We are all mostly related to each other. But weirder still, you’re just about as related to the stranger next to you as to your great×12 grandparents.

Now, there’s another important implication of genomic ancestry studies: Most of the people you are descended from are no more genetically related to you than strangers are. Or to put it another way, your genealogical family tree, which includes all the history of your family going back thousands of years, is much larger than your genetic family tree-the people whom genome sequencing would pinpoint as related to you. 99.9 percent of your genome is the same as that of every other human being (apart from the x and y chromosomes), and that .1 percent of variation in each person gets thinned out pretty quickly across the generations, as each child gets half of each of her parents’ genomes, passes on half to each of her children, and so on. Geneticist Luke Jostins did a nice mathematical analysis and estimated that you have only about a 12 percent chance of being genetically related to an ancestor 10 generations ago; by the time you get to a 14-generation ancestor, the probability is nearly zero.