Image: National GeographicToday, the United Nations announced that the world's population will reach an historic 7 billion people on Oct. 31, 2011. World population hit 1 billion people in 1804. It took 123 years to add the next billion, but less than a century to cruise past the next four billion -- from 2 billion people in 1927 to 6 billion people in 1999. The U.N.'s Population Division also projected that the world will reach just over 10 billion by 2100, a number based on many uncertain assumptions about the future. Demographic projections are often mistaken for predictions, but they …