Run-DMC and Aerosmith’s ‘Walk This Way’ turns 33 this July 4. The changes it made are still reverberating through the music industry.

Right now, Lil Nas X rules the top of the Billboard charts with his trap/country hit “Old Town Road.” There was some initial disagreement over what genre the song belonged to, and then he released a remix stamped with the country imprimatur of a Billy Ray Cyrus feature. The remix has paid off for both artists, with Cyrus enjoying a warm reception at the recent BET Awards, where he performed live with Lil Nas X. Their unexpected collaboration, along with the hit that resulted, is reminiscent of an earlier pairing that disrupted the music industry and American culture: Aerosmith and Run-DMC’s “Walk This Way,” which debuted on July 4, 1986.

I spoke to Geoff Edgers about his book Walk This Way: Run-DMC, Aerosmith, and the Song That Changed American Music Forever. Just like everything else in America, music is infused with racial politics. It shows up in who gets credit and compensation for their art, how the work is considered and awarded by professional organizations, whose music gets played on which radio station, even how individual songs are categorized by genre. The entire notion of “crossing over” describes music that breaks down the boundaries of our still-segregated ears. Edgers’ book examines how one of the most famous rock/hip-hop mashups got made, the repercussions of its commercial success, and what it told us about race and music in America.

The interview has been edited and condensed for length and clarity.

You have this mashup of two groups that are opposite in a lot of ways. One is older white rockers, and the other black kids who are cultural upstarts. I started thinking of Lil Nas X and Billy Ray Cyrus, where again, there’s an aging white artist who’s lending some of his own juice to legitimize the younger one.

It’s a good comparison in some ways. It’s complicated because of how screwy the Billboard charts are and the fact that that song was huge without the Billboard charts putting it on country.

There is something to be said for the fact that Aerosmith for Run-DMC, they were a tool to get on the radio. That was about it. I mean, the song itself was not something they [Run-DMC] loved then or love now, particularly. But it did do exactly that. They simply wouldn’t have gotten on the radio or on MTV without those two scraggly white guys.

You could write a whole book just on artists who’ve had songs blow up that they actually didn’t care for very much.

So much of what does well isn’t our best. And so much of what is our best doesn’t necessarily do well. The reality is ‘Walk This Way’ is a good song, the Run-DMC version is good, but it’s not the best Run-DMC song even on Raising Hell. But it is the most important song on Raising Hell. And it’s got to be the most important song in their catalog.

But that creates a problem for you as a group because you want to be known by your best, and you also don’t want to share the spotlight. So both Aerosmith and Run-DMC, I don’t think, have ever been totally at peace with that version of that song.

Given that we live in the age of the evaporating attention span, were you worried about writing a book about one song?

I’ve had people criticize the book for the long title that they feel is hyperbole. And people will tweet like, ‘This whole book about one song?’ But it’s not really a book about one song; it’s about a lot of different things. Part of it is about rewriting history the way it should be, and not the way the winners wrote it. I don’t have anything against Rick Rubin, but I do think Larry Smith has been forgotten when the guy was basically the Phil Spector of hip-hop. I also think that getting Sha-Rock and Grandmaster Caz and Run-DMC the proper credit they deserve, it’s not out there. The false story that’s been out there is that this famous rock band, Aerosmith, helped a bunch of fledgling rappers build a career. And that just couldn’t be further from the truth.

In fact, if I did that book over, it would be longer. It would be more about that song. I feel like maybe I’ve gotten to think that MTV and radio was more racist than I did when I was putting the book together. And I might have explored that more. But I just felt like I wanted to tell the facts — like, all the reporting — and get that out.

Your book provides a different understanding of ‘Walk This Way.’ You say that the stakes were superlow for Run-DMC. They weren’t walking into a recording session thinking, ‘This is going to put us on some sort of rocket ship.’ They’re annoyed.

It’s a weird split between, on one hand, these guys don’t even know if this song’s going to come out. The producers don’t even know. But then, on the other side, Spin magazine is in the studio that one day, and TV news is there, so somebody knew something was going down that could be important. You know, Run and Darryl rapped terribly that day. I watched that footage that I was able to get from Viacom that hadn’t been released. They weren’t taking it seriously. They weren’t doing a good job. And they had to come later and lay down their vocals again because they didn’t do it right.

You mentioned thinking that your reporting led you to conclude that MTV and radio were a lot more racist than you had originally thought. What the folks from MTV are saying or not saying feels very common. ‘Well, you know, it’s about format.’ No one is being overtly racist, but no one’s thinking about who is excluded by decisions to focus on pop and ignore rap.

I got a little tangled up in the Michael Jackson myth, the story that MTV wouldn’t play Michael Jackson, which I think my reporting shows is not true. But what I didn’t think about enough, or what I’ve come to think about as I put the book together, I decided that it’s true that MTV played African American artists. And it’s true that they would define what African American artists they could play based on the ‘rock’ format or ‘pop’ format. They’d play Lionel Richie. Or Tina Turner. Or Michael Jackson. And their defense was, ‘We’re not racist, we’re not breaking the format.’

Well, the fact is, breaking the format, playing hip-hop, would have been the idea of playing, essentially, an art form built out of African American communities. So if you say you’re going to cut that off completely, that, to me, is getting you into that racist territory.

I guess they weren’t playing, like, Barbra Streisand or Anne Murray because they didn’t fit the format. I’m not sure there were oppressed 55-year-old white singers in Canada who felt like they hadn’t been given a chance, you know? Gordon Lightfoot wasn’t like, ‘God, they’re persecuting me for being Canadian.’ But, I mean, you could make an argument, seriously, that what The Fat Boys and Kurtis Blow and Run-DMC, what they were doing was really important, and the idea that it was cut off from a huge segment of popular culture was criminal.

Why did it take so long to get Run to talk to you?

I could get Darryl [McDaniels] on the phone right now. I assume that [Run] didn’t see the benefit. I assume he also finds it tiring to talk about what he thinks is going to be the same thing over and over again. And it just took forever. I’d talked to all the famous people in the book multiple times by the time I got to him.

What I will say is that he was extremely generous, and I think he was surprised when I brought him this footage he hadn’t seen of the session and let him narrate it. Once he saw that, he was like, ‘Wow, this is amazing stuff,’ and he wanted a copy of it.

What’s your favorite song on Raising Hell?

Probably ‘It’s Tricky.’ Maybe ‘My Adidas,’ you know, maybe. I was 15 when that album came out — that’s when you make your connection to real music. As much as I like something that will come out now and I’ll go, ‘Oh, that’s good,’ I mean I’m an old man. I can’t feel the visceral connection to anything the way I did from 1982 to 1989, I just can’t.