I think the modern answer to the question in the OP comes by reframing it this way, "When did the last common ancestor of all living humans live?" It defines what biologists call the clade of humans. The dominant view among biologists would place the answer at around 200,000 years ago.
I don't this makes sense on the micro scale, with sexual reproduction involved. The last common ancestor of all living humans lived very recently, considering that they weren't the only ancestor of all iliving humans. Wikipedia says that the last common ancestor of all living humans might have beeen as recently as 3000 years ago, but it would be ridiculous to say that that person was the first human.
I'm not sure I get what you're saying. Do believe that we have ancestors that
were "partially human"? That the difference between us and our nonhuman ancestors
is some sort of continuum? That the term "human" is somewhat arbitrary?
It could be similar to a "human's" life in the womb. "Life" doesn't actually begin
at one point in time, but is gradual and at sometime before birth the fetus becomes fully
human. Many people believe that it begins at birth. An alternate theory is that it happens
gradually. At conception the lifeform doesn't look human. Minutes before birth it does.
This seems very similar to human evolution. The "missing" links would be what you
might call partly human.
