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Admin moved page How was human genetics shaped by early human migrations? to How were human genetics shaped by early migrations from Africa
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[[File:Spreading homo sapiens la.svg.png|thumbnail|370px350px|left|Figure 1. Proposed migration patterns out of Africa for modern humans.]]
Early anatomically modern human evolved in Africa, possibly east Africa, at least 200,000 years ago. However, recent finds suggest this might be older. Population bottlenecks, or a reduction in population size, resulted in a relatively less genetically diverse population that would eventually influence all human populations today. Migration out of Africa ensured that the genetic makeup began to be less diverse as humans migrated to other regions and continents.
===Population Bottleneck===
Population bottlenecks often reduce genetic diversity, as a large reduction in population means that subsequent populations will inherit that more limited gene pool. Although modern humans evolved perhaps more than 200,000 years ago, as late as between 80,000-70,000 years ago, a population bottleneck has been proposed for modern humans. There is evidence of major climatic change at around this time, perhaps triggered by a super volcanic eruption activity in Lake Toba, Indonesia, that led to rapid global cooling. Some have suggested that this was the key even event that resulted in a more limited gene pool for modern humans. Others have disputed this finding, and have suggested that the bottleneck may have been triggered much earlier, at about 100,000 years ago or more. <ref>For more on the recent debate, see: Lane, C.S., Chorn, B.T. & Johnson, T.C. (2013) Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka. <i>Proceedings of the National Academy of Sciences.</i> [Online] 110 (20), 8025–8029. Available from: doi:10.1073/pnas.1301474110.</ref> Other work has suggested perhaps by 100,000 years ago variations in modern humans became less, as a limited population began to influence the genetic makeup of many other subsequent human populations.<ref>For more on population bottleneck events that may have shaped how human genetics evolved, see: Cochran, G. & Harpending, H. (2009) <i>The 10,000 -year explosion: how civilization accelerated human evolution.</i> New York, Basic Books.</ref>
Where there is greater diversity, it is found in Africa itself, where different modern African populations show far greater genetic diversity than the rest of modern human populations in all other regions of the globe. Human Humans in general are much less genetically diverse than many other types of animals. Overall, this lack of diversity has suggested a limited number of humans may have been responsible for the variety of populations today, suggesting relatively small numbers, or even population bottlenecks, shaped the population of areas outside of Africa. The relatively greater genetic diversity in Africa suggests humans were evolving like many other species in Africa, creating new populations over time. Climatic effects or crises may have then triggered a large-scale migration out of Africa that subsequently led to the more narrow genetic diversity found today in other regions.<ref>For more on African genetic diversity, see: Campbell, M.C. & Tishkoff, S.A. (2008) African Genetic Diversity: Implications for Human Demographic History, Modern Human Origins, and Complex Disease Mapping. <i>Annual Review of Genomics and Human Genetics.</i> [Online] 9 (1), 403–433.</ref>
===Migration From Africa===
In east Asia, east Siberia, Korea, and Japan, modern humans may have been reached these areas by 35,000 years ago. This population that migrated to these regions also led to a sub-population that became the first colonizers of the Americas. It is not clear when, but between 40,000-16,500 years ago, modern humans migrated over the Beringia land bridge, connecting North America with East Asia in Russia and Alaska today, likely using the ice sheets that formed in the late glacial maximum period when ice sheets and glaciers covered many parts of North America. Recent genetic work has shown that human populations in the Amazon are genetically similar to Australoid populations, or populations that also migrated to east Asia and Australia. In effect, human migrations to the Americas likely traveled along the coastal regions between North and South America.<ref>For more on migrations in east Asia, see: Su, B., Xiao, J., Underhill, P., Deka, R., et al. (1999) Y-Chromosome Evidence for a Northward Migration of Modern Humans into Eastern Asia during the Last Ice Age. <i>The American Journal of Human Genetics.</i> [Online] 65 (6), 1718–1724.; for migrations in the Americas, see: Hey, J. (2005) On the Number of New World Founders: A Population Genetic Portrait of the Peopling of the Americas Andy G. Clark (ed.). <i>PLoS Biology.</i> [Online] 3 (6), e193. Available from: doi:10.1371/journal.pbio.0030193.</ref>
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===Genetic Diversity===
