  We study the evolution of the quark-gluon composition of the plasma created
in ultra-Relativistic Heavy-Ion Collisions (uRHIC's) employing a partonic
transport theory that includes both elastic and inelastic collisions plus a
mean fields dynamics associated to the widely used quasi-particle model. The
latter, able to describe lattice QCD thermodynamics, implies a "chemical"
equilibrium ratio between quarks and gluons strongly increasing as
$T\rightarrow T_c$, the phase transition temperature. Accordingly we see in
realistic simulations of uRHIC's a rapid evolution from a gluon dominated
initial state to a quark dominated plasma close to $T_c$. The quark to gluon
ratio can be modified by about a factor of $\sim 20$ in the bulk of the system
and appears to be large also in the high $p_T$ region.
  We discuss how this aspect, often overflown, can be important for a
quantitative study of several key issues in the QGP physics: shear viscosity,
jet quenching, quarkonia suppression. Furthermore a bulk plasma made by more
than $80\%$ of quarks plus antiquarks provides a theoretical basis for
hadronization via quark coalescence.
