NEW YORK, NY.- After more than four decades, Steve Porcaro is still amazed that his song ended up on the biggest-selling album of all time.

In 1982, when he was a keyboardist in Toto — the band of studio insiders that dominated rock radio with sleekly crafted hits including “Africa” and “Hold the Line” — Porcaro was tinkering with a new tune, a mid-tempo ballad inspired by his attempt to comfort his young daughter after a playground quarrel. The rest of the group wasn’t into it.

But Porcaro kept working on the song at the studio of his Toto bandmate David Paich, the group’s primary songwriter, who was pitching Quincy Jones some rock-oriented material for Michael Jackson’s next album. One day, they put two of Paich’s songs on a cassette for Jones; on the flip side was a rough demo of Porcaro’s ballad.

When Jones heard that tape, it was Porcaro’s tune that entranced him, with its mellow mood and searching chorus: “Why, why?/Tell her that it’s human nature.” With lyrics added by John Bettis, “Human Nature” became a key cut on “Thriller,” which sold 34 million copies in the United States alone and transformed pop music in the 1980s.

“It was a total, absolute fluke,” Porcaro recalled in a recent video interview from his home studio in Los Angeles, which is lined with gold and platinum albums by Toto and Jackson.

“Human Nature” is now part of the latest in the music industry’s big catalog transactions. This week, Porcaro signed a deal, estimated in the low eight figures, to sell the rights to his music to the Jackson estate and the independent music company Primary Wave, they confirmed.

At 66, with a head of thick, snow-white hair, Porcaro said the deal “enables me to do whatever the hell I want to do now for the rest of my life,” including devoting himself to songwriting after he spent much of the 2010s on the road with Toto and scoring the neo-western FX show “Justified.”

“I’m at this point now,” Porcaro added, “where all I want to do is be that guy that just writes songs for myself.”

The deal may also have a payoff for Jackson fans. It includes two long-dormant vault tracks, “Chicago 1945” and “Dream Away,” that Porcaro wrote with the star after “Thriller.” Although versions of the songs have leaked online, he has rarely told the full stories behind them.

Their genesis goes back to the “Thriller” sessions, when Porcaro and other members of Toto — including his brother Jeff, the group’s super-virtuoso drummer — were part of the album’s small army of studio players. As work on the album neared its end, Porcaro said, Jackson asked him to collaborate on songs for his next project with his brothers, which became the Jacksons’ “Victory.”

“Of course I said yes,” Porcaro recalled. He handed Jackson a tape with an upbeat track he had been working on.

A couple of weeks later, Porcaro said, Jackson showed up unannounced at Paich’s home studio, where Porcaro was living. “I got a vocal idea I want to throw on our tune,” Jackson told him. He got on the mic and recorded nine vocal lines — lead parts, harmonies, backups — with lyrics referencing Chicago in the year 1945, including the Cubs losing the World Series. Jackson finished in about 40 minutes and abruptly left.

“Nine vocal passes, start to finish, and he splits,” Porcaro said, still in awe of Jackson at his peak. “It was sung perfectly. I’ve never seen anything like that in my life.”

Days later, about 9 a.m., came another unexpected knock at the door. “Let’s do something,” Jackson said. This time they created a song from scratch: “Dream Away,” a soft ballad with “Human Nature” vibes. After sketching lyrics in a notebook and quickly recording his lead vocal, Jackson once again took off.

Porcaro went back to playing in Toto, where he was the synthesizer whiz at a time when that often meant laborious knob-twiddling to get just the right sounds. His 20-second spotlight on “Rosanna” is “the ultimate synth solo ever,” he said. “That took me over a week to do.”

Despite the group’s blockbuster sales, Toto was derided by rock critics as uncool, overpaid craftsmen who made anything but rock ’n’ roll. (“All chops and no brains,” Rolling Stone once sneered.) Porcaro admits the band spent more time finessing drum sounds than working on lyrics. When bad reviews came, he said, “we’d react to it the only way we knew how. The guys would go, ‘Damn it, we’re going to spend twice as long on the bass drum sound now — we’ll show ’em!’”

But the prolific studio work by the three Porcaro brothers — Steve, Jeff and Mike, a bassist — is a road map through a huge swath of mainstream rock and pop in the 1970s and ’80s, including Steely Dan, Chicago, Boz Scaggs, the Bee Gees, Don Henley and Earth, Wind & Fire. Of the brothers, Steve is the last survivor; Jeff died in 1992, and Mike in 2015. Their father, jazz drummer Joe Porcaro, died in 2020.

Steve left Toto after the group’s 1986 album “Fahrenheit,” and he continued to play on Jackson’s solo albums into the ’90s while developing his career as a film and television composer. He rejoined Toto in 2010 for what was supposed to be a single tour but that turned into a decade of shows before he left again. All the while, he remembered those two unfinished Jackson tracks.

“I was always thinking, ‘Someday I’ll reach out to Michael and finish this stuff,’” Porcaro said. “You always think there’s going to be time for that.” Jackson died in 2009, at age 50.

Porcaro digitized the original tapes, and a few years ago he began working on the tracks, calling in new collaborators — including some who had played on “Thriller,” such as keyboardist Greg Phillinganes and horn arranger Jerry Hey. “Dream Away,” recorded a hair too fast, required a computerized tempo adjustment.

The fine print of Porcaro’s catalog deal is complex. But in essence, Primary Wave and the Jackson estate will split majority ownership of a handful of his songs, including “Human Nature” and the two unreleased Jackson songs; many more pieces, including Porcaro’s film and TV compositions, will go to Primary Wave.

Porcaro will retain a 15% stake in the catalog aside from the two unreleased Jackson recordings, which are being fully acquired by the Jackson estate. Bettis, the lyricist on “Human Nature,” was not involved in the deal.

For the Jackson estate, the transaction is a way to bring in more of Jackson’s work. “We want to control everything Michael’s done,” John Branca, Jackson’s longtime lawyer and co-executor of his estate, said in an interview. Larry Mestel, founder of Primary Wave, noted that the deal increases his company’s stake in Toto, since its portfolio also includes the catalog of Jeff Porcaro. (Primary Wave also owns a minority share of Jackson’s music publishing rights.)

What happens to the two unreleased songs by Porcaro and Jackson is unclear. Branca said the estate is focused on its upcoming Jackson biopic and has no immediate plans to release them.

Jed Weitzman, Porcaro’s manager, said the deal demonstrates how musicians like Porcaro can benefit from the flood of money that in recent years has gone into music catalogs of all kinds, though the public often hears only about the ones involving superstars.

“There are a lot of amazing songwriters that have been left behind in the catalog gold rush,” Weitzman said, “and there is a tremendous opportunity for them right now.”

Porcaro’s deal is also an example of how musicians can benefit from a provision in U.S. law that gives creators the power to eventually recover copyrights they once sold. In 1988, Porcaro sold his catalog to Jackson, in a deal he said he regrets. “I was in a personal financial situation, and I needed the money,” Porcaro said. But last year, Porcaro got those rights back, and began discussions with Primary Wave and the Jackson estate.

The new deal, Porcaro said, gives him the freedom to keep creating his own music.

“The story for me is that I am finally at this age where a lot of people might be thinking about retiring,” Porcaro said. “I’m just getting going.”

This article originally appeared in The New York Times.