Spoiler alert: This story contains details from Thursday's series finale of CBS' "The Big Bang Theory."

It all started – and ended – with "The Big Bang Theory" theme.

The closing shot of Thursday's series finale offered a comfy coda, with the gang eating Chinese takeout in the apartment living room, as they've done hundreds of times, as the show's Barenaked Ladies theme song plays. ("Our whole universe was in a hot, dense state ....").

All familiar elements – except the song didn't quite remain the same. At the request of series co-creator Chuck Lorre, the band's lead singer Ed Robertson composed an acoustic, more melancholy take on a song that documents the start of the universe by what's known scientifically as the Big Bang theory.

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"It's a fantastic little bookend to my entire involvement with this show," says Robertson, whose band joins the cast of the 12-season CBS comedy on Stephen Colbert's "Late Show" Thursday.

The partnership began more than a decade ago after Lorre and co-creator Bill Prady attended a concert by the Canadian band that featured Robertson improvising a song based on Simon Singh's book, "Big Bang: The Origin of the Universe." They asked him to compose the theme of their new CBS comedy, which focused on two Caltech physicists.

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"When I initially sent the song to Chuck and Bill, it was just me on an acoustic guitar singing the song I had written five minutes previous to recording it. It was a quick, scratch demo that I sang directly into my laptop," Robertson says. "I used the built-in microphone in my laptop and sent them this live take of me singing. And they loved it. They wanted to use that exact version and I had to plead to let me get the band into the studio to record."

Although the tone and pace of the new version, composed last month, mark a contrast with the band's "frantic, frenetic theme," Robertson says it strongly connects to the song's origin.

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"I sent (the completed revision) to Chuck and he said something like, 'Tears in my eyes. Perfect. Thank you,'" Robertson says. "Maybe an hour or two later, he e-mailed me back and said, 'I just wrote the last scene and we're going to end with your acoustic version.' I was overwhelmed. I thought, 'Wow! What a beautiful thing this has been in my life.'"

Barenaked Ladies, started by Robertson and Steven Page in their native Ontario in 1988, sold millions of records before the "Big Bang" theme, but the band is now indelibly connected to the series.

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"It has given us access to a whole other audience. It's a major signpost in the band's career," he says. "When we play live, if we were only doing four songs, it's going to be "One Week," "Pinch Me," "If I Had $1,000,000" and "The Big Bang Theory" theme.

Robertson, who will join his bandmates on a 48-stop summer tour with Hootie & The Blowfish, says the song's inclusion in the finale illustrates the respect Lorre, Prady and the "Big Bang" team have for the show's history.

"As much as they're into this story arc of the show and its characters, it's like the story arc of the song is important to them, too," he says. "It's amazing."

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