Thick disks have been observed in many disk galaxies and our Galaxy, the Milky Way, also presents a thick disk whose main spatial, kinematic, and chemical features of this population are well established. However, the origin of this ancient component is still unclear in spite the many studies carried out and several formation scenarios proposed until now. For the first time to our knowledge, we found evidence of a kinematics-metallicity correlation, of about 40–50 km s-1 per dex, amongst thick disk stars at 1 kpc  < |z| < 3 kpc and with abundance −1 <[Fe/H]< −0.5. This finding sets important constraints on the origin of the thick disk in the context of CDM hierarchical galaxy formation mechanisms and of secular evolutionary processes in galactic disks. This result is reported and, preliminary results, based on new N-body high numerical resolution simulations of stellar disks endowed with a bulge inside a dark matter NFW halo, are presented.