Motown. Solo Beatles. Social Protest. 'Saturday Night Fever.'

What did the 1970s look like? Bell-bottoms and leisure suits; Afros and feathered perms; earth tones and platforms. All bets were off. Do your own thing.

So what did the '70s sound like? A look at the 253 songs that reached No. 1 on the Billboard Hot 100 reveals a similar patchwork of styles. There were story songs ("The Night Chicago Died," "Billy Don't Be a Hero") and instrumentals ("A Fifth of Beethoven," "Theme From 'S.W.A.T.' "). There were classics by such giants as the Rolling Stones, Smokey Robinson and David Bowie, and there were novelty songs like "The Streak" and "Kung Fu Fighting"-the Pet Rocks of the pop charts.

It was a range that mirrored the variety shows on the three-and only three-networks at the time. However, with closer examination, some crucial patterns in a changing musical universe can be detected. Since so many of the rock icons of the '60s aspired to something beyond hit singles, a feeling compounded by the deaths of Jimi Hendrix, Janis Joplin and Jim Morrison as the decade began, the shadow of the '60s is actually much smaller than might be expected. Each of the former Beatles did hit No. 1 in the '70s (Paul McCartney six times, John Lennon once), but this list feels overwhelmingly like a fresh start.

In fact, if there was one juggernaut from the '60s that loomed the largest, it was the superstar stable at Motown Records. Though history generally indicates that Motown's early-'60s first wave was its incomparable pinnacle, only to be pushed aside by the heavier funk of Stax, Sly Stone and James Brown, the company produced no less than 25 No. 1 singles in the '70s, including a remarkable 15 between 1970 and 1973.

Motown's renaissance is indicative of perhaps the most notable development on this list, which is the unprecedented integration that was happening atop the charts. During the first half of the decade, black and white artists easily and consistently passed the No. 1 slot back and forth. Looking at this list and seeing Elton John grab the ring from Stevie Wonder, or Fleetwood Mac hand off to Marvin Gaye, it's a reminder of a time that felt as close to one pop nation, indivisible, as it ever would.

But where white artists were generally retreating from the social protest of the '60s, opting instead for the more introspective work of Carole King and James Taylor, R&B of the early '70s unflinchingly reflected the turmoil of Watergate, Vietnam and urban blight. Even in hindsight, the fact that such songs as "War," "You Haven't Done Nothing" and "Papa Was a Rollin' Stone" were No. 1 singles is simply astonishing (though the triumph of Helen Reddy's "I Am Woman" deserves some recognition as well).

This historic display of unity, though, would be ruptured by the single event that rumbled through the '70s and changed everything, the Big Bang, the tectonic shift from which pop music still reels: the birth of disco. The impression today is that the 1977 release of "Saturday Night Fever" was the line in the sand, but looking at this list, it's clear that it was just the tipping point. Such proto-disco singles as "Love's Theme" and "TSOP" were already at the top of the chart in 1974. "Disco Lady" and even "Disco Duck" were No. 1s before John Travolta donned that white suit.

Still, as rock critics championed punk and new wave (which had zero impact at this stratosphere beyond Blondie and, perhaps, the Knack), "Fever" was the story of the decade. With four No. 1s-plus a stack of additional hits and radio staples, on its way to sales topping 15 million-it would define a pop moment forever. And as disco swallowed more and more of the top 40, music's audience began to splinter in ways that would become permanent.

For a few years, black and white artists, rock and dance music, were able to happily co-exist, but the roads were diverging. No black performers reached No. 1 between August 1976 and January 1977, or between October 1977 and May 1978. By October 1982, there were three weeks when not one record by an African-American artist was in the top 20 of the singles or albums chart-the first time there was such a sharp division since the '40s.

"Saturday Night Fever" also indicated something else to come: It marked the emergence of the "event" record-not just an album that got huge on its own steam, like "Rumours" or "Hotel California," but an all-out media blitz designed for mega-platinum scale. The 1981 launch of MTV was coming up quick. And in the final months of 1979, "Don't Stop 'Til You Get Enough" returned Michael Jackson (who had, of course, kicked off 1970 with four straight chart-toppers with the Jackson 5) to No. 1. As he was reaching the pinnacle in October, a curious song called "Rapper's Delight" had crept into the lower reaches of the top 40.

The divisions between decades often seem arbitrary. But as the '70s ended, and Jimmy Carter handed over the White House to Ronald Reagan, it was clear that the world was ready for a brand-new beat.