Stellar mass of zCOSMOS 20k galaxies selected for our study (secure flags, no broad-line AGN) plotted as a function of redshift. The blue dashed lines represent the mass completeness limits of the global galaxy population (solid), the population of early-type (dashed) and the late-type galaxies (dot-dashed). The black boxes mark the 61 mass-redshift bins that define the stacked galaxies we studied with the diagnostic diagrams.