African Subjugation Part Two: The Equatorial Conundrum

Obviously southern (equatorial) civilizations lagged way behind northern "Western" civilizations economically and technologically heading into the twentieth century, which led to subjugation, which led to the backwards and broken societies we have today. What I don't understand exactly is how, when, and why the equatorial part of the world fell behind. The only explanation I've heard is that the colder weather of the North forced its peoples to develop the technology to combat the cold while simultaneously restricting them from depending on purely agrarian pursuits. Anyone who could shed some more light on this topic and/or recommend a book or other source of information would be greatly appreciated. Anthropology, geography, and history majors - we're looking at you.