"Chances are, if you have a famous ancestor far enough back that finding out about them is a surprise, you share them with a small city of other people. And the farther back you go, the truer that is. In 2004, statistician Joseph Chang, computer scientist Douglas Rohde, and writer Steve Olson used a computer model of human genetics to show that anyone who was alive 2,000-3,000 years ago is either the ancestor of everyone who’s now alive, or no one at all. Think about that: If a person alive in 1,000 BCE has any descendants alive today, they have all of us—even people from different continents and isolated populations. This line of thought led to the revelation that everyone of European heritage alive today is a descendant of Charlemagne, who ruled over much of Europe as the first Holy Roman Emperor...
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."
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