The world population reached 1 billion for the first time in 1804.
It then took 123 years for the number of people to double, to 2 billion, in 1927.
It took just 72 years for that number to triple, to 6 billion people, in 1999.
The world population has experienced continuous growth since the end of the Black Death around the year 1400.
Annual births have reduced to 140 million since their peak at 173 million in the late 1990s, and are expected to remain fairly constant.
The world population is projected to reach seven billion by late 2011, and around eight billion by 2025.
By 2045-2050, the population of humans on earth is currently projected to reach about nine billion.
Hopefully, some of us will be in space or other planets by then.