" Another way to comprehend it is this: From the time of our beginning as a species (about 200,000 years ago) until the year 1804, human population rose to a billion; between 1804 and 1927, it rose by another billion; we reached 3 billion in 1960; and each net addition of a billion people, since then, has taken only about thirteen years. In October 2011, we came to the 7-billion mark "

― David Quammen , Spillover: Animal Infections and the Next Human Pandemic