We only need to go back to the year 1250 to have more direct ancestors than the number of human beings who have ever lived.

From The Book of the Dead by John Mitchinson.

The first thing that strikes you about the Dead is just how many of them there are. The idea you hear bandied about that there are more people living now than have ever lived in the past is plain wrong—by a factor of thirteen. The number of Homo sapiens sapiens who have ever lived, fought, loved, fussed, and finally died over the last hundred thousand years is around 90 billion.

Ninety billion is a big number, especially when you’re trying to write a book with a title that implies it covers all of them. But it all depends how you look at things. Ninety billion is big, but also small. You could bury everyone who has ever lived, side by side, in an area the size of England and Scotland combined. Or Uruguay. Or Oklahoma. That’s just 0.1 percent of the land area of the earth. And if you piled all the dead people who have ever lived on to an enormous set of scales, they would be comfortably outweighed by the ants that are out there right now, plotting who knows what. It’s all a question of perspective.

The Dead are, literally, our family. Not just the ones we know we are related to: our two parents, four grandparents, and eight great-grandparents. Go back ten generations and each of us has a thousand direct relatives; go back fifteen and the number soars to more than thirty-five thousand (and that’s not counting aunts and uncles). In fact, we only need to go back to the year 1250 to have more direct ancestors than the number of human beings who have ever lived. The solution to this apparent paradox is that we’re all interrelated: the further back you go, the more ancestors we are likely to share. The earliest common ancestor of everyone living in Europe lived only about six hundred years ago, and everyone alive on the planet today is related to both Confucius (551–479 BC) and Nefertiti (1370–1330 BC). So this is a book of family history for everyone.