  The contribution to proton helicity from the spins of quarks and gluons at
small Bjorken $x$ can be conveniently studied through the small-$x$ helicity
evolution. Recently, it has been employed in a global analysis that includes
polarized inclusive and semi-inclusive deep-inelastic scattering measurements
at small $x$. The analysis demonstrates an excellent agreement between the
evolution and the data, but its predictions contain large uncertainties owing
to the large number of free parameters in the initial conditions. The latest
development aims to mitigate this issue by employing the valence quark model in
the polarized proton target in order to replace most of the free parameters in
the initial conditions by the physical information from the proton model.
