Ciara Curtin in Scientific American:
Little is known about this distant past and how many of us there might have been, but by the time of the agricultural revolution in the Middle East in 9000 B.C., Earth held an estimated five million people.
Between the rise of farming and the height of Roman rule,population growth was sluggish; at less than a tenth of a percent per year, it crawled to about 300 million by A.D. 1. Then the total fell as plagues wiped out large swathes of people. (The “black death” in the 14th century wiped out at least 75 million.) As a result, by 1650 the world population had only increased to about 500 million. By 1800, though, thanks to improved agriculture and sanitation, it doubled to more than one billion. And, in 2002 when Haub last made these calculations, the planet’s population had exploded, reaching 6.2 billion.
To calculate how many people have ever lived, Haub followed a minimalist approach, beginning with two people in 50000 B.C.—his Adam and Eve. Then, using his historical growth rates and population benchmarks, he estimated that slightly over 106 billion people had ever been born. Of those, people alive today comprise only 6 percent, nowhere near 75 percent.