  The fact that the spins of the quarks in the proton, as measured in deep
inelastic lepton scattering, only add up to about 30$\%$ of the spin of the
proton is still not understood after 30 years. We show that our newly developed
model for the quark and gluon momentum distributions in the proton, based on
quantum fluctuations of the proton into baryon-meson pairs convoluted with
Gaussian momentum distributions of partons in hadrons, can essentially
reproduce the data on the proton spin structure function $g_1^P(x)$ and the
associated spin asymmetry. A further improved description of the data is
achieved by also including the relativistic correction of the Melosh
transformation to the light-front formalism used in deep inelastic scattering.
However, this does not fully resolve the spin puzzle, including also the
neutron spin structure and the spin sum rules. These aspects can also be
accounted for by our few-parameter model if the conventional SU(6) flavor-spin
symmetry is broken, giving new information on the non-perturbative bound-state
nucleon.
