[Link] First almost fully-formed human [foetus] brain grown in lab, researchers claim

I meant it in the sense you understood first. I don't know what to make of the other interpretation. If a concept is well-defined, the question "Does X match the concept?" is clear. Of course it may be hard to answer.

But suppose you only have a vague understanding of ancestry. Actually, you've only recently coined the word "ancestor" to point at some blob of thought in your head. You think there's a useful idea there, but the best you can for now is: "someone who relates to me in a way similar to how my dad and my grandmother relate to me". You go around telling people about this, and someone responds "yes, this is the brute fact from which the conundrum of ancestry start". An other tells you you ought to stop using that word if you don't know what the referent is. Then they go on to say your definition is fine, it doesn't matter if you don't know how someone comes to be an ancestor, you can still talk about an ancestor and make sense. You have not gone through all the tribe's initiation rituals yet, so you don't know how you relate to grey wolves. Maybe they're your ancestors, maybe not. But the other says : "At least, you know what you mean when you claim they are or are not your ancestors.".

Then your little sisters drops by and says: "Is this rock one of your ancestors?". No, certainly not. "OK, didn't think so. Am I one of your ancestors?". You feel about it for a minute and say no. "Why? We're really close family. It's very similar to how dad or grandma relate to you." Well, you didn't include it in your original definition, but someone younger than you can definitely not be your ancestor. It's not that kind of "similar". A bit of time and a good number of family members later, you have a better definition. Your first definition was just two examples, something about "relating", and the word "similar" thrown in to mean "and everyone else who is also an ancestor." But similar in what way?

Now the word means "the smallest set such that your parents are in it, and any parent of an ancestor is an ancestor"..."union the elders of the tribe, dead or alive, and a couple of noble animal species." Maybe a few generations later you'll drop the second term of the definition and start talking about genes, whatever.

My "fuzziest starting point" was really fuzzy, and not a good definition. It was one example, something about being able to "experience" stuff, and the word "similar" thrown in to mean "and everyone else who is conscious." I may (kind of) know what I mean when I say a rock is not conscious, since it doesn't experience anything, but what do I mean exactly when I say that a dog isn't conscious?

I don't think I know what I mean when I say that, but I think it can help to keep using the word.

P.S. The final answer could be as in the ancestor story, a definition which closely matches the initial intuition. It could also be something really weird where you realize you were just confused and stop using the word. I mean, the life force of vitalism was probably a brute fact for a long time.