Rock and roll is a genre of popular music that originated and evolved in the United States during the late 1940s and early 1950s, primarily from a combination of African American blues, country, jazz, and gospel music. Though elements of rock and roll can be heard in records of the '20s and '30s, rock and roll didn't get its name until the 1950s.
It is possible, with the help of a little hindsight, to find rock roots at almost every stratum of American folk and popular music during the mid-Thirties. ... Rock & roll was an inevitable outgrowth of the social and musical interactions between blacks and whites in the South and Southwest. ... Rock might not have developed out of a self-contained African-American tradition, but it certainly would not have developed had there been no African-Americans.
Early rock & roll derived the lion's share of its energy and inspiration from black music. In the Fifties "rock & roll" was often taken to be simply a new name for "rhythm & blues," or "R&B," the music industry's generic term for any popular music primarily produced and consumed by African Americans. The R&B scene of the early Fifties was diverse enough to accommodate the suave stylings of a Charles Brown, the earthy, fiery Chicago blues of a Muddy Waters and everything in between.
It has often been said that rock & roll was the result of cross-breeding between rhythm & blues and country & western music. That's a large part of the equation, of course, but hardly the entire picture. Gospel music, swing jazz, jump blues combos, country swing bands, Tin Pan Alley publishers - they were just some of the other key building blocks of the music.
By the mid-thirties, popular music had come to an evolutionary fork in the road. One path, built on blues and jazz, would eventually lead to rock. The other, the increasingly conservative pop of the same era, would become the music that rock would react against. The music that led most directly to rock and rhythm and blues existed well outside the mainstream, far away from the Broadway stage, Hollywood studios, and -after 1950- network television.
There are a great number of opinions as to what could be called the first "rock & roll" record; indeed, an entire book (the fine 'What Was the First Rock & Roll Record?') has been written on the subject. ... Whatever it was, and whenever it became a style, by 1954 there were several record in the Top 30 that couldn't from a latter-day vantage point, be called anything but rock & roll.
Ask the proverbial person on the street to name the top acts in popular music during the fifties, and you're likely to get a list that reads like the maiden class of inductees into the Rock and Roll Hall of Fame: Chuck Berry, Ray Charles, Sam Cooke, Fats Domino, the Everly Brothers, Buddy Holly, Jerry Lee Lewise, Elvis Presley, Little Richard. ... The reality was quite different. Of these first-generation rock stars, only Elvis topped the singles charts on a regular basis.
With Elvis leading the way, rock & roll soon established itself as the younger generation's popular music. The media did much to popularize the new sound. Newspapers and magazines did features on rock & roll. Radio promoted the new sound as a way to attract new listeners. ... Television was equally important in popularizing rock & roll. ... The most influential TV show of all was Dick Clark's "American Bandstand," which premiered on ABC-TV on August 5, 1957.
Many Americans believed that rock 'n' roll was an irritant that provoked conflict between parents and teenagers and increased antisocial behavior. Acknowledging that there was no simple, causal equation between enjoying Elvis and arranging a rumble, they remained convinced that rock 'n' roll reinforced the most worrisome aspects of youth culture: antagonism to adult authority and expectations; conformity to peer-group norms; and an ephemeral, erratic emotional intensity.
Psychiatrist Francis Braceland thought [rock music] all the more dangerous because it was "a communicable disease." A few went further, declaring the music of teenagers a tool in a conspiracy to ruin the morals of a generation of Americans. In their  best-seller 'U.S.A. Confidential', journalists Jack Lait and Lee Mortimer linked juvenile delinquency "with tom-toms and hot jive and ritualistic orgies of erotic dancing, weed-smoking and mass mania, with African jungle background."
Rock 'n' roll isn't just another form of music - it's an indelible part of the human experience. It may well be the oldest form of cultural expression in human history. It didn't spring up like some Atom Age mutant in the 1950s; it simply shook off the dust of centuries of repression, took on a new incarnation, and picked up where it left off.