The formation and evolution of early-type galaxies constitutes a long-standing and crucial problem in cosmology. In all hierarchical clustering models within a lambda-dominated Cold Dark Matter cosmology, the massive early-type galaxies seen now are expected to have formed through the merger of smaller galaxies over time. However, most of the studies of the evolution of the scaling relationships with redshift are compatible with an epoch of formation of the stars in these galaxies at z>2 and passive evolution since then. Separating the time of the formation of their stars from the time of their assembly, a partial answer is that most of the stars in the most massive ellipticals were formed at high redshift. In this scenario the physical origin for the observed trends would lie in a systematic decrease, with the mass of the galaxy, of the relative amount of dissipation experience by the baryonic mass component during the galaxy assembly. We test this scenario by analysing the stellar population gradients in a sample of early-type galaxies observed with the Keck telescope and comparing them with numerical simulations of colliding galxaies. I will also discuss some problems on the determination of reliable stellar population parameters.