Reminds me of my Structuralism 101 course at university. Everything is relative, the act of reading changes meaning, etc etc. Fun stuff to play with, but real world application remains in that one is identifying inhibiting factors to information transmission rather than absolute limits.
So yes, one can find an arbitrary line for the first human, but there will be a point on the continuum where you can say "this is distinctive to the other" based on the differences. A t-rex is not a chicken after all. There's always room for further refinement and further knowledge, and there's always a good case not to overly stress differences which are little more than cosmetic or just extend the range in variation.