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In biology, the most recent common ancestor (MRCA) of any set of organisms is the most recent individual from which all organisms in a group are directly descended. The term is often applied to human genealogy. The MRCA of a set of individuals can sometimes be determined by referring to an established pedigree. However, in general, it is impossible to identify the specific MRCA of a large set of individuals, but an estimate of the time at which the MRCA lived can often be given. Such ''time to MRCA'' (TMRCA) estimates can be given based on DNA test results and established mutation rates as practiced in genetic genealogy, or by reference to a non-genetic, mathematical model or computer simulation. Assuming that no genetically isolated human populations remain, the human MRCA may have lived 2,000 to 4,000 years ago.〔 This estimate is based on a non-genetic, mathematical model that assumes random mating and does not take into account important aspects of human population substructure such as assortative mating and historical geographical constraints on interbreeding.〔 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. To avoid confusion, last common ancestor (LCA) or the equivalent term concestor is sometimes used in place of MRCA when discussing ancestry between species. The term ''MRCA'' may also be used to identify a common ancestor between a set of organisms via specific gene pathways. The TMRCA in the case of gene pathways will be different depending on how restrictive the choice of genes is. Choosing a very restrictive gene such as mtDNA, which is inherited only along the female lineage Mitochondrial Eve or the male equivalent Y-chromosomal Adam are examples of such MRCAs, which yields TMRCA estimates orders of magnitude further back in time than TMRCA if any possible line of descendancy is allowed. Such genealogies in reality trace ancestry of individual genes, not organisms. As a result, TMRCA estimates for genetic MRCAs are necessarily greater than those for MRCAs of organisms. == MRCA of all living humans == Tracing one person's lineage back in time for a few generations in principle forms a binary tree of parents, grandparents, great-grandparents and so on. However, the number of individuals in such an ancestor tree grows exponentially and very soon exceeds the population from which the ancestors were drawn. A human alive today would, over 30 generations (going back to about the High Middle Ages), have 230 or about 1.07 billion ancestors, more than the world population at the time.〔See the chapter ''All Africa and her progenies'' in 〕 Thus it is obvious that there is multiple counting and the individual is descended from some of these ancestors through more than one line: pedigree collapse changes the binary tree to a directed acyclic graph. Consider the formation, one generation at a time, of the ancestor graph of all living humans with no descendants. Start with living people with no descendants at the bottom of the graph. Adding the parents of all those individuals at the top of the graph will connect (half-) siblings via one or two common ancestors, their parent(s). Adding the next generation will connect all first cousins. As each of the following generations of ancestors is added to the top of the graph, some of the relationships between more and more people are mapped (second cousins, third cousins and so on). Eventually a generation may be reached where one of the many top-level ancestors is the MRCA from whom it is possible to trace a path of direct descendants all the way down to every living person in the bottom generations of the graph. The MRCA of everyone alive could have co-existed with a large human population, each of whom either has no living descendants or is an ancestor of only some of the people alive today. Therefore, the existence of an MRCA does not imply the existence of a population bottleneck, let alone a "first couple". The MRCA of the current population can change over time as people die. It is incorrect to assume that the MRCA passed all of his or her genes (or indeed any gene) down to every person alive. Because of sexual reproduction at every generation, an ancestor passes half of his or her genes to each descendant in the next generation. Save for inbreeding, the percentage of genes inherited from the MRCA becomes smaller and smaller in individuals at each generation, sometimes decreasing to zero (at which point the Ship of Theseus paradox arises), as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Most recent common ancestor」の詳細全文を読む スポンサード リンク
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