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Author Topic:   Common Ancestor Terminology
Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 4 of 15 (672901)
09-12-2012 10:01 AM
Reply to: Message 1 by Hideyoshi
09-11-2012 9:42 PM


The individual elephant living today in the Serengeti and the human living in Brooklyn today may have never gotten any closer than a cousin of a cousin of a cousin of a cousin of a cousin and all separated in existence by a hundred years or more.
I'm not sure how this is meant to work. If you and I are related by being descended from two people who were cousins, then those two cousins must have had a common grandfather, and who is our common ancestor. Or if they were second cousins, they had a common great-grandfather, who was our common ancestor. Or if they were third cousins twice removed, then someone was the great-great-grandfather of one of them and the great-great-great-great-grandfather of the other ... and so on.
In general, I don't see how you could have a scenario where two people (or elephants, or whatever) are related but don't have a common ancestor. What else is relatedness, when you get down to it?
(Obviously for the purposes of this discussion, being "related by marriage" doesn't count.)
So my brother and I have a common ancestor (my mother, to name but one); my father and I have a common ancestor (his father) my aunt and I have a common ancestor (my grandfather). My niece and I have a common ancestor (my grandfather again). My great-aunt and I have a common ancestor (my great-grandmother) ... and so on. You seem to be imagining that there's some degree of consanguinity by which two people (or elephants, or whatever) could be blood relatives but not have a common ancestor. Well, what is it?
Edited by Dr Adequate, : No reason given.

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 Message 1 by Hideyoshi, posted 09-11-2012 9:42 PM Hideyoshi has not replied

Replies to this message:
 Message 5 by Taq, posted 09-12-2012 10:45 AM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 6 of 15 (672923)
09-12-2012 12:17 PM
Reply to: Message 5 by Taq
09-12-2012 10:45 AM


Yes, but that's another question.
My question is, how can two people (or organisms) be related without having some individual who is their common ancestor? So far as I can see, they must do so simply by definition of "related" There has to be some point --- some individual --- at which their family trees meet, or they aren't related.
The author of the OP seems to be suggesting otherwise. He says: "The individual elephant living today in the Serengeti and the human living in Brooklyn today may have never gotten any closer than a cousin of a cousin of a cousin of a cousin of a cousin and all separated in existence by a hundred years or more." But surely he's wrong --- the closest that two organisms' family trees can get can never be distant cousinship, 'cos if they get that close then there must also actually be an individual common to both.
Edited by Dr Adequate, : No reason given.

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 Message 5 by Taq, posted 09-12-2012 10:45 AM Taq has replied

Replies to this message:
 Message 8 by Taq, posted 09-13-2012 11:52 AM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 10 of 15 (673046)
09-13-2012 12:11 PM
Reply to: Message 8 by Taq
09-13-2012 11:52 AM


Yeah, but he's still wrong.
With sexually reproducing species it makes no sense to talk about an individual as a common ancestor, at least from a genetic standpoint.
No, not really. It would, of course be wrong to suggest that such an individual was unique, which I didn't. But they did exist. If humans and chimps (for example) have a species that is our common ancestor, then necessarily there was at least one individual in that species which was our common ancestor, because two individuals can't be related unless they have at least one common ancestor.

This message is a reply to:
 Message 8 by Taq, posted 09-13-2012 11:52 AM Taq has replied

Replies to this message:
 Message 11 by Taq, posted 09-13-2012 12:51 PM Dr Adequate has replied
 Message 15 by Hideyoshi, posted 09-15-2012 12:25 PM Dr Adequate has not replied

  
Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 12 of 15 (673076)
09-13-2012 2:51 PM
Reply to: Message 11 by Taq
09-13-2012 12:51 PM


I fully agree that there would be such an individual, and probably more than one.
But is that really what we are talking about when we reference a common ancestor?
Well, we mean both. When we say that chimps and humans had a common ancestor, then we mean that there was some individual who was a common ancestor of both. And, of course, vice versa. We cannot say one without meaning (as a matter of logical necessity) the other.
And this is the only point that I'm trying to clear up. Admit that I'm right, which I clearly am, and we have no quarrel.

This message is a reply to:
 Message 11 by Taq, posted 09-13-2012 12:51 PM Taq has replied

Replies to this message:
 Message 13 by Taq, posted 09-13-2012 4:06 PM Dr Adequate has replied

  
Dr Adequate
Member (Idle past 285 days)
Posts: 16113
Joined: 07-20-2006


Message 14 of 15 (673123)
09-14-2012 6:07 AM
Reply to: Message 13 by Taq
09-13-2012 4:06 PM


My problem is my own biases in how I view it, so I would be happy to agree with that. I tend to stress the population over the individual common ancestor ...
And so would I, but not to the point of denying the individual common ancestor. That's all that I'm trying to clear up.
You are mostly right.
Yeah, I mostly am.

This message is a reply to:
 Message 13 by Taq, posted 09-13-2012 4:06 PM Taq has not replied

  
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