We study the evolution of the quark-gluon composition of the plasma created in ultra-Relativistic Heavy-Ion Collisions (uRHICʼs) 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→Tc, the phase transition temperature. Accordingly we see in realistic simulations of uRHICʼs a rapid evolution from a gluon dominated initial state to a quark dominated plasma close to Tc. The quark-to-gluon ratio can be modified by about a factor of ∼20 in the bulk of the system and appears to be large also in the high pT 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.