Ska is one of the longest-running styles of popular music, and it has a rich history, one that spans over half a century, six continents, and countless examples of cultural exchange. It was born in Jamaica in the 1950s as a mix of American rock and roll, R&B, and jazz with a style of Jamaican folk music known as mento, and it started to gain popularity in other countries in the 1960s, with the success of Millie Small's ska cover of "My Boy Lollipop" in 1964, the ska showcase at New York City's World's Fair that same year, and the influence on international superstars like The Beatles, who pulled influence from ska on the middle 8 of 1964's "I Call Your Name" and had a ska-infused hit with "Ob-La-Di, Ob-La-Da" in 1968. (If you don't believe me, there's an interview with John Lennon where he talks about the influence of ska.)
Ska was characterized by its emphasis on the upbeat (if you're counting "one and two and three and four" along to a ska song, the emphasis is on the "and"), and this became a constant in popular Jamaican music, as ska slowed down by the late 1960s and morphed into rocksteady, before slowing down even further and morphing into reggae. Most of the big-name reggae pioneers -- like Toots & the Maytals, Jimmy Cliff, and Bob Marley & The Wailers -- started out playing ska.