The world’s population reached seven billion in October 2011. The sixth billion was reached in 1999 and it is significant that the seventh billion took the same number of years (12) to add as the sixth. This is relevant because prior to that there had been a progressive shortening of the time taken to add billions to the human population. The first billion was reached in 1804, taking many thousands of years of human evolution to achieve. Thereafter successive billions were added in 123, 32, 15, and 13 years respectively.
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