Identical ancestors point
In genetic genealogy, the identical ancestors point (IAP) or all common ancestors (ACA) point is the most recent point in a given population's past where each individual then alive turned out to either be the ancestor of every individual alive now or has no currently living descendants. This point lies further in the past than the population's most recent common ancestor (MRCA).
To illustrate the concept, a set of full siblings has an IAP one generation back: their parents. Similarly, double first cousins have an IAP two generations back: the four grandparents.
Considering all humans alive today and moving back in time, we eventually arrive at the MRCA to all humans. The MRCA had many contemporary companions. Many of these contemporaries had descendant lines to some people living today, but not to all people living today. Others did not have any children, or had descendants, but all descendant lines are now fully extinct.
Going further back, the ancestors of the MRCA are also common ancestors to all humans, just not the most recent. As we move further back in time, other common ancestors will be found on other lines, resulting in more and more of the ancient population being common ancestors. Eventually the point is reached where all people in the past population fall into one of two categories: they are common ancestors, with at least one line of descent to everyone living today, or, they are the ancestors of no one alive today, because their lines of descent are completely extinct on every branch. This point in time is termed the 'identical ancestors point'.
Chang also showed that in a large, well mixed population of size N, we only have to go 1.77 log2(N) generations in the past to find the time when everyone in the population (who left descendants) is an ancestor to the entire population.
The identical ancestors point for Homo sapiens has been the subject of debate. In 2004, Rohde, Olson and Chang showed through simulations that the Identical Ancestors Point for all humans is surprisingly recent, on the order of 5,000-10,000 years ago. Ralph and Coop, considering the European population and working from genetics, came to similar conclusions for the recent common ancestry of Europeans.
All living people share exactly the same set of ancestors from this point back, all the way to the very first single-celled organism. However, people will vary widely in how much ancestry and genes they inherit from each ancestor, which will cause them to have very different genotypes and phenotypes. In 5000 BC, at around the ACA point, a modern day Japanese person will get 88.4% of his ancestry from Japan, with most of the remainder from China or Korea. The modern Japanese person will get just 0.00049% of his ancestry from Norway. A modern day Norwegian, meanwhile, will get over 92% of his ancestry from Norway and over 96% of his ancestry from Scandinavia. A modern day Norwegian will get 0.00044% of his ancestry from 5000 BC Japan. Thus, even though the Norwegian and Japanese person share the same set of ancestors, these ancestors appear in their family tree in dramatically different proportions. A Japanese person in 5000 BC will likely appear trillions of times in a modern day Japanese person's family tree, but might appear only one time in a Norwegian person's family tree. A 5000 BC Norwegian person will similarly appear far more times in a typical Norwegian person's family tree than he will appear in a Japanese person's family tree. 
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.
- Chang, Joseph T., ‘’Recent Common Ancestors of All Present-Day Individuals’’
- 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. "Based on the results of a series of computer models, it seems likely that our most recent common ancestor may have lived between 2,000 and 5,000 years ago. This is, perhaps, one tenth to one one-hundredth the length of time to our most recent common ancestors along solely male or solely female lines, which have been the target of considerable recent interest. The point beyond which everyone alive today shares the same set of ancestors is somewhat harder to predict, but it most likely falls between 5,000 and 15,000 years ago, with a significantly more recent date for the point at which we share nearly the same set."
- Ralph, Peter and Coop, Graham, The Geography of Recent Genetic Ancestry across Europe, PLOS Biology, May 7, 2013, “For instance, we estimate that someone from Hungary shares on average about five genetic common ancestors with someone from the United Kingdom between 18 and 50 generations ago. Since 1/r(36) = 5.8×107, we would conservatively estimate that for every genetic common ancestor there are tens of millions of genealogical common ancestors. Most of these ancestors must be genealogical common ancestors many times over, but these must still represent at least thousands of distinct individuals.”
- 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.
- Spencer, Goeff (December 2002). "Background on Comparative Genomic Analysis" (Press release). National Human Genome Research Institute. Retrieved 28 November 2015.
- 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"