  Nucleon structure function ratios r(x) = F2A(x)/F2D(x) measured in the range
of atomic masses A larger or equal 4 are analyzed with the aim to determine the
pattern of the x and A dependence of F2(x) modifications caused by nuclear
environment. It is found that the x and A dependence of the deviations of the
r(x) from unity can be factorized in the entire range of x. The characteristic
feature of the factorization is represented with the three cross-over points
x_i, i = 1 -- 3 in which r(x) = 1 independently of A. In the range x lager than
0.7 the pattern of r(x) is fixed with x_3 = 0.84 +/- 0.01.
  The pattern of the x dependence is compared with theoretical calculations of
Burov, Molochkov and Smirnov to demonstrate that evolution of the nucleon
structure as a function of A occurs in two steps, first for A less or equal 4
and second for A larger than 4. The long-standing problem of the origin of the
EMC effect is understood as the modification of the nucleon structure in the
field responsible for the binding forces in a three-nucleon system.
