Both parties have strong trade union links - the JLP to the Industrial Trade Union, the PNP to the Trade Union Congress. And both parties and both unions date back to the series of (originally wild) strikes and race/political riots that gripped Jamaica and several other British Caribbean possessions in 1937ff.
Both were very much the creation of one man, the JLP's founding saint being Alexander Bustamante, the PNP's Norman Manley. Ironically, not only were both fairly light-skinned, middle-class mulattoes that Jamaicans would call "White" in day-to-day parlance, they were also first cousins. (official racial statistics, 2001 - 90% Black, 7% Mulatto (self-identified), 1% South Asian, 1% White, 1% East Asian. Going by blood quantum, if that could be determined, the Mulatto share would likely be about twice as high or higher. At the time of the abolition of slavery, the island was 90% slave, 5% free black or mulatto, and 5% white)
From what I can make out, Bustamante was already the treasurer of a fledgling trade union when the riots begun, which was transformed and greatly expanded during this period. Manley was a journalist at the time, and founded the first Jamaican political party, named the People's National Party (as opposed to a party for the white or moneyed elite that had held the franchise until then) in 1938. Bustamante was imprisoned between 1940 and 1942, and was released upon striking a (sensible) deal with the British authorities. He founded the Jamaica Labour Party in 1943 - Labour because it was led by union leader, Bustamante - which was basically conceived as the more moderate, but still anti-establishment, version of the radical PNP. Radicals within the trade union then walked out and set up the PNP-allied rival organization, which is the state of affairs to this day.
Anyways... in the first Universal Suffrage elections, in 1944, the JLP won 22 out of 32 fptp seats, the PNP just 4. Bustamante became Chief Minister of Jamaica (no, actually he didn't become that until 8 years later when the position was officially created. But until then, everybody knew that the Minister of Communications was the de-facto CM...) and remained so until
1955, when the PNP won elections on a program of full independence - not for Jamaica, but for a very decentralized country consisting of all the British Caribbean colonies. This state (the West Indies Federation) existed from 1958 to 1962 (no, actually, it existed until 1978. But after 1962, its dependency on the UK was much greater and it only included a number of smaller islands, most of which are still British colonies today). The JLP then switched to support of Jamaican (alone) Independence. In 1961, Manley decided to let the people decide and called a referendum on withdrawal from the WIF (the government called the referendum, then campaigned for a no vote...) where the ayes had it. Manley oversaw withdrawal from the Federation, then resigned. The JLP won the 1962 elections. By this time, Manley and Bustamante were septagenarians.
Anyways, Bustamante resigned as PM in 1967, then his chosen successor Donald Sangster (another light-skinned guy) keeled over from a heart attack two months later, and little-known (but Black) Hugh Shearer became PM. Norman Manley died in 1969, and the leadership of his party transferred to his son, Michael.
It was at this time that JLP and PNP stopped trying just being the better [insert other party's name here] and battles became fiercely ideological.
Michael Manley was very pan-third-worldish, non-aligned, in practice later pro-Cuban. And he placed a large punitive tax on foreign (read: US) operated bauxite mines. (Jamaica is, like, the world's second or third largest producer of bauxite, needed to make aluminum.) He never dared to nationalize them, though. During the runup to the 1972 election campaign, which he won, he visited Emperor Haile Selassie, the result being that Rastafarians, who until then had shared the Jehova's Witnesses approach to politics, became a PNP vote bank, and have remained so ever since.
Manley's big rival Edward Seaga, US-born of Christian Lebanese parents, who had been minister of finance under Shearer and became leader of the JLP in 1974, was very pro-American (duh, "US-born") and anti-communist. He also pulled in political donations etc from the US, of course a highly inflammatory strategy.
Both were master demagogues, and both quickly started calling on gangster support - there is little dispute that the JLP started it, though; but that's not to say the PNP was any better in following years.
Conditions in Kingston, especially, in the runup to the 76 elections, were bordering on civil war. Things calmed down a little after Manley's convincing reelection, but got worse and worse from about 1978 on. In the 1980 elections (about 900 people were killed during the, erm, campaign), the PNP was routed, winning just 9 out of 60 seats, and Seaga became PM. When Seaga called early elections in the wake of the US invasion of Grenada which he enthusiastically supported, the PNP boycotted them, and the JLP won all 60 seats - 54 were uncontested, and turnout was under 1% in most of these.
Since then, the situation has mellowed a lot. Seaga fell out of favor with his American donors - he could never deliver on his promise to eliminate the bauxite tax due to the threat of rebellion within his party - and Manley moderated his socialist rhetoric. In 1988, the government badly mishandled the catastrophic hurricane Gilbert, and in the elections in early 89 (postponed due to Gilbert, I think), the PNP walked to a convincing win. Certain downtown Kingston "garrison" constituencies were now fiefdoms of the parties (more PNP, but some -including Seaga's own seat - JLP) where opposition supporters didn't dare go vote, but the rest of the country has known free and fair elections since. Manley resigned due to cancer in 1992 (and died in 97) and his successor P.J. (for Percival James) Patterson (who is Black) has been very moderate, and has won three elections against Seaga.
The last (in 2002) was the closest of these, with 52.2% to 47.2% of the vote and 34 to 26 seats. In these elections, a lot of effort was put into ensuring fairness in the Kingston "garrisons" - Jimmy Carter involved himself immensely - with the result that winner's percentages went down from about 95-100% to about 75-85% ... and in one case to just 59%.
In 2004 Seaga finally resigned the JLP leadership, and his successor Bruce Golding has tried to overtake the PNP on the left with populist demands ... of course this is nowhere as difficult as it would have been 20 years ago.
Really, the one remaining ideological issue on the island is Cuban relations. Cooperation with Cuba in the field of medical care, begun under Manley, interrupted under Seaga, once again existing today, has largely been a success story. The JLP opposes it. Cuba is very close to Jamaica; in the old pre-Castro days, fishermen would often just hop over to the other island. If you're a tourist, touts will sell you the trip to the top of Blue Mountain Peak with the promise that, on a clear morning just after daybreak, you can see Cuba in the distance. Once you've been, old touts will admit that they've only seen Cuba once in their lifetime and have been to the peak several hundred times.
