There's no such thing as a common ancestor and the whole process of biological evolution precludes such a critter from having existed.
Oh. So you saying that my cousin in Massachusetts, My cousin in Florida and I do NOT have a common ancestor in Grandad?
So if common ancestors do not exist who was Grandad?
How did we come about? Special creation?
Isn't it true that every living thing has an ancestor from a few generations ago? And that ancestor had an ancestor from a few generations prior to that? How far back do you propose we go before we run out of ancestors? Then what came before that?
I'd bet that if we had the complete genealogies back far enough you and I could find a common ancestor between us. Got any French in your background?
So show me how the the Miacis line did not radiate into ursine, canine and feline lineages and is thus NOT the common ancestor for these groups.
there have been many descended lines from what we can agreeably consider Neanderthal to modern humans.
This is not what the thousands of professional (in your lexicon "pop-culture") anthropologists over the last decades say. And the latest genetic data (2010) says that H. Neanderthalensis was in fact a separate species from H. sapiens.
But this doesn't answer any of my questions about the definition of "common ancestor".
the answer to your question depends on where one wants to put the cut off for 'species'.
Put it where ever you like. My question remains. Was there no "ancestor" three, two, one generation(s) prior to this arbitrary line? If not then where did the first organism after this arbitrary line come from?
None of this obfuscation answers my questions:
What are these two notions of "common ancestor" I used and how do they differ?
What is this third definition you are using, how does it differ from the one(s) I used and how would anyone have known this difference in context with your message?
my firm (evidence-backed) belief that there have been many descended lines from what we can agreeably consider Neanderthal to modern humans.
Let me ask an additional set of questions:
What evidence? Since you disagree with copious amounts of archeological and genetic evidence by thousands of life-long experts over many decades, I assume this is pretty powerful stuff and you have published this evidence somewhere, yes?
Finally, what are your credentials in this field? Or any field? You do have them, yes? Or are you just full of ...
First of all, Jon, "common ancestor" is not an analogy for some other concept. It is the direct concept itself. The concept of direct lineage.
Second, in the case of my family, we have a direct demonstrable link between the various later individuals and a single specific earlier individual from which we stem. That is our common ancestor.
Third, as we proceed deeper into time the specific information of exact genealogy, between say you and I, may be lost and the best we could hope for is to identify a population subset from which we both stem. European? French? Mediterranean coast? Marseille region? The "common ancestor" concept is as valid and meaningful in this instance as in the above and is not analogous to any other concept but is a concept of direct lineage, though, in this case, can only get as specific as to identify a sub-population, a founder population, of common origin.
When we get to bears, cats and dogs we find that the best we can get, the last direct link between these that can be identified, is not an individual or even as fine as a sub-population, but the more granular overall population of an early carnivore we call miacis about 60 million years ago. The concept still holds and is valid. Miacis is the founder population, the earliest "common ancestor," to these lineages.
This is the standard definition of "common ancestor" used in the field.
So what is your definition of "common ancestor"? And why would you choose a definition totally at odds with the standard in the field?
Are you trying to deny nested hierarchy, common descent?
We, instead, have to eventually decide a cutoff, but doing so requires us to admit that our cutoff point is somewhat arbitrary and not necessarily representative of a true genetic and breeding relationship between the beasts in question.
What are you talking about? If this is your view of the use of "common ancestor" in the field it is a bad one.
Take any two species. Trace their lineages back to the species or genus where those lineages converge and, by definition, you have found the earliest common ancestor between the two. And, yes, this does indeed show the "true genetic and breeding relationship between the beasts in question."
Not when the 'common ancestor' being discussed is of the "idealised ... with no further genetic flow" type.
No such thing. This "definition" of "common ancestor" is strictly within your own head and is fallacious.
I just think it faulty to believe that there could be common parents to an entire species. Seems too Adam and Eve to me, even if we are talking about an entire generation of parents, rather than just two people.
Then stop thinking like that. This is not what the definition of "common ancestor" entails.
I said that just one sentence after. You're quote-mining now, and misrepresenting my position. I don't do dirty debating.
As the communicator I obviously did not make my intent clear to the receiver. I will attempt to do better.
When this is the meaning folk begin putting on to the term, it becomes necessary to point out that their notion of 'common ancestor' is a fantasized non-existent entity/species/generation.
A "common ancestor" of the "idealised ... with no further genetic flow" type does not exist. That you perceive this definition as the one being used in this thread is strictly within your own head and is fallacious.
Whether the comparison is between two individuals, two closely related sub-populations, two related species, two broadly separated species, makes no difference. Follow the lineages back until they converge. This is the most recent "common ancestor."