Natalie Solent observes the following
I don't really think that there have been any Arabs or Ashanti for many decades who wouldn't recognise a pair of trousers. Who is there left? A few tribes in Papua New Guinea, perhaps. About ten to fifteen years ago I saw a film of such a tribe being contacted for the first time, but I haven't heard of any such event since then. In our lifetimes the world finally did or will become one. Future historians will be able to state the exact day it happened.
Guns, Germs and Steel author Jared Diamond told a story in one of his other books about setting out to the most remote corner of some Pacific island or other, to meet what he thought were the most isolated people in the country, and possibly in the world. After a couple of days of pushing through dense jungle, he reached his destination, and was greeted warmly by a local. Diamond's sense of achievement at reaching this place was dampened somewhat by the fact that the person who greeted him was wearing a University of Wisconsin sweatshirt.
I doubt there are any people at this point who have had no contact with civilization. For one thing, we simply know so much more about the surface of the earth than we did even 15 years ago. Everything inch of the earth's surface is photographed and mapped in detail. This is a new condition. Until 10 to 15 years ago, once in a while we heard stories about contact between populations of people who had been cut off from civilization for a while and it looked like a first contact, but even in these cases, investigations would often suggest that the true nature of the encounter was blurrier. It would turn out that someone had left the tribe in question 40 years ago and came back seven years later with stories of civilization, or similar. People are too determined to trade with one another and to explore to stay cut off for long, unless there are truly dramatic geographical barriers in the way. I am not sure we will ever know the exact day when we all became one, because I think the process was more gradual than sudden.
It seems that the last substantial population of people to have been cut off from the rest of the world were the highland peoples of New Guinea, of who the outside world was completely unaware until the 1930s. This was a case when there were dramatic geographical barriers. There may have been small isolated groups after that, but World War II pretty much spelled the end even for these, I suspect.