"No matter the languages we speak or the color of our skin, we share ancestors who planted rice on the banks of the Yangtze, who first domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forests of North and South America, and who labored to build the Great Pyramid of Khufu," Olson and his colleagues wrote in the journal Nature.

How can they be so sure?

Seven years ago one of Olson's colleagues, a Yale University statistician named Joseph Chang, started thinking about how to estimate when the last common ancestor of everybody on Earth today lived. In a paper published by the journal "Advances in Applied Probability," Chang showed that there is a mathematical relationship between the size of a population and the number of generations back to a common ancestor. Plugging the planet's current population into his equation, he came up with just over 32 generations, or about 900 years.

Chang knew that answer was wrong because it relied on some common, but inaccurate, assumptions that population geneticists often use to simplify difficult mathematical problems.

Chang's calculations essentially treated the world like one big meet market where any given guy was equally likely to pair up with any woman, whether she lived in the next village or halfway around the world. Chang was fully aware of the inaccuracy — people have to select their partners from the pool of individuals they have actually met, unless they are entering into an arranged marriage. But even then, they are much more likely to mate with partners who live nearby. And that means that geography can't be ignored if you are going to determine the relatedness of the world's population.

A few years later Chang was contacted by Olson, who had started thinking about the world's interrelatedness while writing his book. They started corresponding by e-mail, and soon included in their deliberations Douglas Rohde, a Massachusetts Institute of Technology neuroscientist and computer expert who now works for Google.

The researchers knew they would have to account for geography to get a better picture of how the family tree converges as it reaches deeper into the past. They decided to build a massive computer simulation that would essentially re-enact the history of humanity as people were born, moved from one place to another, reproduced and died.