Why do people with a broadly European ancestry have less genetic variability than those of a broadly African ancestry PDF

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A thorough look on why people of European ancestry have less genetic variability than Africans. ...

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Why do people with a broadly European ancestry have less genetic variability than those of a broadly African ancestry?  No two humans are genetically identical. On average, in DNA sequence, each human is 99.9% similar to any other human. Even monozygotic twins (who develop from one zygote) have infrequent genetic differences due to mutations occurring during development.  Africa is where modern humans evolved and is the starting place for the global expansion of our species. African populations also have the highest levels of genetic and phenotypic variation among all humans. Modern humans evolved in Africa ∼200 kya (thousand years ago) and migrated from Africa within the past 50–100 kya, successfully colonizing most of the terrestrial parts of the globe. There were at least several "out-of-Africa" dispersals of modern humans, possibly beginning as early as 270,000 years ago, and certainly during 130,000 to 115,000 ago via northern Africa. The most significant "recent" wave took place about 70,000 years ago, via the so-called "Southern Route", spreading rapidly along the coast of Asia and reaching Australia by around 65,000–50,000 years ago. While Europe was populated by an early offshoot which settled the Near East and Europe less than 55,000 years ago. This is called the “Out-of-Africa” migration. "Recent African origin," or Out of Africa II, refers to the migration of anatomically modern humans (Homo sapiens) out of Africa after their emergence at c. 300,000 to 200,000 years ago, in contrast to "Out of Africa I", the migration of archaic humans from Africa to Eurasia between roughly 1.8 to 0.5 million years ago. In Europe, these modern humans displaced the Neanderthal (Homo neanderthalensis) as well as other species of the Homo genus and were eventually found on all continents. This means that modern humans have diverged not that long ago, and it also explains the low number of genetic differences among human. At each step, a small group of migrants took a fraction of humanity's genetic diversity with it, creating a series of population bottlenecks.  Consistent with the “Out of Africa” model of modern human origins, analyses of genetic data indicate that Africans have higher levels of genetic diversity than non-Africans. A 10-year study published in 2009 led by a geneticist, Sarah Tishkoff, demonstrated startling diversity on the continent of Africa, shared ancestry among geographically diverse groups and traces the origins of Africans and African Americans. Researchers studied 121 African populations, four African American populations and 60 non-African populations for patterns of variation at 1327 DNA markers. The study traced the genetic structure of Africans to 14 ancestral population clusters that correlated with ethnicity and shared cultural and/or linguistic properties. The research team demonstrated that there is more genetic diversity in Africa than anywhere else on earth. They also determined that the ancestral origin of humans was probably located in southern Africa, near the South Africa-Namibian border, corresponding to the current San homeland.

 According to a 2000 study of Y-chromosome sequence variation by Peter Underhill, modern extant human Y chromosomes trace ancestry to Africa and that the descendants of the derived lineage left Africa and eventually replaced archaic human Y chromosomes in Eurasia. The study also shows that a minority of contemporary and Khoisan (San or bushmen) are the descendants of the most ancestral patrilineages of anatomically modern humans that left Africa 35,000 to 89,000 years ago.  Ancient 'Out of Africa' migration left stamp on European genetic diversity. The DNA of European-Americans appears to carry proportionately more harmful genetic changes than that of African-Americans, because they emerged from a smaller and less diverse population. A study published in Nature led by Prof Carlos Bustamante of Cornell University, New York State, shows that the proportion of single letter spelling variations in the human genetic code that are probably harmful and unique to that particular population are significantly higher in the European-Americans (16 per cent) than in the African-American sample (12 percent) his team analysed. While showing overall less genetic variation, had proportionately more amino acid changes and proportionately more harmful amino acid single nucleotide polymorphisms than the African sample. His team speculates that this is a consequence of a "bottleneck" - a huge decline in numbers - that Europeans experienced at about the time of the migration out of Africa, around 45,000 years ago. "What we may be seeing is a 'population genetic echo' of the founding of Europe,".  Another study by Lohmueller and his colleagues show there is proportionally more deleterious genetic variation in European than in African populations. They mainly concentrated on the distribution of what are generally known as single nucleotide polymorphisms (SNPs for short). The data showed that genetic diversity was lower in the European population than in the African population; however, there was a higher ratio of non-synonymous SNPs. That is why the researchers assumed that the higher ratio of nonsynonymous SNPs in the European population correlated with the different effect of natural selection acting on the Africans who migrated from Africa and populated the rest of the world and those who remained in Africa. The researchers tried to simulate the observations using diverse demographic migration models. All models in which the populations experience what is known a bottleneck effect had a higher number of non-synonymous SNPs compared to models simulating constant or expanding populations. A genetic bottleneck refers to a population that consists of only a few individuals, therefore entailing the loss of genetic variation. Since in evolutionary terms this expansion happened only recently, the time elapsed has not been sufficiently long for the high number of deleterious effects to be eliminated by natural selection. This also explains why Africans have a higher variation. When the emigrating group separated from the larger group, only part of the gene pool was taken along. Interestingly, mutations in the human genome were significantly more common in populations that were farther from our African origin (The farther from Africa, the more of them there are).

 Through whole-genome sequencing of three Vindija Neanderthals, a draft sequence of the Neanderthal genome was published by a team of researchers led by Richard Green in 2010. Neandertals are on average closer to individuals in Eurasia than to individuals in Africa. Furthermore, individuals in Eurasia today carry regions in their genome that are closely related to those in Neandertals and distant from other present-day humans. Revealed that Neanderthals shared more alleles with Eurasian populations than with Sub-Saharan African populations. According to Green et al. (2010), the observed excess of genetic similarity is best explained by recent gene flow from Neanderthals to modern humans after the migration out of Africa. This provides evidence that archaic human species (descended from Homo heidelbergensis) having interbred with modern humans outside of Africa. Green et al. (2010) estimated the proportion of Neanderthal-derived ancestry to be 1–4% of the Eurasian genome but it was later revised to a higher 1.8–2.6% and it was noted that East Asians carry more Neandertal DNA (2.3–2.6%) than Western Eurasians (1.8–2.4%) in Prüfer et al. (2017)....