Not sure if this question can be answered but I've always been intrigued as to what the population density would look like if people returned to the country of thier birth. What would the most populous country be for example?
How far back do you mean? Eventually we all came from Africa. If you mean just first generation immigrants, then it wouldn't make a huge difference. If you go back further then you start to run into the question of how to handle the offspring from marriages between people from different countries.