been substantially (by a factor of about ten) more massive at birth. This conclusion comes from the evidence that 1G stars are the minority population in most GCs and that only a fraction of 1G stars is turned into 2G stars. As a consequence, the proto GCs should have lost the majority of their 1G stars into the Galactic halo thus providing a major contribution to the assembly of the Galaxy. In this case, 1G and 2G stars could form in environments with different densities and follow a different dynamical evolution during the early phases of cluster formation.

The present-day 1G/2G ratio among stars with different masses, depends on the initial mass functions of the distinct stellar populations and on their dynamical evolution (see Vesperini et al.2018). As a consequence, the evidence that the population ratios are almost constant along the $\sim$0.15-0.80 $\mathcal{M}_{\rm \odot}$ mass interval provides strong constraints to the early phases of GC formation.

Alternative scenarios of formation of multiple populations in GCs suggest that there would have been only one episode of star formation, but a fraction of stars would have successively accreted material processed and polluted by massive stars of the same generation (e.g.Bastian et al.2013; 