The 5th Dimension recall the unusual way 'Aquarius / Let the Sunshine In' came together

In summer 1969, 300,000 people filled Harlem's Mount Morris Park over six weekends for the Harlem Cultural Festival, a free event showcasing Black music and culture.

Despite its all-star lineup of Nina Simone, Stevie Wonder and Mahalia Jackson, the festival was largely overshadowed by Woodstock that same summer and didn't return the next year. TV producer Hal Tulchin filmed the full concert series, but networks weren't interested in airing it,and the footage was shelved for decades.

Those who played the event never forgot about it, though.

"It crossed our minds from time to time," says Marilyn McCoo, lead vocalist of the original 5th Dimension. "We'd say, 'Do you remember when we did that festival in Harlem? Boy, I wonder whatever happened to that.'"

So imagine McCoo's surprise when musician-turned-filmmaker Questlove reached out to her and her husband, co-lead singer Billy Davis Jr., to let them know the footage had been unearthed and that he was making a documentary. The couple's moving reaction watching their performance back is featured in "Summer of Soul (...Or, When the Revolution Could Not Be Televised)," which is now in theaters and streaming on Hulu.

"We had no idea that would be part of the film," Davis says. "But seeing yourselves after 50 years was, quite naturally, very emotional."

The 5th Dimension, which formed in 1965, were riding high on the success of their first No. 1 hit, "Aquarius / Let the Sunshine In," when they were offered a spot at the Harlem Cultural Festival.

"It sounded like a great idea," McCoo says. "We thought it'd be a cool experience, and it'd be nice to go out there and sing a few songs for the audience in Harlem that might never see us live. Sometimes the tickets were too expensive and they might not be able to afford it, but this was free."

Plus, "that was a good time to show up on stage when you've got the No. 1 record," Davis says. "Everybody was jumping up and down, and that just gave us more energy, to know we were accepted there."

In the documentary, Davis, 83, and McCoo, 77, sweetly recount the unusual way they stumbled into recording "Aquarius / Let the Sunshine In," a medley of two songs from the Broadway musical "Hair." While performing in New York, Davis lost his wallet in a taxi, which just so happened to be found by a producer of "Hair." The man called Davis at his hotel and came to see The 5th Dimension play live, before inviting the group to come see the controversial rock musical, which had an anti-war message and on-stage nudity.

"Everybody was trying to get in and see 'Hair,' because it was one of the first big musical productions where everybody was getting naked," Davis says with a laugh. "We wanted to go see it, too, because nobody believed that was actually happening."

By intermission, they knew they wanted to record opening number "Aquarius." They called up their producer, Bones Howe, who told them that other artists had covered the track to little success.

"We were not discouraged at all," McCoo says. "We said, 'But the 5th Dimension didn't do it!' "

Howe came back a couple of weeks later with the idea of combining "Aquarius" with the show's finale "The Flesh Failures (Let the Sunshine In)."

"He said, 'I think if we put those two songs together, maybe we'll have something there,'" McCoo says. "And that's the way it started."

In addition to that medley, The 5th Dimension also recall playing the bluesy "Don'tcha Hear Me Callin' to Ya?" at Harlem Cultural Festival. Because they had to catch a flight to play another show, they were not able to stick around and watch any of the other artists perform that day.

But "it was something we were so proud to be a part of," Davis says. "Everybody on that lineup became very famous through the years, like B.B. King. Some of them were famous already, like Sly and the Family Stone and Gladys Knight & the Pips. It was a great lineup, and just a beautiful, beautiful time."

McCoo and Davis, who married in 1969, continue to make music as a duo, releasing their social justice-inspired album "Blackbird" earlier this year, featuring covers of Paul McCartney and John Lennon. The R&B/pop legends received a star on the Hollywood Walk of Fame in 1991 as members of The 5th Dimension, and will soon be receiving their second.

"We're going to be the first African-American couple to have a star on the Hollywood Walk of Fame," McCoo says. "We're so blessed."

This article originally appeared on USA TODAY: 'Summer of Soul': 5th Dimension got 'very emotional' making new doc