Identical ancestors point

In genetic genealogy, the identical ancestors point (IAP) is that last point in a given population's past where each individual then alive turned out to be either the ancestor of every individual alive now or to have no currently living descendants. This point lies further in the past than the population's most recent common ancestor (MRCA).

To illustrate the concept, the IAP of one generation unites the population of full siblings; an IAP of two generations unites the population of double first cousins. An IAP of three and four generations would unite the population of quadruple second cousins and octuple third cousins, respectively.

The MRCA had many contemporary companions of both sexes. Many of these contemporaries had direct descendants, but not all of them left an unbroken link of descendants all the way down to today's population: in other words, some of these contemporaries are ancestors of no one in the current population. The other contemporaries of the MRCA are the ancestors of some but not all of the current population.

Because ancestors of the MRCA are by definition also common ancestors, we can continue to find (less recent) common ancestors by pushing further back in time to find more and more ancestors that are the ancestors of all people alive today. Eventually we will reach a point in the past where all humans can be divided into two groups: those who have no descendants today and those who are common ancestors of all living humans today. This point in time is termed the identical ancestors point. Even though each living person receives genes in dramatically different proportions from these ancestors from the identical ancestors point, all living people share exactly the same set of ancestors from this point back, all the way to the very first single-celled organism.

The identical ancestors point for Homo sapiens has been the subject of debate. Some mathematical models yield surprisingly recent dates, a mere few thousand years ago.

Note that the MRCA and his/her ancestors did not pass all their genes down to every person alive today. Because of sexual reproduction, an ancestor only passes "half" of his or her genes on to the next generation. (Technically, it is half of those genes in which the parents differ. All members of a species are genetically very similar; even fruit flies share about half of their genes with humans, and so both parents share the vast majority of their genetic material, and so most of their genes are passed on to their offspring.) The percentage of genes inherited from the MRCA becomes smaller and smaller at every successive generation, as genes inherited from contemporaries of MRCA are interchanged via sexual reproduction.

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