A.J. Jacobs has received some strange emails over the years, but this note was perhaps the strangest: “You don’t know me, but I’m your eighth cousin. And we have over 80,000 relatives of yours in our database.”

That’s enough family members to fill Madison Square Garden four times over. Who are these people, A.J. wondered, and how do I find them? So began Jacobs’s three-year adventure to help build the biggest family tree in history.

Jacobs’s journey would take him to all seven continents. He drank beer with a US president, found himself singing with the Mormon Tabernacle Choir, and unearthed genetic links to Hollywood actresses and real-life scoundrels. After all, we can choose our friends, but not our family.

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