Re: Date for Last Common Ancestor?

Susan S. Chin (susansf@netcom.com)
Mon, 12 Aug 1996 13:20:22 GMT

Stephen Barnard (steve@megafauna.com) wrote:
: Susan S. Chin wrote:
: >
: > Stephen Barnard (steve@megafauna.com) wrote:

: [a bunch of stuff clipped for brevity]

: >
: > : 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?
: >

: The "last" part is crucial to the concept of "last common ancestor". I you leave
: out "last" then we have common ancestors going all the way back to the origin of
: life. In my scenario the erstwhile LCA remains a CA, but no longer the LCA.
: That's what is a little peculiar about the property of being an LCA.

: Steve Barnard

The "last" part of LCA is crucial in identifying when the split between
lineages occurred. At that point of splitting, branching, whatever you
want to call it, there is an ancestral species known as the LCA. If there
is further splitting or speciation which later occurs, that is totally
irrelevant to the original LCA which has already been identified wrt Homo
sapiens sapiens or any other species. Further splitting produces LCA's for
OTHER species which result from the split, but has nothing to do with the
original LCA. So if anything, LCA's are relative to what 2 organisms you
are comparing. The ancestral relationship to their LCA though is absolute.

In the original post, I believe the question was LCA for Homo sapiens
sapiens or mankind as we know it today. Therefore, when that crucial
event occurred which gave rise to modern man, it is assumed that it
occurred only once, on a species wide unit of evolution. But in any
case, we have our LCA. Further events which occur are irrelevant,
unless you are categorizing different races into various subspecies of
H.s.s. And even then, the LCA of H.s.s. as a species doesn't change.

It sounds to me like we have a disagreement over the unit of evolution
here. I'm referring to the LCA as a species, or a population of the
species which gave rise to modern man. Once this LCA appears, any later
purported LCA's are really artificially recognized units of evolution.

Susan

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                                             susansf@netcom.com