chicken would also be able to defined as it’s ancestor
This isn’t the case, and there’s a mathematical theorem describing this called the Intermediate Value Theorem. Basically, if you have a function describing a line you can draw without picking up your pencil, at some point along that line the value takes on every value on that line. Makes sense, right?
If I draw a line separating Chicken-birds from Not-chicken-birds, and show the evolutionary path leading from non-chicken to chicken, at some point it crosses that line. We don’t have to know where that point is, we just know it crosses the line at some point.
At that point, wherever it is, we have a bird that meets the criteria of “chicken” hatching from an egg laid by a bird that doesn’t.
Besides, this is all pretty moot. We actually know when and where chickens originated. They originated about 3000 years ago in China and India after being domesticated from Southeast Asian Red Junglefowl.