Michael Jackson's “This Is It” is a simple song whose history is a lot less simple. Pieced together after his death from an old demo with new instrumentation and backing vocals, it's been nominated for a Grammy for Best Male Pop Vocal Performance—an honor that would be hard to imagine if Jackson were still with us.

His recording is a decent response to the public hunger for just one more Michael Jackson record (a hunger that's sure to be met many more times over the next few decades—he apparently left a lot of rejected recordings in the vaults). It's not one of his great performances, although it's nice to hear him applying his prodigious gifts but not his obsessive perfectionism to a song. It does have a lot of his signature vocal gestures, though. (Has any other pop singer ever made so much of his unvoiced breathing as an expressive device?) And it's satisfying to hear his brothers' less showy voices harmonizing behind his, just one more time.

As it turns out, “This Is It” was co-written back in 1983 by Jackson with another singer-songwriter who found fame at an early age: Paul Anka. (Here's Anka singing the first hit he wrote, “Diana” — he was only 16 years old when it went to #1 in 1957.) Jackson's vocal on the released version of “This Is It” is apparently the one from his and Anka's early-'80s demo. How did this long-abandoned song end up providing the title for what would've been Jackson's farewell tour? That's not clear.

In fact, the release of the This Is It album wasn't the first time Anka and Jackson's composition had been heard by the public. That was back in 1991, on Sa-Fire's album I Wasn't Born Yesterday, on which it appears under the title "I Never Heard."

Sa-Fire isn't a name you hear a lot these days, but in the late '80s she was one of the marquee names of Latin freestyle— a strain of dance music that flourished for a few years, somewhere between hip-hop and house. Her big uptempo records were 1986's “Don't Break My Heart” and 1988's “Boy, I've Been Told.” To hear them is to instantly be transported to that era of New York radio, if you happen to have lived through it the first time. (The same goes for Sa-Fire's hair and outfit in the “Don't Break My Heart” video, and especially her hat in the “Boy, I've Been Told” clip.) Her biggest pop hit, though, was “Thinking of You," which probably tricked a number of listeners into thinking it was a Madonna ballad.

Sa-Fire's “I Never Heard” isn't one of her career peaks either; she sings it more as if she's trying to establish the emotionally wounded persona that was at the core of a lot of freestyle songs than as if she's getting something out of this particular song's melody or words, and the instrumentation's chintziness makes the recording wear out its attractiveness long before it's over. Still, if you're willing to indulge in a bit of imaginary history, you can pretend that the two recordings were made in the order in which they were released. Jackson, always an omnivorous listener who absorbed musical ideas just as easily as dancing technique, might have heard an obscure freestyle album track from a couple of decades earlier, and thought: there's a beautiful song in there; I bet I could interpret it as an understated jam and get something really good out of it. (That would be a better story, if only it were true.)

After Jackson's version of “I Never Heard”/“This Is It” appeared, the song came full circle: since late 2009, Paul Anka has been performing it himself at his concerts, as a downtempo ballad that eventually segues into his band accompanying Jackson's demo recording.

Anka was always more a songwriter and presence than a great singer, and time hasn't been kind to his voice—the performance doesn't really come alive until the prerecorded tape comes in. But in Anka's interpretation, you can hear another dimension to the song: a vehicle not for a voice, like Jackson's, or for an extroverted display of pained intensity like Sa-Fire's, but for an inward-looking performance—the sound of someone surprised and shaken by love.