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#40
09-24-2012, 09:52 PM
 Blake Member Join Date: Mar 2001 Posts: 11,049
Quote:
 Originally Posted by Chessic Sense 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.

I can't make sense of this.

To start with, I can't see any possible way that every kid would share at least one parent with every other kid. It seems mathematically impossible since there are a minimum of 4 children, but a maximum of 2 permutations of parents

John and Jane have a kid: Bob
Kath and Ken have a kid: Carol

Kath and John then have a kid: Ted
Jane and Ken then have a kid: Alice

Ted doesn't share any common parentage at all with with Alice. Ted's parents are Kath and John. Alice's parents are Jane and Ken. Ted and Alice are genetically utterly unrelated.

In order to find a path from each living person to every other living person within this group, you will need to go back and find an ancestor that is shared by both Ted and Alice. If John's Grandfather and Ken's Great Grandfather are the same person, Engelbert, then you will have a path linking all these people. But that would make that Engelbert the MRCA .

If we mapped out every lineage going back 4 generations, the only way we I can see that we could have enough people to connect everyone in at least one manner is if the MRCA occurred within four generations.

I'm guessing I'm missing something here.

Perhaps you are working form a position that "step sibling" or adopted sibling or other relationships that have no objective or genetic basis are sufficient to establish a relation chain. But I don't think that's what the OP wants. Since he is talking about incest, relationships that only exist culturally, with no genetic relationship at all, hardly seem to fit.