In 2011 I wrote about an amusing article in the 9 September 1978 Washington Post in which the reporter, Henry Allen, began thinking about the number of ancestors he had 10 generations ago. He figured that each generation had 2 parents, so the 10th generation would have 2^10 or over a thousand members. At 25 years per generation he started imagining the expanding number of direct ancestors he had going back through history, achieving astronomical numbers, which did not seem reasonable. As reporters do, he started interviewing people to get to the bottom of the problem. See the Ancestors Problem.

One thing that did come out of the exercise is that I began researching the idea of Most Recent Common Ancestor (MRCA). It led to a number of approaches from statistical to molecular evolution markers. I was hoping to write more about it when I had the time.  An example of the statistical approach is the article by Joseph Lachance, “Inbreeding, Pedigree Size, and the Most Recent Common Ancestor of Humanity,” J Theor Biol. 2009 November 21; 261(2): 238–247. Online 2009 August 11. doi: 10.1016/j.jtbi.2009.08.006

(Update 5/10/2019) James Grime has a Youtube video that goes into MRCAs in more detail, which is entitled, “EVERY baby is a ROYAL baby” (https://www.youtube.com/watch?v=Fm0hOex4psA) and is based on the research paper by Joseph T. Chang in 1999 and Note in Nature 2004:

• Chang, Joseph T., “Recent Common Ancestors Of All Present-Day Individuals,” 12 June 1998 (published in Advances in Applied Probability, Vol. 31, No. 4 (Dec., 1999), pp. 1002-1026) (http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/Ancestors.pdf)
• Rohde, Douglas L. T., Steve Olson, and Joseph T. Chang, “Modelling The Recent Common Ancestry Of All Living Humans,” Nature, Volume 431, 2004, pages 562–566 (http://www.stat.yale.edu/~jtc5/papers/CommonAncestors/NatureCommonAncestors-Article.pdf)

In a related video James Grime looks in the opposite direction from ancestors toward descendants, and in particular when a line has no further descendants, that is, goes extinct. It is called “Will your name become extinct?” (https://www.youtube.com/watch?v=z34XhE5oRwo) The video has an appendix, “Math of Extinction (extra)” (https://www.youtube.com/watch?v=zomuAufWXr0), which proves a statement James Grime made in the extinction video.