A Diffusion Wave out of Africa
This paper proposes that the worldwide transition to an anatomically modern human form was caused by the diffusive spread from Africa of a genotype — a coadapted combination of novel genes — carrying a complex genetic advantage. It is suggested that the movement out of Africa was not a migration but a "diffusion wave" — a continuous expansion of modern populations by small random movements, hybridization, and natural selection favoring the modern genotype. It is proposed that the modern genotype arose in Africa by a shifting-balance process and spread because it was globally advantageous. It is shown that the genotype could have spread by directionally random demic diffusion, but only under conditions involving a low rate of interdeme admixture ("interbreeding") and strong selection. This mechanism is investigated using a quantitative model that suggests explanations for many puzzling aspects of the genetic, fossil, and archaeological data on modern human origins. The data indicate significant genetic assimilation from archaic human populations into modern ones. A morphological advantage of the modern phenotype — possibly reducing childbirth mortality — is proposed as the cause of the transition. The evidence of this and previous human "revolutions" suggests that the shifting-balance process, proposed by Sewall Wright, was particularly important in human evolution — possibly because human populations had a small-deme social structure with low interbreeding rates that allowed it to operate. This may explain the relative uniqueness of human evolution.
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On the Diffusion-Wave Model for the Spread of Modern Humans
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Mixed Signals of Expansions Refute an Exclusively African Descent for Modern Humans
Ten years ago evidence from genetics gave strong support to the “recent Africa origin” (rao) view of the evolution of modern humans, but subsequent large bodies of data not only have failed to support the rao model, they do not support any simple model of human demographic history. In this paper we study a process in which modern humans originate in Africa, then spread across the world by local demic diffusion, hybridization, and natural selection which fixes an advantageous gene combination characterizing anatomical modernity. Numerical simulations of this process replicate many of the seemingly contradictory features of the genetic data and suggests that as much as 80% of nuclear loci have assimilated genetic material from non-African archaic humans.
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Population History and Natural Selection Shape Patterns of Genetic Variation in 132 Genes
Identifying regions of the human genome that have been targets of natural selection will provide important insights into human evolutionary history and may facilitate the identification of complex disease genes. Although the signature that natural selection imparts on DNA sequence variation is difficult to disentangle from the effects of neutral processes such as population demographic history, selective and demographic forces can be distinguished by analyzing multiple loci dispersed throughout the genome. We studied the molecular evolution of 132 genes by comprehensively resequencing them in 24 African-Americans and 23 European-Americans. We developed a rigorous computational approach for taking into account multiple hypothesis tests and demographic history and found that while many apparent selective events can instead be explained by demography, there is also strong evidence for positive or balancing selection at eight genes in the European-American population, but none in the African-American population. Our results suggest that the migration of modern humans out of Africa into new environments was accompanied by genetic adaptations to emergent selective forces. In addition, a region containing four contiguous genes on Chromosome 7 showed striking evidence of a recent selective sweep in European-Americans. More generally, our results have important implications for mapping genes underlying complex human diseases.
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Genomics refutes an exclusively African origin of humans
Ten years ago, evidence from genetics gave strong support to the "recent Africa origin" view of the evolution of modern humans, which posits that Homo sapiens arose as a new species in Africa and subsequently spread, leading to the extinction of other archaic human species. Subsequent data from the nuclear genome not only fail to support this model, they do not support any simple model of human demographic history. In this paper, we study a process in which the modern human phenotype originates in Africa and then advances across the world by local demic diffusion, hybridization, and natural selection. While the multiregional model of human origins posits a number of independent single locus selective sweeps, and the "out of Africa" model posits a sweep of a new species, we study the intermediate case of a phenotypic sweep. Numerical simulations of this process replicate many of the seemingly contradictory features of the genetic data, and suggest that as much as 80% of nuclear loci have assimilated genetic material from non-African archaic humans.
PDF file
This paper proposes that the worldwide transition to an anatomically modern human form was caused by the diffusive spread from Africa of a genotype — a coadapted combination of novel genes — carrying a complex genetic advantage. It is suggested that the movement out of Africa was not a migration but a "diffusion wave" — a continuous expansion of modern populations by small random movements, hybridization, and natural selection favoring the modern genotype. It is proposed that the modern genotype arose in Africa by a shifting-balance process and spread because it was globally advantageous. It is shown that the genotype could have spread by directionally random demic diffusion, but only under conditions involving a low rate of interdeme admixture ("interbreeding") and strong selection. This mechanism is investigated using a quantitative model that suggests explanations for many puzzling aspects of the genetic, fossil, and archaeological data on modern human origins. The data indicate significant genetic assimilation from archaic human populations into modern ones. A morphological advantage of the modern phenotype — possibly reducing childbirth mortality — is proposed as the cause of the transition. The evidence of this and previous human "revolutions" suggests that the shifting-balance process, proposed by Sewall Wright, was particularly important in human evolution — possibly because human populations had a small-deme social structure with low interbreeding rates that allowed it to operate. This may explain the relative uniqueness of human evolution.
PDF file
On the Diffusion-Wave Model for the Spread of Modern Humans
PDF file
Mixed Signals of Expansions Refute an Exclusively African Descent for Modern Humans
Ten years ago evidence from genetics gave strong support to the “recent Africa origin” (rao) view of the evolution of modern humans, but subsequent large bodies of data not only have failed to support the rao model, they do not support any simple model of human demographic history. In this paper we study a process in which modern humans originate in Africa, then spread across the world by local demic diffusion, hybridization, and natural selection which fixes an advantageous gene combination characterizing anatomical modernity. Numerical simulations of this process replicate many of the seemingly contradictory features of the genetic data and suggests that as much as 80% of nuclear loci have assimilated genetic material from non-African archaic humans.
PDF file
Population History and Natural Selection Shape Patterns of Genetic Variation in 132 Genes
Identifying regions of the human genome that have been targets of natural selection will provide important insights into human evolutionary history and may facilitate the identification of complex disease genes. Although the signature that natural selection imparts on DNA sequence variation is difficult to disentangle from the effects of neutral processes such as population demographic history, selective and demographic forces can be distinguished by analyzing multiple loci dispersed throughout the genome. We studied the molecular evolution of 132 genes by comprehensively resequencing them in 24 African-Americans and 23 European-Americans. We developed a rigorous computational approach for taking into account multiple hypothesis tests and demographic history and found that while many apparent selective events can instead be explained by demography, there is also strong evidence for positive or balancing selection at eight genes in the European-American population, but none in the African-American population. Our results suggest that the migration of modern humans out of Africa into new environments was accompanied by genetic adaptations to emergent selective forces. In addition, a region containing four contiguous genes on Chromosome 7 showed striking evidence of a recent selective sweep in European-Americans. More generally, our results have important implications for mapping genes underlying complex human diseases.
PDF file
Genomics refutes an exclusively African origin of humans
Ten years ago, evidence from genetics gave strong support to the "recent Africa origin" view of the evolution of modern humans, which posits that Homo sapiens arose as a new species in Africa and subsequently spread, leading to the extinction of other archaic human species. Subsequent data from the nuclear genome not only fail to support this model, they do not support any simple model of human demographic history. In this paper, we study a process in which the modern human phenotype originates in Africa and then advances across the world by local demic diffusion, hybridization, and natural selection. While the multiregional model of human origins posits a number of independent single locus selective sweeps, and the "out of Africa" model posits a sweep of a new species, we study the intermediate case of a phenotypic sweep. Numerical simulations of this process replicate many of the seemingly contradictory features of the genetic data, and suggest that as much as 80% of nuclear loci have assimilated genetic material from non-African archaic humans.
PDF file
