“When his first request for an interview with former president George HW Bush was rejected, AJ Jacobs resorted to a desperate plea: “Couldn’t you do it for family?” – pointing out that they were (distant) cousins.

The plea worked. Bush gave him an interview and even posed for a photo – an image that sits alongside others, including Daniel Radcliffe, Olivia Munn, and Ricky Gervais, in Jacobs’ new book, It’s All Relative. The book examines the promise and challenges of the world family tree – the dream of some geneticists and genealogists to connect everyone who’s ever lived, be they Neanderthal or celebrity.

AJ Jacobs and Donny Osmond.

The world family tree is an obsessive – some might say absurd – undertaking. That makes it right up Jacobs’ alley. For previous bestsellers, he has tried to follow Biblical law to the letter, live by every bit of health advice out there, and read the whole Encyclopedia Britannica. Now he has brought his completist approach to genealogy – hence the attempt to track down as many of his distant cousins as possible.

Jacobs first learned about the world family tree via an email from a total stranger. It read: “You don’t know me, but I’m your eighth cousin. And we have over 80,000 relatives of yours in our database.” Even more impressively, Jacobs’ new cousin, who lived in Israel, had rounded up 3,000 of them for a reunion – close to a world record. Surely, Jacobs thought, with a year to plan and his myriad contacts, he himself could draw at least that many, putting him in the record books.

Plus, he thought, in this age of political strife, the reunion would be an exercise in inclusion and togetherness; in crossing lines of race and nationality… If only he could pull it off. The intricacies of planning such an event, though, are a hard ask for someone who, by his own admission, is an anxiety-prone and somewhat misanthropic introvert. “It could be huge,” Jacobs writes of his planned Global Family Reunion. “It could be historic. It could be a nightmare.”

Jacobs began by pinging 100 third, fourth, and fifth cousins via 23andMe, an ancestry service. Responses were mixed. “You got some people who would email me back and say, ‘Don’t ever contact me again. I have enough family, and I hate most of my family already.’” But a surprising number said they were game for the party; several even offered to bring potato salad for 3,000.

Christina Hendricks.

Jacobs was hopeful about smashing the record. After all, as long as you’re willing to do the work, you can figure out how you’re connected to anyone who’s ever lived. Going all the way back, we all share two “uber-grandparents, the real Adam and Eve,” known by geneticists as Y-Chromosomal Adam and Mitochondrial Eve. And there are a ton of sites such as Geni and Ancestry helping genealogists to make more meaningful connections. If Jacobs has billions of cousins out there, surely, he thought, 3,000 will come to his party.

When I meet him for lunch, I expect he will tell me we’re third or 18th cousins. Sure enough: “I looked you up,” he says. “I haven’t found a relationship path between us – yet. But I did once have a collection of air sickness bags!” (My grandfather invented the air sickness bag.)

But why gather information about dead strangers just because they’re on our family tree? Why should we care about someone simply because they share some DNA with us?

The idea of the world family tree, he says, gives him a profound sense of belonging to something larger than himself. It’s the ultimate social network and it has to be worth making these connections – it may even hasten world peace.

AJ Jacobs with Olivia Wilde.

He understands that some of us might be wary of reaching out to strangers. Still, as far as he’s concerned, it’s worth the risk – for the crazy stories, if nothing else. For instance, he learned that, around 1900, his fourth-great-aunt was about to be wed in an arranged marriage in Poland but the groom didn’t show up. No matter: some family members walked outside, found a single man, and said, “Would you like to get married today?” Thus Jacobs acquired his fourth-great-uncle.

Marriages, of course, are the connective tissue of family trees, and he likes to remind people that all those vertical lines on a family tree represent sex. “Think about your 2,000th great-grandma having sex with a Neanderthal,” he says. “You want to talk about a teenage rebellion: having sex with a different species!”

Of course, not all the lines on a tree represent marriage, or even sex, especially these days. There’s gay marriage, reproductive technology, adoption, step-parenthood… The decline of the nuclear family makes genealogy harder. Jacobs argues it also makes it more exciting. He cheers creative family structures and celebrates racial intermarriage: “I was very excited about Prince Harry’s engagement to Meghan Markle. I am very into the whole idea of intermarriage breaking down these DNA silos.”

AJ Jacobs with George and Barbara Bush.

Throughout the book, Jacobs offers a “Reunion Countdown”. For every bit of good news (the literary critic Henry Louis Gates, Jr, agrees to speak at the reunion! Sister Sledge will sing their hit We Are Family!), there is bad news. At one point, he learns that a French family has held a reunion of 4,500 relatives, raising the attendance target to a figure thousands more than Jacobs has been able to pin down so far. He despairs.

He pushes through with his own plan, though, partly because it’s too late to recover his deposits, but mostly the world peace thing. And he also thinks it’ll be good for his own children. A 2001 study by doctors Marshall Duke and Robyn Fivush discovered that knowing family stories helps build resilience in children. In particular, when they hear what’s called an “oscillating narrative” – the family was rich and then poor and then rich, but still stuck together as a family – that helps them to see themselves and their crises in perspective.

Inspired by the “oscillating narrative” model, Jacobs stresses mixed-bag stories to his three sons (he has 11-year-old twins and a 13-year-old): “I talk to my kids now all the time about our ancestors’ failures, but also my own failures – and I think that they think I am a total failure, because I maybe overdo it. But it’s so important. I grew up thinking that my father was flawless, and it’s stressful.”

AJ Jacobs with John Legend.

On the world peace front, while Jacobs was working on the book, a Harvard study found that Israelis and Palestinians were nicer to each other in a memory game after being told that they were genetically related. When someone cuts in front of him at the bookshop, he now thinks, “OK, that’s not cool, but he’s my cousin. He and I share a grandfather way back. And we’ll have descendants in common. So maybe I should give him the benefit of the doubt.”

The morning of the reunion, Jacobs woke to the sound of rain. The field where the event was being held became a marsh. Then there was the question of attendance. For most of the day he didn’t know how many people showed up. “I should have held it in a cozy restaurant back home,” he says.

But when Sister Sledge started performing We Are Family, the crowd all started waving their I Am A Cousin signs. Jacobs found himself onstage with the band, dancing. As he looked out at the crowd, he saw a truly random group – among them a rabbi, a minister, a gay standup comedian, the actor Marilu Henner, dozens of Mormon missionaries, the film-maker Morgan Spurlock, historical re-enactors, and people. Just lots of people: “Heteros, adoptees, sperm donors, diblings. It’s a Cobb salad of humanity.”

In the end, that day, he would break a few records: most simultaneous reunions (more than 40), most people around the world celebrating family at the same time (close to 10,000). But, although they had 3,800, the main event would not take the biggest reunion prize. Like so much with family, it was… complicated. The Global Family Reunion was, he says, “horrible. And great. It was the best worst day of my life.”

• It’s All Relative, Adventures in the Ups and Downs of the World’s Family Tree by AJ Jacobs (Oneworld, £12.99), is available for £11.04 from guardianbookshop.com