Theory of Relativity
The following are excerpts from "The Royal We" by Steve Olson, which was published in the May 2002 issue of The Atlantic Monthly and, more recently, in The Best American Science and Nature Writing 2003. It can be read in its entirety here.

The idea that virtually anyone with a European ancestor descends from English royalty seems bizarre, but it accords perfectly with some recent research done by Joseph Chang, a statistician at Yale University. The mathematics of our ancestry is exceedingly complex, because the number of our ancestors increases exponentially, not linearly. These numbers are manageable in the first few generations—two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents—but they quickly spiral out of control. Go back forty generations, or about a thousand years, and each of us theoretically has more than a trillion direct ancestors—a figure that far exceeds the total number of human beings who have ever lived.

In a 1999 paper titled "Recent Common Ancestors of All Present-Day Individuals," Chang showed how to reconcile the potentially huge number of our ancestors with the quantities of people who actually lived in the past. His model is a mathematical proof that relies on such abstractions as Poisson distributions and Markov chains, but it can readily be applied to the real world. Under the conditions laid out in his paper, the most recent common ancestor of every European today (except for recent immigrants to the Continent) was someone who lived in Europe in the surprisingly recent past—only about 600 years ago. In other words, all Europeans alive today have among their ancestors the same man or woman who lived around 1400. Before that date, according to Chang's model, the number of ancestors common to all Europeans today increased, until, about a thousand years ago, a peculiar situation prevailed: 20 percent of the adult Europeans alive in 1000 would turn out to be the ancestors of no one living today (that is, they had no children or all their descendants eventually died childless); each of the remaining 80 percent would turn out to be a direct ancestor of every European living today.

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Chang's model has even more dramatic implications. Because people are always migrating from continent to continent, networks of descent quickly interconnect. This means that the most recent common ancestor of all six billion people on earth today probably lived just a couple of thousand years ago. And not long before that the majority of the people on the planet were the direct ancestors of everyone alive today. Confucius, Nefertiti, and just about any other ancient historical figure who was even moderately prolific must today be counted among everyone's ancestors.

Toward the end of our conversation Humphrys pointed out something I hadn't considered. The same process works going forward in time; in essence every one of us who has children and whose line does not go extinct is suspended at the center of an immense genetic hourglass. Just as we are descended from most of the people alive on the planet a few thousand years ago, several thousand years hence each of us will be an ancestor of the entire human race—or of no one at all.