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Old 09-24-2012, 09:24 PM
Chessic Sense Chessic Sense is offline
Join Date: Apr 2007
Posts: 6,662
All this talk of MRCA is unnecessary. That unknown person is the mother or father of all humanity, but that's not what the OP is asking about. The OP wants to know what generation, i.e. what set of ancestors, would be enough to find some path from each living person to every other living person.

The difference is that a single ancestor needn't be present in every "relation chain" from living person to living person, like the MRCA demands. If we mapped out every lineage going back only, say, 4 generations and had enough people to connect everyone in at least one manner, then we'd say everyone is 2nd cousins even if the MRCA isn't in that generation.

An example would be two couples who have kids and then switch partners to have more kids. Every kid would share at least one parent with every other kid, but no common ancestor would exist within this set. And since humanity's female MRCA never knew the male MRCA, you can't disqualify this set of kids just because they're not full siblings.