I was listening to Brad DeLong's lectures for his 20th Century economic history class. In the intro class he says that the 20th century would be the century of economics - economic growth, economic wars, economics-based ideologies. I'm sure that he would consider population a sub-phenomenon of the economic story, but to me the outstanding fact is the growth in population.
In 1900 the global population was 1.6B. By 2000 it was over 6B. It took us 100,000 years to get to 1.6B and 100 years to quadruple that. Most/many of us were richer than our counterparts in 1900, but I'm still struck by how many more of us there are.