(Marvin Joseph/The Washington Post/Judith Thompson Hamer, brother-in-law Tim Eastman and sister Carolyn Thompson Brown find an item they gave to the museum. )

Carolyn Thompson Brown and her sister, Judith Thompson Hamer, stood in the Heritage Hall of the National Museum of African American History and Culture and listened as the museum’s founding director, Lonnie Bunch III, thanked them for what they’d done.

“You have turned this building into a museum,” he told them. “And this museum, thanks to you, can tell the unvarnished truth.”

What the Thompson sisters had done was offer up a few photos of their ancestors.

Nearby was Shelia “The Diva” Lewis, one of the original “Soul Train” dancers, who had donated several outfits she’d worn on the show. Then there was Bernice Cosey Pulley, who arrived two hours early to wait for the doors to open. She will turn 90 this week, and as the second black woman to graduate from the Yale Divinity School, she contributed an oral history. Al Ashley, 81, traveled from Little Rock to see his 1940 Booker T. Washington postage stamp on display. Danielle Spencer David, who played Dee on the 1970s sitcom “What’s Happening!!,” gave television scripts and cast photos.

There were musicians and art collectors. Broadcasters, historians and great-great-grandsons. There were women in towering heels and men hunched over in wheelchairs. And each of them — the more than 3,000 invited to Saturday night’s preview — had given something to help bring the museum into existence.

“This is the house you built,” Bunch told them. “Welcome home.”

And the first order of business for the Thompson sisters was to find their way around the house. It is a five-story, 400,000 square-foot building that displays 3,000 artifacts. Carolyn and Judith knew their photos would be in a gallery dealing with the great migration of blacks from the South to the Northeast, but that’s all they knew.

The history the Thompson sisters were concerned with was that of their great-great-grandfather, Philip Johnson. He’d been a slave in Virginia for 50 years before the Emancipation. But after the Civil War, he bought land with his wife, Maria, and they sat for a photograph wearing heavy clothes, faint smiles and the mantle of freedom.

Their daughter married a Pullman porter from North Carolina and together the couple moved to New York City. They had a house and a rental property and a son named Frank, who would one day become father to Carolyn and Judith, the two women now looking for their past, newly enshrined in the annals of American history.

“This is not an elevator,” a museum staffer said as the women prepared to head to the lower level. “It’s a time machine. We’re going back to the year 1400.”

Carolyn and Judith — and the husband and daughter they had in tow — did not have time to look at the other exhibits. The slave cabin or the shackles that their great-great-grandfather might have known too well. The receipt of sales for a teenage slave. The advertisements for human auctions.

The Thompson sisters wove their way through crowds of people who could be heard exclaiming, “There it is!” and “Found it!” of their donated artifacts. But it seemed Carolyn and Judith had gone too far. Or maybe not far enough. Or maybe their things didn’t make it on display after all?

“It should be in here,” the women said simultaneously. Nothing. Time to backtrack.

And then: “There’s the banner!” Carolyn said. It was red with gold lettering and said “Cornell.” Their father had graduated from the university in 1925. When his daughters were little girls, he brought them to the school and wrote each of their names on a chalkboard with the year he expected them to graduate. And they both did, right on time.

“How about that,” Carolyn said, staring into the illuminated glass case. “You can see it’s a little moth-eaten, but hey — that’s all right. My father would be so happy.”

“He’d be so proud of the museum,” Judith added. And of its demonstration that “We’re really here. We’re here to stay. We made contributions.”

Across the hall were framed photos of their grandparents, whose race nearly prevented them from buying property in Queens in the 1930s, just as it had for Judith in Connecticut in the 1970s.

Staring at the face of her grandmother, which looks so much like own, Carolyn reflected, “It makes you feel an obligation to the ancestors.” That’s why she wants her children and grandchildren to come here — to see the collective shoulders on which they stand. And to understand that it’s their duty, now, to thrive.

After several more minutes of searching, they found it: a ­larger-than-life photo of their great-great-grandparents, who lived in bondage but died free. It seemed as if the couple were staring directly at Carolyn and Judith, who between them have two PhDs, five children and seven grandchildren. And who, in that moment, were standing three blocks from the White House, where a black man serves as president, in a gleaming new museum devoted to the history and culture of their people.

“It’s wild,” Carolyn said, staring back. “You think, if only they could have known how the story would turn out.”