In the late 1950's, there were musicians who were working with rock and folk here in the States. It was a natural progression. Here's my take. Folk music became a divided camp between the academic or traditionalists and the young people coming along who were learning to play folk music. The young people preferred to be creative and not restricted by folk "authorities". It was a natural evolution here because there was interest in R and B and folk music combined by younger people. Bo Didley, B.B.King, Chuck Berry were not unknown in the folk communities and in fact Elvis was a folk fan. If you listen to the Rootop Singers doing "Walk Right In" my friend Erik Darling did a lot to build an accoustic music sound with a rockish beat. At the time it was not a popular thing to do...combine folk and rock but it was being done before it hit the charts. I remember hearing rock-styled music by black musicians playing electric guitar who had an interest in the folk music being played by basically white folks around them. Eventually, it would go across the Pond with the skiffle revival.