The abundance of r-process elements of stars in the Milky Way (MW) provides clues to clarify the early evolutionary history of galaxies. Astronomical high dispersion observations show that metal-poor stars in the MW halo have large star-to-star scatters in the distribution of r-process elements. Neutron star mergers (NSMs) are one of the most promising sites of r-process. Previous chemical evolution studies, however, suggested that the merger timescale of NSMs is too long to reproduce the observed scatters. In this study, we performed a series of N-body/hydrodynamic simulations of the MW progenitor galaxies. We show that the scatters can be explained by NSMs due to the slow chemical enrichment of such galaxies. This result suggests that stars in the MW halo formed with a star formation rate of less than 10^-3 Msun/yr. We also find that the dynamical time of halos affects the early evolutionary history of galaxies. Our results demonstrate that the future observations of r-process elements in metal-poor stars will be able to constrain the early chemo-dynamical evolution of the Local Group galaxies. (Special SFB guest seminar. Host: D. Kruijssen)