Identical Ancestors Point

In genetic genealogy, the identical ancestors point (IAP) is that 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 living descendants at all. This point lies further in the past than the population's most recent common ancestor (MRCA).

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

The MRCA had many contemporary companions of both sexes. Many of these contemporaries left direct descendants, but not all of them left an unbroken link of descendants all the way down to today's population. That is, some contemporaries are ancestors of no one in current population. The rest of contemporaries of MRCA may claim ancestry over a subset of current population, but not the entirety of 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 more and more ancient common 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 left 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 estimated to between 15,000 and 5,000 years ago, with an estimate of the human MRCA living about 2,000 to 5,000 years ago, that is, estimating the IAP to be about three times as distant as the MRCA. Note that both the matrilineal and the patrilineal human MRCAs are far more remote still, dating to some 150,000 and 90,000 years ago, respectively.

It is incorrect to assume that the MRCA and his/her ancestors passed all their genes down to every person alive today. Because of sexual reproduction, at every generation, an ancestor only passes half of his or her genes to the next generation. 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. As the human genome consists of roughly 232 base pairs, the genetic contribution of a single ancestor through a single line may be flushed out of an individual's genome completely after 32 generations, or roughly 1,000 years.