After many and many research, I've finally discovered who is the virtuoso bassist who made the bassline for the pop 60's song Aquarius/Let The Sunshine In, sung by the 5th Dimension. Joe Osborn, born in 1937, though little known to the public, Osborn's work is widely admired by fellow musicians.
This old guy is solid as a rock. He kicks ass. Joe Osborn's instrument throughout most of his recording career was a 1960 Fender "stack-knob" Jazz Bass. His style is distinctive, with a resonant, bright tone produced, in part, by his use of a plectrum (pick).
Many producers and arrangers chose to spotlight his contributions by mixing the bass line more prominently than had been customary, and incorporating brief bass solos into their arrangements. His playing can be readily heard on records by such well-known groups as the Mamas and the Papas, the Association, and the Fifth Dimension. Osborn's contribution to Simon and Garfunkel's "Bridge Over Troubled Water" is particularly compelling, but his best-known creation is probably the funky obbligato that powers the Fifth Dimension's version of "Aquarius/Let the Sunshine In."
It is safe to say that if you were listening to the radio in the '60s and early '70s, then you were listening to Joe several times an hour, all day long. Some of Osborn's fondest memories from that era took place in a recording studio he built in the garage of his North Hollywood home. Joe worked with a variety of young, would-be stars in those days, including Jan & Dean, Ry Cooder, Taj Mahal and Kenny Rogers. It was there he recorded the young, undiscovered Karen and Richard Carpenter; he began developing his signature upper register bass lines and melodic slides while playing with them. " 'Let the Sunshine In' is probably one of the most talked about bass lines," Osborn said, "but one of my personal favorites is The Carpenters' 'For All We Know.' " Throughout this song you can hear the Osborn-original creative slide technique.