Re: Date for Last Common Ancestor?

Geoffrey Norman Watson (
Thu, 15 Aug 1996 13:33:26 +1000

On Sun, 11 Aug 1996, T&B Schmal wrote:

> I wrote:
> I am interested in bracketing the dates for the last common ancestor of
> humankind. Would a good guess be somewhere between "Eve" and the
> appearance of modern man - say, between 200 and 50 Kya? It seems to me
> that dates outside this range would be impossible.
> Tom Schmal
> CHESSONP asks:
> >
> > 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.
> What I ment by LCA is tough to define in genetic terms, but from a lineage
> point of view, it is a person (well, really a pair of persons) from which
> all of us living are directly descended, ie, each of us can trace our
> ancestory to that one person. Now, we can also all trace our ancestory to
> that person's mother but she would not be the last common ancestor, she
> would be the second-to-last.

The main problem with this definition of LCA is what possible interest or
significance can it have? Each part of the human DNA has a history which,
since lineages die out at random, may be traceable to a single origin - an
LCA in this sense. However there are possibly a large number of such LCAs
and their contibutions are only to a small fragment of our DNA - and in
most cases our copy has been modified by mutation anyway.

The only place for an LCA is on a Trivial Pursuit card :).

The significance of LCA analysis is in the process of deciphering the
lineages, and the tree that results. This may give us important information
about our history as a species.

Geoffrey Watson