  Perturbative NLO and NNLO QCD evolutions of parton distributions are studied,
in particular in the (very) small-$x$ region, where they are in very good
agreement with all recent precision measurements of $F_2^p(x,Q^2)$. These
predictions turn out to be also rather insensitive to the specific choice of
the factorization scheme ($\bar{\rm MS}$ or DIS). A characteristic feature of
perturbative QCD evolutions is a {\em{positive}} curvature of $F_2^p$ which
increases as $x$ decreases. This perturbatively stable prediction provides a
sensitive test of the range of validity of perturbative QCD.
