I don't like this because it's not addressing the actual saying. Obviously the saying is about chicken eggs specifically.
But I've always felt obviously the egg came first. The first chicken was born in an egg, so the egg came first. That egg could have been produced from a creature with a mutation which caused it to produce the first chicken egg when it is not itself the exact same species.
Ah, but when that line of tiny change is so arbitrary... Is it a true chicken until it grows up and fulfils its destiny? Is it a chicken based purely on its genetic code, so the egg whence it hatched is a chicken egg; or is it truly a chicken when it becomes a chicken..... meh, I write this far and find I still agree with you: even in that case the egg it hatched from becomes a chicken egg by virtue of the chicken it grew into.
Hatch or grow. Because once you're asking those questions, is the first chick truly the first chicken?
"Is a juvenile defined by what it currently is or what it will/might become?" And, "is chicken-ness an innate quality of the animal, or in relation to the animal fulfilling/presenting (or being able to fulfil) some chicken-ness?"
I believe this is correct as I read in a book somewhere that it was a kind of proto-chicken if you will, that laid an egg of which came a the first chicken.
The more interesting question is how long did it take for the first BBQ Chicken.
But I think it's not about chicken at all. People just don't know which creature on earth laid the first egg, so the chicken is just a stand-in. As chicken are the species we most associate with eggs for obvious reasons.
What came first: the first egg or the first egg-laying creature? Has to be the egg-laying creature, but then how did that get born?