It was probably so gradual they’ll never pinpoint it. Rather like there aren’t really clear boundaries between races.
It is like trying to pinpoint the exact location your walk through the forest became a walk through the swamp. There was no line, the forest just gradually becomes more and more swampy until you are no longer in a ‘swampy forest’ but more in a ‘foresty swamp’ and eventually - all swamp.
Looking for an exact line of demarcation, looking at your footprints and saying “this step was in a forest, but this step was in a swamp” - is sort of silly - but people like things to fit into neat little boxes, despite this often being inapplicable to reality.
Careful there.
You-know-who will be by shortly to assert that there are no transitional forms...