Re: Date for Last Common Ancestor?

Susan S. Chin (susansf@netcom.com)
Mon, 12 Aug 1996 05:37:12 GMT

Stephen Barnard (steve@megafauna.com) wrote:
: Susan S. Chin wrote:
: >
: > Stephen Barnard (steve@megafauna.com) wrote:
: > : T&B Schmal 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.
: > : >
: > : > Is this on track and can this range be narrowed? Suggestions welcomed.
: > : >
: > : > Tom Schmal
: > :
: > : The attribute of being "the last common ancestor of humankind" has a
: > : very peculiar property. That person can only be identified long after
: > : the fact, and the identity and date of that person is liable to be
: > : changed radically at any time. For example, if a new disease were to
: > : decimate the world's human population, with only say 10% of the people
: > : surviving because of an inherited resistance, then the date of the last
: > : common ancestor would probably be moved forward in time considerably.
: >
: > : Steve Barnard
: >
: > Whether or not we will ever determine who and when this last common
: > ancestor occurred, I don't see how it can arbitrarily be moved forward in
: > time due to a scenario of catastrophic decimation of 90% of humankind. The
: > assumption is that by that time, the LCA had already split off to form
: > these various populations.
: >
: > Even if you subscribe to the theory of multiple and separate evolutionary
: > roots of mankind per geographic regions, at one point, there was
: > a species of organism from which Homo sapiens sapiens arose.
: >
: > The "Eve" that we hear about as the "mother of us all" is really a
: > population which contained the genes which ultimately resulted in all of
: > humakind as we see it today. It was not meant to imply (though of course
: > it does anyway) that we all arose from one female individual, but from a
: > population of individuals with those genes.
: >

: It's really very simple. The mutation that gave rise to the immunity to the
: disease could easily, and probably would have, occured much later than the
: erstwhile LCA. Therefore, the new LCA would be that person who first exhibited
: the mutation.

In this scenario though, the populations that exist, those with and those
without the immunity, are *already* commonly descended from the Last
Common Ancestor. The new LCA you are referring to, the one with the
beneficial mutation shares a common ancestry with those unfortunate ones
who didn't "survive." The emphasis should be on the Common Ancestor part
of LCA, not on Last, since that might create some confusion. In your
scenario, the Common Ancestor remains the same, regardless of the
mutation. That Common Ancestor is the LAST Common Ancestor. No more after
that. So, is that your point as well?

: This has nothing to do with the "theory of multiple and separate evolutionary
: roots of mankind per geographic regions", which I consider to be highly
: unlikely. I also don't quarrel with the concept that there *is* an LCA, which
: is clearly the case from purely logical arguments. The point is that the
: attribute of being the LCA has some very peculiar semantic properties.

: Steve Barnard

-- 

                                             susansf@netcom.com