The historian and cartography Bill Rankin, who maintains the blog Radical Cartography, created the fascinating histograms above, which show the world’s population distributed by longitude and latitude. (Apparently, it's been kicking around the Internets for a while, but was new to me.)
A couple of striking things: First, and most obvious, there are an awful lot of people in India and China. Second, the map shows that the majority of the world’s population lives in the Northern Hemisphere–where most of the land is—and the temperate climate zone. I would be interesting to check back in a century to see if the bump had moved due to climate change.
On the longitude map, what stands out the most is the third largest spike, located around 30° east. This isn’t the most obvious population belt, but the cities of St. Petersburg, Kiev, Odessa, Ankara, Cairo, Khartoum, Juba, Kampala, Kigali, Lusaka, Harare, Pretoria, and Durban are all within about two or three degrees of 30° east. That’s a crowded meridian!
Part of the Nile, along which most of the people in heavily-populated Egypt live, flows roughly along this line, but other than that, your guess about what this means is as good as mine.