Precursors

Although strongly influenced by traditional African, American jazz and old-time rhythm and blues, reggae owes its direct origins to the progressive development of ska and rocksteady in 1960s Jamaica. An important factor in this development was the influence of Rastafari, with Rasta drummers like Count Ossie contributing to seminal recordings, bringing the influence of these rhythmic patterns into the music.
Ska arose in the studios of Jamaica in the late 1950s; it developed from the earlier mento genre. Ska is most easily characterized as a quarter note walking bass line, accentuated guitar or piano rhythms on the offbeat, and a drum pattern that places the emphasis on the 3rd beat of the bar. It is very memorable for its jazz-influenced horn riffs. Jamaica gained its independence in 1962 and ska became the music of choice for Jamaican youth seeking music that was their own. It is also worth noting that ska gained some popularity among mods in Britain.
There have been many interesting theories as to why Jamaican musicians slowed the ska sound to make rocksteady, including the singer Hopeton Lewis simply being unable to sing his hit record "Take It Easy" at a ska tempo. By 1968, many musicians had begun playing the tempo of ska slower, while utilizing more syncopated bass patterns and smaller bands. This new, slower sound was called rocksteady, a name solidified after the release of a single by Alton Ellis. The rocksteady style is most often indistinguishable from reggae, although reggae tends to focus lyrically more on lyrics based on black consciousness, Rastafari and the effects of poverty. Some reggae also introduced a much slower tempo than rocksteady. The "double skank" guitar strokes on the offbeat were also part of the new reggae style.