Nigeria is the 7th most populated country in the world, but I have no idea why. What are some geographic/historical reasons why this might be? Compared to other countries in Africa with roughly the same climate, such as the CAR or other nations, why is Nigeria the most populated? Were there any great empires which enabled more efficient ways to create food? Was the geography a factor?
In terms of climate, Nigeria is much like India, the second most populous country in the world after China. (India is barely more than a fourth the size of China so it is much more densely populated on a per square mile basis.) That is, both India and Nigeria have hot tropical climates. And most of each country has abundant rainfall (except for the northern reaches of both countries), that is, between 30 and 60 inches a year. Because of these facts, something like 30% of Nigeria's economy is agriculture-based, an unusually high percentage, even for a developing country.
Nigeria is also distinguished from other countries such as Zaire (formerly the DRC), insofar as most of Nigeria's land (like that of India) is "grassland," rather than forest, because most of Zaire, gets 60-80 inches of rain, even more than most of Nigeria. This makes Nigeria and India (but less so Zaire) highly suitable for crop growing, which can support a large population.
Put another way, Nigeria probably has the "best" rainfall pattern of any country in Africa, well balanced between "yellow," "green" and "blue" regions. Most other countries have either extremes of red and orange (dry) or blue (too wet).