Top Song of 1974: "Rock Your Baby" by George McCrae
And disco arrives to the party. Or rather, starts the next phase of the party. The immediate appearance of the four-on-the-floor beat (here carried by the bass, not the drums) signals that rock is on its way out as a summer pop chart topping genre, and that dance music is ascendant. With the odd blip coming in the '80s, that's pretty much how things would continue until hip-hop/R&B took the dance music mantle.
As an example of the type of music that inspired the "Disco Sucks" movement, what's startling about "Rock Your Baby" is how innocuous it is. It's no more vapid than, say, "Purple People Eater." It's a solidly constructed pop song, and McCrae puts some serious falsetto on display. The way that his voice floats over the airy bed of keyboards and short, bright guitar riffs, all floating along on bouncing sea of bass, give the song a feeling of a cruise ship ride in perfect weather - all nonchalant relaxation.
What's different about it is that the grit and sloppiness of rock, evident from "Rock Around the Clock" all the way through "Bad Bad Leroy Brown", is gone. Even tight, safe-sounding pop/rock like "I Feel The Earth Move" has a visceral immediacy to it that "Rock Your Baby" lacks. The edges (the percussive piano chords of "I Feel The Earth Move", the hint of gospel desperation in the backing vocals of "Tossin' and Turnin'", the raw sexual primacy of Elvis's vocal delivery), are all sanded down.
Top Song of 1975: "Do The Hustle" by Van McCoy
And here I claimed that disco was appearing. I stand corrected. This is the first straight-up out-of-central-casting Disco with a capital D song. It's got all the signifiers; the four-on-the-floor drums, the string glissandos, the funk guitars that float in and out, the whispered over-the-top woman whispering "Do It" while the crowd shouts "Do The Hustle!" It's almost time-capsule stereotypical; the seventies answer for "Heartbreak Hotel."
All I can say is, there's a reason that the '70s got a lot of grief for the musical developments. After a frenzied pace of musical innovation that saw rock artists go from three-chord rockabilly songs about drag racing to things like Pet Sounds and The White Album, this song sounds almost painfully simplistic and retrograde. Rock managed to stand on its own, the sound of disco is the sound of music being moved back to the background - necessary for dancing, but no way would you put it on in your headphones and soak it in.
That's my rock and roll bias, though. I can hear the way that this marks a radical shift in its own right - "Do The Hustle" may not point the way forward quite as elegantly as "Rock Your Baby", but it still serves notice that what constituted pop music was taking a sharp turn. The strings point the way towards a re-incorporation of synthesized sounds, and the instantly recognizable disco rhythm announces that henceforth if you want to chart in the summertime, your chances are a lot higher if someone can dance to your music, and not just boogie a little bit to some gospel piano.