Fusion enjoyed peak popularity during the seventies due to the popularity of bands like Mahavishnu Orchestra, Weather Report, and Return To Forever, as well as musicians like Jeff Beck, Stanley Clarke, and Herbie Hancock. Today, it's popularity lives on primarily due to the influence it continues to wield in genres like hip-hop.
While pinpointing an origin for fusion is no easy task, many would point to the landmark Miles Davis album Bitches Brew as a primary influence. Others would probably cite influences as diverse as Carlos Santana, Jimi Hendrix, and Sly And The Family Stone. A reasonable argument could also be made that British progressive rock bands like Yes, Porcupine Tree and King Crimson owe the improvisational, genre-crossing freedom they continue to enjoy at least in part to the enduring impact of fusion.