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Re: Date for Last Common Ancestor?
Stephen Barnard (steve@megafauna.com)
Mon, 12 Aug 1996 07:45:27 -0800
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
> --
> susansf@netcom.com
You seem to be confusing two separate uses of the concept of Last Common
Ancestor. (Sorry, I don't mean to sound condescending, but I think you
really are mixing up two different things.)
1. The concept is used to define the relatedness of species by looking at
the LCA of two species. For example, chimps are thought to be more closely
related to humans than are gorillas because we share a more recent LCA with
chimps (probably).
2. The sense in which Mitochondrial Eve was our last female common ancestor
has nothing to do directly with the beginning of our species. Her
immediate ancestors were just as human as all of her descendants.
When used for the purpose of relating two species, it seems like the LCA is
absolute, as you say. It does have a another peculiar property, however.
It can only be identified well after the fact. For example, suppose a
lineage splits into two, leading eventually to two separate species. For a
long time after the split the members of the two lineages will remain in
the same species and will be capable of mixing reproductively in principle.
It will only be when the two species have finally split reproductively that
their LCA will be meaningful for relating them as species.
Steve Barnard
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