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The MRCA of a set of individuals can sometimes be determined by referring to an established pedigree. In general, however, it is impossible to identify the specific MRCA of a set of individuals, but an estimate of the time at which the MRCA lived can often be given; such estimates can be given based on DNA test results and established mutation rates, or by reference to a non-genetic genealogical model. The term MRCA is usually used to describe a common ancestor of individuals within a species. It can also be used to describe a common ancestor between species (i.e. the species founder). To avoid confusion, Last Common Ancestor (LCA) or the equivalent term concestor, coined by Richard Dawkins , is sometimes used in place of MRCA when discussing ancestry between species. MRCA OF TWO INDIVIDUALS The most recent common patrilineal ancestor of any two males, and the most recent common matrilineal ancestor of any two individuals can be estimated by Genealogical DNA Test s. The tests use Mitochondrial DNA for matrilineal inheritance or Y-chromosome -DNA for patrilineal inheritance. MRCA OF ALL LIVING HUMANS The existence of an MRCA does not imply existence of a Population Bottleneck or First Couple . The MRCA of everyone alive today could have co-existed with a large human population, most of whom either have no living descendants today or else are ancestors of a subset of people alive today. This seemingly paradoxical phenomenon can be easily explained, if the nature of Lineage is taken into account. When tracing human lineage back in time, most people look at parents, grandparents, great-grandparents and so on. The same approach is often taken when tracing descendants via children and grandchildren. This approach is misguided, as the numbers of ancestors and descendants Grows Exponentially as generations are added to the lineage tree. In 80 generations, the number of ancestors is 280, more than a trillion trillion.See the chapter ''All Africa and her progenies'' in 1 This simple calculation does not take into account the fact that every Fertilisation is really a fertilisation between ''distant Cousin s''. The ancestry tree is not really a Tree , but a Directed, Acyclic Graph . One can place all living people at the bottom of the graph and ancestors above their descendants. As each generation of ancestor is added at the top of the graph, one of the many top-level ancestors will eventually become the MRCA from whom it is possible to trace a path of direct descendants all the way down to every living person at the bottom of the graph. |
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