DNA and Father's Day

In Bill Bryson’s 2003 book, A Short History of Nearly Everything, he writes, “If your two parents had not bonded when they did—possibly to the second—you wouldn’t be here.” Your existence also depends upon countless exact bondings between your grandparents, great-grandparents, all of your forefathers back thousands of years.

Bryson has counted up all the people required to make you, You. He says that “if you count back sixty-four generations, to the time of the ancient Romans, the number of people on whose cooperative efforts your existence depends has risen to a giant number, a 1 followed by...