Tracing one person’s lineage back in time forms a binary tree of parents, grandparents, great-grandparents and so on. However, the number of individuals in such an ancestor tree grows exponentially and will eventually become impossibly high. For example, an individual human alive today would, over 30 generations, going back to about the High Middle Ages, have 230 or about 1.07 billion ancestors, more than the total world population at the time.

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