Chuck Berry.

Chuck Berry is a great answer, but Linda Joy already said that. From another perspective, Adolf Hitler (half joking here). Why would I say that, you ask? Well, in the pre-WWII music economy, everything had been building up slowly to Big Band. There was Tin Pan Alley, which grew into Vaudeville, which grew into Swing Jazz, which grew into Big Band, but then, WWII started, and musicians could no longer gather nor tour in large enough groups to constitute Big Bands, and the innovations made during the war, necessary for survival, made the in-roads after the war for electric amplification. With all of the industrialization during the war, big machines in urban areas influenced loud churning beats. This led to what people in the late 1940's called "Cat Music." It was a mixture of Jazz-Blues and Country with a little bit of Swing and a little bit of Rock and Roll attitude. One of those Cat Music guys was Chuck Berry, a blues musician who started playing in country bands and assimilated some country music into his style. Another was Elvis Presley, who was a country musician who assimilated some blues into his style. Whether it was country mizxed with blues or blues mixed with country, the combination was Rock and Roll, and it the thing that defined the 1950's.