A special note to preface: The criticism contained herein is not intended to challenge the fact of biological evolution. However, the terminology often used by public proponents does seem to be lacking in consistency with the natural world.
When Charles Darwin entered the fray in the ongoing discussion about common descent, it was a discussion that already borrowed terminology from more mundane human geneaology. When social custom demanded worrying about property and violence and loyalty; a term like descendant and ancestor was intended to settle the matter once and for all. If Queen Bonnie owned Tulipsville in ye olden times and you could show the direct and real and sequential descendant of Queen Bonnie to all concerned; that was that, Tulipsville was yours. Transferred to evolutionary usage, if it could be determined and shown that the descendants of a small mammal that lived and loved in Cretaceous North America survived to present day as a human being living in Brooklyn, that would be that. Direct, Real, Sequential.
On the other hand, going in the reverse order doesn't seem so convenient with the terminology at hand. Specifically, "the Common Ancestor" of, say, a human living today in Brooklyn and an African Elephant named Mogri living in the Serengeti today. Much can be inferred toward that end on a very general trend, more primitive primates here, and more primitive proboscidean there. And yet this only gets you so far. In the case of a descendant, you can safely throw out a great majority of the individuals as unimportant (not unimportant on ethical grounds, but just for the bookkeeping) , say the various siblings and such. But for an ancestor, if you try to ignore any of the individuals, you lose all of your accuracy. The closest you may be able to get is a population of potentially breeding organisms. The genes shared in common between a human and elephant of today did not necessarily come from an individual in that population, but could have been contributed by distant relatives in that population who are not connected in any direct linear fashion. The individual elephant living today in the Serengeti and the human living in Brooklyn today may have never gotten any closer than a cousin of a cousin of a cousin of a cousin of a cousin and all separated in existence by a hundred years or more.
Despite all this, when was the last time anyone described to the public or expected the public to assume that the term "the Common Ancestor" was actually plural, not necessarily distinct, and possibly metaphorical in nature? Is there simply a weakness in the definitions of our language, and we need a new word? Or does the existing terminology need to be readdressed and properly defined to those outside the biological sciences?
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