  Using the deepest and most complete observations of distant galaxies, we
investigate the progenitors of present-day large spirals. Observations include
spatially-resolved kinematics, detailed morphologies and photometry from UV to
mid-IR. Six billions years ago, half of the present-day spirals were starbursts
experiencing major mergers, evidenced by their anomalous kinematics and
morphologies. They are consequently modeled using hydrodynamics models of
mergers and it perfectly matches with merger rate predictions by
state-of-the-art-{\Lambda}CDM semi-empirical models. Furthermore imprints in
the halo of local galaxies such as M31 or NGC5907 are likely caused by major
merger relics. This suggests that the hierarchical scenario has played a major
role in shaping the massive galaxies of the Hubble sequence. Linking galaxy
properties at different epochs is the best way to fully understand galaxy
formation processes and we have tested such a link through generated series of
simulations of gas-rich mergers. Mergers have expelled material in galactic
haloes and beyond, possibly explaining 60% of the missing baryons in Milky-Way
(MW) mass galaxies. A past major merger in M31 might affect drastically our
understanding of Local Group galaxies, including MW dwarves. We also propose
future directions to observationally constrain the necessary ingredients in
galaxy simulations.
