CHIMA IROH - Academia.edu (original) (raw)

Papers by CHIMA IROH

Proceedings of The National Academy of Sciences, Feb 5, 2013

Accuracy of evolutionary analysis of populations within a species requires the testing of a large... more Accuracy of evolutionary analysis of populations within a species requires the testing of a large number of genetic polymorphisms belonging to many loci. We report here a reconstruction of human differentiation based on 100 DNA polymorphisms tested in five populations from four continents. The results agree with earlier conclusions based on other classes of genetic markers but reveal that Europeans do not fit a simple model of independently evolving populations with equal evolutionary rates. Evolutionary models involving early admixture are compatible with the data. Taking one such model into account, we examined through simulation whether random genetic drift alone might explain the variation among gene frequencies across populations and genes. A measure of variation among populations was calculated for each polymorphism, and its distribution for the 100 polymorphisms was compared with that expected for a drift-only hypothesis. At least two-thirds of the polymorphisms appear to be selectively neutral, but there are significant deviations at the two ends of the observed distribution of the measure of variation: a slight excess of polymorphisms with low variation and a greater excess with high variation. This indicates that a few DNA polymorphisms are affected by natural selection, rarely heterotic, and more often disruptive, while most are selectively neutral.

When I read this book's predecessor, Culture and the Evolutionary Process, not long after it came... more When I read this book's predecessor, Culture and the Evolutionary Process, not long after it came out 20 years ago, it seemed to me to offer a new and more productive future for my discipline, archaeology. There were many features that were attractive but one of the most refreshing was its lack of dogmatism. Like many other humanities and social science disciplines, in the mid 1980s archaeology, in Britain at least, was going through a period of major upheaval. In the case of archaeology the so-called processual approach that had been pioneered by authors such as Lewis Binford in the 1960s, with its emphasis on culture as adaptation and its rejection of history, was being challenged by views that emphasised the meaning of artefacts rather than their function, and cultural uniqueness arising from specific histories rather than universal adaptive processes. You had to choose one camp or the other; there was no room for fence-sitting. The evolutionary approach of dual inheritance theory advocated by Boyd and Richerson rejected this dilemma. Adaptation was important but so were specific histories. It could not merely be assumed that adaptive problems called into existence their own solutions. The dynamics of culture itself were important. Moreover, the very features of human culture that made it adaptive also opened up the possibility of maladaptive developments. If one accepted this perspective, establishing the importance of adaptive payoffs or the specifics of particular histories in understanding patterning in the archaeological record was not a matter of making a dogmatic commitment a priori but something to be elucidated in particular cases as a result of empirical work.

DR CHIMA IROH, Feb 21, 2013

The analysis of mitochondrial DNA (mtDNA) has been a potent tool in our understanding of human ev... more The analysis of mitochondrial DNA (mtDNA) has been a potent tool in our understanding of human evolution, owing to characteristics such as high copy number, apparent lack of recombination, high substitution rate and maternal mode of inheritance. However, almost all studies of human evolution based on mtDNA sequencing have been confined to the control region, which constitutes less than 7% of the mitochondrial genome. These studies are complicated by the extreme variation in substitution rate between sites, and the consequence of parallel mutations causing difficulties in the estimation of genetic distance and making phylogenetic inferences questionable. Most comprehensive studies of the human mitochondrial molecule have been carried out through restriction-fragment length polymorphism analysis, providing data that are ill suited to estimations of mutation rate and therefore the timing of evolutionary events. Here, to improve the information obtained from the mitochondrial molecule for studies of human evolution, we describe the global mtDNA diversity in humans based on analyses of the complete mtDNA sequence of 53 humans of diverse origins. Our mtDNA data, in comparison with those of a parallel study of the Xq13.3 region in the same individuals, provide a concurrent view on human evolution with respect to the age of modern humans.

Proceedings of The National Academy of Sciences, Feb 5, 2013

Accuracy of evolutionary analysis of populations within a species requires the testing of a large... more Accuracy of evolutionary analysis of populations within a species requires the testing of a large number of genetic polymorphisms belonging to many loci. We report here a reconstruction of human differentiation based on 100 DNA polymorphisms tested in five populations from four continents. The results agree with earlier conclusions based on other classes of genetic markers but reveal that Europeans do not fit a simple model of independently evolving populations with equal evolutionary rates. Evolutionary models involving early admixture are compatible with the data. Taking one such model into account, we examined through simulation whether random genetic drift alone might explain the variation among gene frequencies across populations and genes. A measure of variation among populations was calculated for each polymorphism, and its distribution for the 100 polymorphisms was compared with that expected for a drift-only hypothesis. At least two-thirds of the polymorphisms appear to be selectively neutral, but there are significant deviations at the two ends of the observed distribution of the measure of variation: a slight excess of polymorphisms with low variation and a greater excess with high variation. This indicates that a few DNA polymorphisms are affected by natural selection, rarely heterotic, and more often disruptive, while most are selectively neutral.

When I read this book's predecessor, Culture and the Evolutionary Process, not long after it came... more When I read this book's predecessor, Culture and the Evolutionary Process, not long after it came out 20 years ago, it seemed to me to offer a new and more productive future for my discipline, archaeology. There were many features that were attractive but one of the most refreshing was its lack of dogmatism. Like many other humanities and social science disciplines, in the mid 1980s archaeology, in Britain at least, was going through a period of major upheaval. In the case of archaeology the so-called processual approach that had been pioneered by authors such as Lewis Binford in the 1960s, with its emphasis on culture as adaptation and its rejection of history, was being challenged by views that emphasised the meaning of artefacts rather than their function, and cultural uniqueness arising from specific histories rather than universal adaptive processes. You had to choose one camp or the other; there was no room for fence-sitting. The evolutionary approach of dual inheritance theory advocated by Boyd and Richerson rejected this dilemma. Adaptation was important but so were specific histories. It could not merely be assumed that adaptive problems called into existence their own solutions. The dynamics of culture itself were important. Moreover, the very features of human culture that made it adaptive also opened up the possibility of maladaptive developments. If one accepted this perspective, establishing the importance of adaptive payoffs or the specifics of particular histories in understanding patterning in the archaeological record was not a matter of making a dogmatic commitment a priori but something to be elucidated in particular cases as a result of empirical work.

DR CHIMA IROH, Feb 21, 2013

The analysis of mitochondrial DNA (mtDNA) has been a potent tool in our understanding of human ev... more The analysis of mitochondrial DNA (mtDNA) has been a potent tool in our understanding of human evolution, owing to characteristics such as high copy number, apparent lack of recombination, high substitution rate and maternal mode of inheritance. However, almost all studies of human evolution based on mtDNA sequencing have been confined to the control region, which constitutes less than 7% of the mitochondrial genome. These studies are complicated by the extreme variation in substitution rate between sites, and the consequence of parallel mutations causing difficulties in the estimation of genetic distance and making phylogenetic inferences questionable. Most comprehensive studies of the human mitochondrial molecule have been carried out through restriction-fragment length polymorphism analysis, providing data that are ill suited to estimations of mutation rate and therefore the timing of evolutionary events. Here, to improve the information obtained from the mitochondrial molecule for studies of human evolution, we describe the global mtDNA diversity in humans based on analyses of the complete mtDNA sequence of 53 humans of diverse origins. Our mtDNA data, in comparison with those of a parallel study of the Xq13.3 region in the same individuals, provide a concurrent view on human evolution with respect to the age of modern humans.