When Gell-Mannlater introduced quarks as constituents of all observablehadrons, p; he remarked in jest that the hadronic bootstrap principle still holds: No observable  hadron is more elementary than any other (vacuously so,  since none of them are elementary). Gell-Mann liked the bootstrap principle but remarked that it need notbe confined to a purported opposition betweenQFTand dispersion methods  (S-matrix theory).

The Gell-Mann matrices  are to SU(3) what Pauli matrices  are to SU(2).

Gell-Mann had tried to brute-force his way into a suitable group with up to 7 generatorsduring his 1959-60 stay at the Collège de France, especially in the fall of 1959. 

In the Fall of 1960, Dick Block (Richard Earl Block, b. 1931,  Ph.D.1956) was a young assistant professor of mathematics at Caltech,  who had recentlycompleted his Ph.D. dissertation on Lie Algebras at Chicago, underA. Adrian Albert (1905-1972). Block pointed out to Gell-Mann that he was re-inventing the wheel: The problem was about simple Lie groups in canonical form and the solution was well-known...

Therefrom came Gell-Mann's interest in SU(3) which worked beautifully [sic].  Gell-Mann was the first to call gluons the  8  vector bosons associated to the above generators (the word itself had been introduced by Edward Teller (1908-2003)  in an unrelated context). 

At the time, George Zweig (1937-)  was working at CERN, just after getting his doctorate,  on a similarscheme based on four aces  rather than three quarks.

Yuval Ne'eman (1925-2006) also came to the same conclusion around the same time,  as he obtained hisPh.D. underAbdus Salam (1926-1996)at Imperial College London (Ne'eman was military attaché  at the nearby Israeli embassy).

The discovery of the  ,  in February 1964, can be construed as the experimental confirmation of  SU(3)  symmetry in hadrons. Both Gell-Mann and Ne'eman were involved. This paved the way for the quark  proposal shortly thereafter.

This was the first direct experimental evidence for quarks.

The principle is also valid for weak interactions but it's far more noticeable withhigh-energy strong interactions. (A quark of one flavor can transform into a quark of another flavor only through a weak interaction.)