Modern Human Migrations: The First 200,000 Years
In a way, the DNA sequences contained within each of our cells can be thought of as the text in microscopic book. This text is amazingly long, approximately one million pages if 6,000 letters fit on both sides of a page. This DNA contains the codes used t
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In a way, the DNA sequences contained within each of our cells can be thought of as the text in microscopic book. This text is amazingly long, approximately one million pages if 6,000 “letters” fit on both sides of a page. This DNA contains the codes used to build RNA and proteins, which in turn assemble all the tissues in our bodies and direct the chemical reactions of life. We inherit copies of this DNA genome book from our parents and pass on copies to our children. However, in each generation, small changes are made, approximately 175 letters out of the genome, which are also passed on to our descendants. We can infer how closely people are related by comparing these types of inherited changes, which are shared between individuals or groups of individuals. Most of these changes are of no functional consequence, such as using either American or British spelling for color/colour; the meaning is still the same. However, occasionally, there is a functional change that can result in a phenotypic difference among individuals.
Humans Have Surprisingly Little Genetic Variation If two sequences of human DNA are randomly picked and compared for the same gene region, on average only one “letter” out of a thousand may be different (Li and Sadler 1991). Comparing this diversity to the estimated mutation rate per generation predicts that the DNA sequences are separated by approximately 20,000 generations
F.A. Reed (*) Department of Evolutionary Genetics, Max Planck Institute for Evolutionary Biology, August-Thienemann-Straße, 24306 Pl€ on, Germany Current Affiliation: Department of Biology, University of Hawai‘i at Ma¯noa, 2540 Campus Road, Honolulu, HI 96822, USA e-mail: [email protected] M. Messer et al. (eds.), Migrations: Interdisciplinary Perspectives, DOI 10.1007/978-3-7091-0950-2_29, # Springer-Verlag Wien 2012
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on average from a shared common ancestor.1 This may seem like a lot, but it is much smaller than one of our closest living relatives, the chimpanzees, which have a few times more genetic variation than do modern humans (e.g. Gagneux et al. 1999; Kaessmann et al. 2001). On average, the great apes differ at every few hundred DNA positions, which suggest they are separated by a correspondingly greater average number of generations necessary to accumulate a larger amount of variation in the population.2 Furthermore, the common ancestor of humans and chimpanzees, ~5 million years ago, is inferred to have had much more genetic variation than modern humans (Chen and Li 2001). So modern humans have much less genetic variation and are much more closely related to each other, than we might otherwise expect.
Most Genetic Variation Is Shared Among Human Populations Genetic studies have repeatedly found that the vast majority of genetic variation, over 80%, is found across many human populations (Barbujani et al. 1997), whether these populations are defined according to current ethnic affiliation or ancestral geographic region of origin. Overt physical features such as skin pigmentation, which may
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