I think, as humans, we like to classify things into groups: human and inhuman, good and evil, black and white, pile and not-pile. We’ve evolved to see clear distinctions between things because it’s useful to see a rock as distinct from the ground so that you don’t trip on it. However, the real world isn’t as clearly defined as we like to think it is. At the quantum level, for example, there are no real distinctions between one object and another, and an observer at that scale couldn’t tell you where the rock ended and the ground began.
Similarly, we looked for a “missing link” in biology for a very long time before we realized that there was no clear distinction between humans and their ancestors. There was no point where we became human, just as there isn’t a point where a pile becomes not-a-pile, because “human” and “pile” are internal labels that we use to divide the world into discrete pieces, even when there really isn’t a clear dividing line.
I know that only tangentally answers the question, but it’s the best I could do – this is my third re-write!