Hard to say. You see, throughout the course of history Europe, India and China concentrated over two thirds of the world's population. The first to go through medical advances and an industrial revolutions would face a demographic explosion with no precedents and would have to vent it somewhere. So I guess North America would be incredibly populous, probably over twice as much. Also depending on how things went, Siberia and Central Asia would have been effectively colonized instead of merely conquered by an empire as it happened here.
Mmm... so many implications. I might write an alternate history timeline about this.
That being said, South and Central America would probably end up like Africa today, perhaps a little better. That depends on how fast their populations would grow after the threat of tribalism and diseases were removed.
