Re: Date for Last Common Ancestor?

Stephen Barnard (
Mon, 12 Aug 1996 16:57:24 -0800

> "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"
> This does not prove that the cardinality of the sequence of sets {S_k}
> coverges to one. To establish the existence of a "Last Common Ancestor"by
> logical means only will require quite a lot more than you have supplied,
> starting with a clear definition of what is the "Last Common Ancestor" for
> a species.

I suppose that in strict mathematical terms it doesn't. That would
require a strictly decreasing cardinality of sets.

>From a practical point of view I'm satisfied with it. The group that
included the putative mitochondrial Eve would very probably have been
pretty small. But you never know. :-) I've seen speculation in this
newsgroup that relies on *much* flimsier assumptions.

Steve Barnard