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Old 09-24-2012, 10:03 AM
Hari Seldon Hari Seldon is offline
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Join Date: Mar 2002
Location: Trantor
Posts: 11,081
Even MRCA can be misleading since a group of people could all be 50th cousins at most, while not having a common ancestor much earlier than that. Of course, any two would have to have a common ancestor within the past 50 generations, but that is not the same as the whole population having a common ancestor within 50 generations.

Perhaps an example would help. Imagine just three people, Alice, Bob, and Charles. Alice's parents are Don and Edith. Bob's parents are Edith's brother Frank and Gloria. Charles's parents are Gloria's brother Hank and Don's sister Irene. Assuming no other relationship, Alice and Bob, Bob and Charles, and Charles and Alice are all first cousin's but they have no ancestor closer than three generations ago.