With Earth’s population now at 7 billion, how many people have ever lived?

Every time we pass another population milestone — at the moment, we’re tallying a whole-Earth total of 7 billion — the question arises: How many people have ever lived on the planet?

The Population Reference Bureau, or PRB, has an answer for that. (Make your guess now; the answer will appear lower down.) But it’s heavily qualified, and there’s a story behind it.

The story starts sometime in the 1970s, when a notion came into common conversation that the population was growing so fast that 75 percent of the people who had ever lived on Earth were currently alive. The origin of the idea is murky, but it came amid new fears of a “population explosion”; the organization Zero Population Growth had been founded in 1968.

But the idea sounded fishy to serious demographers, including Carl Haub of PRB. In 1995 he wrote an article examining what he has called “a ridiculous statement” and trying to come up with a more realistic demographic alternative. The article, updated in 2002 and again last month, can be read on his group’s Web site, www.prb.org.

Haub begins by adopting the United Nations’ estimate of 50,000 B.C. as the likely date for the beginning of the human race (“population: 2,” says the accompanying chart), then estimates changing birth rates and average length of life, accounting for major disasters such as the Black Death and factoring in historic population estimates.

His total count of everyone who has ever lived: 108 billion.

Haub is the first to admit this is a highly speculative enterprise. “Semi-scientific it must be, because there are, of course, absolutely no demographic data available for 99 percent of the span of the human stay on Earth,” he writes. “Still, with some speculation concerning prehistoric populations, we can at least approach a guesstimate of this elusive number.”

— Nancy Szokan