Why does evolutionary science seem to be

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Do all woman possess the same line? How do we know for a fact that these lines die out?

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There are actually many mitochondrial lines or variants that co-exist now. One can look at maternal lineages among, say ethnic groups, to see the distribution of these lines. A mitochondrial lineage can indeed go extinct. If your mother is an only child and produces a son as her only progeny, her mitochondrial genome will not be passed on to the next generation. Thus, that line (especially if it is a rare or unique variante) becomes extinct. Similarly, if a male who is an only child produces only daughters, his Y chromosome will never make it into the next generation..his copy is extinct.

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And if so how long does it take lines to die out? What is there life time?

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No way to know how long it takes to die off..it will vary. If the population remains large and the lineage is well represented in the population, it will not become extinct for many generations if at all. If you have a rare mitochondrial genotype and are a member of a small group, the chances are high that your genotype will be lost from the population.

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If we came from a population and the life time was approxiamately the same for each line why would there be just one

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If the population was large and there were many lineages (as we see currently) there is no shrinkage in the number of lineages...there is actually expansion of lineages. If something happens and the population shrinks, lineages will be lost as those containing a particular mitochondrial genotype die or fail to reproduce.

Hi Loudmouth,
Here is one reference from total mtDNA genome sequencesNature. 2000 Dec 7;408(6813):708-13. Related Articles, LinksErratum in:
Nature 2001 Mar 29;410(6828):611.Comment in:
Nature. 2000 Dec 7;408(6813):652-3.Mitochondrial genome variation and the origin of modern humans.Ingman M, Kaessmann H, Paabo S, Gyllensten U.Department of Genetics and Pathology, Section of Medical Genetics, University of Uppsala, Sweden.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.