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 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 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 mathematically estimated to have been between 15,000 and 5,000 years ago. It is estimated that the human MRCA lived about 2,000 to 5,000 years ago, so the IAP is estimated to be about three times as distant in the past as the MRCA. However, this estimate does not take into account isolated populations, many of which do indeed still exist today (See uncontacted people). Note that both the matrilineal and the patrilineal human MRCAs are far more remote still, dating to some 200,000 and 338,000 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, an ancestor only passes half of his or her genes on 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. Since 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 the descendants' genome completely after 32 generations, or roughly 1,000 years.
- See the chapter All Africa and her progenies in Dawkins, Richard (1995). River Out of Eden. New York: Basic Books. ISBN 0-465-06990-8.
- Rohde DL, Olson S, Chang JT (September 2004). "Modelling the recent common ancestry of all living humans". Nature 431 (7008): 562–6. doi:10.1038/nature02842. PMID 15457259.
- Rohde, DLT , On the common ancestors of all living humans. Submitted to American Journal of Physical Anthropology (2003), p. 27.
- Dawkins, Richard (2004). The Ancestor's Tale, A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company. ISBN 0-618-00583-8.
- Identical ancestors point - explaining the relationship of "most recent common ancestor" and "identical ancestors point"