Would I notice something not quite right if I saw a living Homo Sapiens from like 40,000 years ago? If so, which is the threshold beyond which the resemblance with the contemporary humans is perfect?

Answer 1

No, there would be no noticeable difference between in and someone from 40,000 years ago. If it were 100,000 years ago, the almost certainly yes—but mostly in terms of what you could not see: things like the inability to think like we do and the capacity for language.

So when did that change occur? Best guess is around 70,000 years ago.