The thing is that species aren't that clear cut but exist on a spectrum. There is no first chicken as little as there is a first blue shade on a color gradient. Sure, you can draw the line somewhere but even when clearly defined as ancestor of all modern chickens, you can't really go down to the individual level.
That is what I mean, the scooe of the question is too narrow.
"It's a thought experiment, there is no real answer, blah blah blah..."
The question is only deep among those who constrain themselves to what they believe the question asks, just widen the scope and you have the logical sollution.
The flaw of the question is assuming there is a clear dividing line between species. Evolutionary change is a continuous process. We only have dividing lines where we see differences in long dead ones in the fossil record, or we see enough differences in living ones. The question has no answer, only a long explanation of how that isn't how any of this works.