humans occurs in the archaeological record in the Near East. Only after this point do anatomically and behaviorally modern human remains become widespread in Eurasia. It is unclear what precipitated the tremendous population growth associated with this second occupation of the Near East and subsequent dispersal (7); possibly, cultural advances accumulated to a “tipping point” that supported extreme demographic growth (8), or anatomical changes that are not reflected in the paleoanthropological record [e.g., neuroanatomy (9)] occurred in the ancestral population. The geographic range expansions of humans outside of Africa were almost certainly associated with climatic fluctuations (10); however, the Great Expansion was an unprecedented demic success that occurred when the climate remained substantially colder than the previous interglacial. As discussed later, we know that the ancestral population for this expansion was African, but the rate of growth and structure of the ancestral population remain poorly understood.
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Fig. 2.
Schematic of a serial found effect. We illustrate the effect of serial founder events on genetic diversity in the context of the OOA expansion. Colored dots indicate genetic diversity. Each new group outside of Africa represents a sampling of the genetic diversity present in its founder population. The ancestral population in Africa was sufficiently large to build up and retain substantial genetic diversity.
The third assumption is that there have been no dramatic postexpansion bottlenecks that differentially affected populations from which the serial migration began. If the source population for the expansion suffered a severe bottleneck that reduced its genetic diversity, we should see a poorer linear fit to the decline of heterozygosity with distance from Africa, or erroneously assign a population with higher genetic diversity as the source population. It is this third assumption we believe deserves additional consideration.
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L. L. Cavalli-Sforzaa,1, and
Marcus W. Feldmanb,2
Edited by C. Owen Lovejoy, Kent State University, Kent, OH, and approved September 25, 2012 (received for review July 19, 2012)
http://www.pnas.org/content/109/44/17758.full
This idea of diversity is correct, but it is probably not due to "genetic drift, isolation and random mutations ". First of all there is no such thing as "random mutation". Genetic drift could have influenced some hypothetical populations, but the diversity among African people is probably due to different origins for each African/Black population (Khoisan, Pygmy, etc) and later mating among the diverse Black populations. Just as there formerly existed homo erectus, homo habilis etc., there was homo australian, homo khoisan, homo pgymy and homo Sub-Saharan African. This view is supported by the skeletal history of anatomically modern humans in Africa.
Thusly, there was more than one out of Africa (OoA) event in which Black people left Africa. The first OoA event was led by homo Australians who left Africa sometime before 100kya, homo Khoisan 45kya, homo pygmy 15kya and finally homo Sub-Saharan Africans 6kya.
This is the only way to explain African diversity. Henn's models for the "Great Expnasion" lack any foundadtion because he is modeling a hypothetical OoA event in which there was a single 'Great Expansion', when there is no evidence that the so-called Caucasian and Mongoloid populations existed anywhere on earth before 6kya. The first Caucasian skeletons do not appear in the archaeological record before 1500BC, Classical Mongoloids (Indonesian,Japanese, Filipino type) around 4000BC, and contemporary East Asian (Chinese, Korean) 1000BC. The late appearance of these populations, and the record of different African populations originating at different times in history provides no support to Henn's 'Great Expansion' theory. It lacks congurence because the theory is based on the idea of the constancy of habitation of the various contemporary populations in the regions they now inhabit, when the historical and archaeological records fail to support any analogy between the present location of populations in the Americas, Eurasia, and even Africa to the populations that inhabited these regions hundreds, let alone thousands of years ago. This lack of analogy is due to population shifts resulting from migrations and war. These factors would have been the major influence on genetic shifts, instead of isolation, genetic drift and genetic mutation.
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