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Re: Date for Last Common Ancestor?
Jane Andrews (mja1002@cus.cam.ac.uk)
Tue, 13 Aug 1996 16:16:17 +0100
On Mon, 12 Aug 1996, Stephen Barnard wrote:
> Susan S. Chin wrote:
> >
> > I did read about my Eve as a population and not an individual
> > ancestor recently, but don't recall the source.
> > > >
then stephen replied (amongst other things)
>
> Here's the logic that demonstrates that there is a single Last Common
> (female) Ancestor:
>
> Let S_1 be the set of all people whom are alive today. Let S_2 be the
> set of mothers of the members of S_1. In general, let S_k be the set
> of mothers of S_k-1. The size of these sets is nondecreasing (S_k <=
> S_k-1), because everyone has only one mother, but some mothers have
> more than one child. When the size of the set reaches 1 then we have
> arrived at the mitochondrial Eve.
>
> Steve Barnard
>
>
I with Susan on this one. Unfortunatly I can't remember the reference
either (I can go and look it up tomorrow if anyone's interested) but I
remember reading a paper which idicated that the proposed "eve" could have
come from an ancestoral population of up to 10 000 individuals. Her
mitochondrial sequence has been preserved by random genetic drift and all
the others lost by chance, stochastic processes. This means that there
is no need to invoke a huge bottle neck reducing, the effective
female population to one, in order to explain the apparent single female
ancestor.
Steve points out that mitochondrial DNA is not the whole story. Nuclear
DNA is inherited from both parents and so follows different patterns to
mtDNA. An individuals nuclear DNA has many more ancestors than his or her
mtDNA. Thus all those individuals in the ancesoral population whose mtDNA
was lost by drift may have contributed nuclear DNA to later individuals
carrying the remaining mtDNA - perhaps nuclear sequences that were not
present in any of the ancestoral population who did carry the surviving
mtDNA. Thus individuals who are not represented by an "Eve" character (ie
the didn't have the ancestoral mtDNA sequence) have contributed to the
genetic vartion of the desendent populations (us!).
I'm not sure that I've explained myself very clearly, but I hope that this
makes sence.
Jane Andrews.
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