In the last 20 years, genealogy websites have attracted more than 15 million customers by promising insights into your past. Maybe you’ll uncover a secret infidelity or be reunited with a long-lost cousin, like when Larry met Bernie on Finding Your Roots. It’s deeply personal, affecting stuff. But when your family tree contains thousands, millions, even tens of millions of people, it’s no longer a personal history. It’s human history.

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