Human Evolution Research Paper

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Africa" to "Out of Beringia": Can Population Genetics Explain the Mechanisms underlying the formation of Distinct Cultures and Linguistic Groups?

The "Out of Africa" Theory

The "Out of Africa" theory has played a major influential role in how other population genetics studies are approached, conducted, and interpreted (reviewed in: Campbell & Tishkoff, 2010). According to this theory populations of anatomically modern (H. sapiens) humans left the African Continent sporadically over a period of 10's of thousands of years and these outward migrations constitute the genetic origins of all non-Africans. The results from these studies in world population genetics paint a fascinating history of our species' gradual and sporadic entry into the rest of the world, a history that only a few decades ago was more theory than fact.

The "Out of Africa" theory gained support when researchers compared genetic markers in the mitochondrial DNA (mtDNA) from Africans and non-Africans. The mtDNA is maternally inherited, which made it significantly less complicated to trace macro-migratory patterns during the evolutionary history of our species. Today, population genetic studies utilize a variety of genetic markers to map evolutionary lineages for humans and other species, including mtDNA, single nucleotide polymorphisms, inversions and transversions, INDELs (insertions/deletions), nuclear microsatellites, X-chromosome, and the Y-chromosome. With the complete sequence of both the autosomal and mitochondrial genomes now known the ability to reveal the finer details of our evolutionary history is at hand.

Homo sapiens evolved approximately 150,000 to 200,000 ya (years ago) in Africa based on the fossil record and genetic history of our species. By 100,000 ya, H. sapiens arrived in the Near East from Africa. Sometime between 40,000 and 80,000 ya another migration out of Africa entered Eurasia. Recent research supports the theory that the primary exit point from the African Continent for these populations was East Africa, across the Bab-el-Mandeb straight at the mouth of the Red Sea, and then along the south Asia coastline to Australia/Melanesia. Fossil records dating back to 55,000 ya in Australia support this conclusion. Although migration through North Africa and the Nile Valley could have been another major exit point, the genetic evidence points to East Africa as the only one.

The genetic diversity among Africans is much greater than among non-Africans. This is due to a much larger effective population size in Africa, which is currently estimated to be between 2,300 and 15,000 based on sequence analysis of small regions of autosomal DNA. By comparison, the effective population size for non-Africans is predicted to be between 300 and 7,500. This difference supports the theory that Africans migrated out of Africa to give rise to Non-Africans, creating a bottleneck that restricted the genetic diversity of non-Africans. Detailed analysis of short tandem repeat polymorphisms, INDELs, and single nucleotide polymorphisms for 121 geographically-distinct regions in Africa revealed 14 genetically-distinct ancestral population clusters currently in existence today. With few exceptions, these genetic ancestral clusters also define distinct linguistic and cultural populations. This finding suggests that these 14 genetic ancestral clusters became isolated for extended periods in evolutionary history, long enough for them to experience genetic drift and adaptation, and to develop unique languages and cultures. In Africa at least, population genetics seems to provide a reasonable explanation for how various ethnically-distinct groups have evolved.

"Out of Beringia" Theory

Relatively recently in evolutionary time, H. sapiens is believed to have crossed the Bering Strait from Asia about 21,000 ya during the coldest part of the last ice age (reviewed in: Sandoval et al., 2009). This theory is supported by both mtDNA lineage analysis of Native Americans and the archeological evidence, but several alternative theories are still being investigated. Based on this theory, intrepid Asian adventurers are believed to have settled on the North American side of the land bridge formed during the last ice age. The mtDNA evidence further suggests that there was only one major migration event into Beringia from Asia. This may have been due to the loss of the land bridge as the ice age began to end the sea levels rose. The five founder lineages therefore arrived in the New World at about the same time.

The mtDNA evidence also revealed that this event represented an extreme example of an evolutionary bottleneck, since only five founder genotypes survived to…[continue]

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