In 1999, the human population passed six billion. In 1985, it passed five billion. In 1962, it passed three billion. In 1800, it passed one billion. In 1 AD, the world's population, according to the censuses taken by the governments of that time, was only 250 million. At the current human population growth rate, considering wars and famines and all such variables, it would take approximately 5,000 years to get the current population from two original people.
Taken from:
How Old Is The Earth?
There are also many other websites that bring up those same figures.
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