Re: Date for Last Common Ancestor?

Stephen Barnard (
Mon, 12 Aug 1996 03:34:49 -0800

Susan S. Chin wrote:
> Stephen Barnard ( 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