Over the next three decades, the West African nation's population is expected to soar even more: From 216 million people this year to 375 million, the United Nations says, making Nigeria to the fourth most populous country in the world after only India, China and the United States.And More than 15 million people in Lagos are already competing for basic amenities of life.Tuesday marks the UN projection for when the world's population is expected to hit 8 billion people, though officials are careful to note it's not a precise milestone.