Identical ancestors point  

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Kunstformen der Natur (1904) by Ernst Haeckel
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Kunstformen der Natur (1904) by Ernst Haeckel

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, all 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'.

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Unless indicated otherwise, the text in this article is either based on Wikipedia article "Identical ancestors point" or another language Wikipedia page thereof used under the terms of the GNU Free Documentation License; or on original research by Jahsonic and friends. See Art and Popular Culture's copyright notice.

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