  The evolution of the nucleon structure as a function of atomic mass A is
considered for the first time for the lightest nuclei, D, 3H, 3He and 4He, with
an approach based on the Bethe-Salpeter formalism. We show that the pattern of
the oscillation of the structure functions ratio r^A(x) = F_2^A/F_2^N(D) varies
with A by changing the position of the cross-over point x_3 in which r^A(x) =
1, unlike the pattern for nuclei with masses A larger than 4, where only the
amplitude of the oscillation changes. In particular we find that the pattern of
F_2(x) modifications is controlled with the values (1 - x_3) = 0.32 (D/N), 0.16
(3He/D) and 0.08 (4He/D). The obtained results follow from the relativistic
consideration of the nuclear structure and allow us to define a whole class of
modifications of the partonic distributions in the nucleon bound in a nucleus.
The EMC effect is explained as a particular case of the considered class.
