  Using the deepest and the most complete set of observations of distant
galaxies, we investigate how extended disks could have formed. Observations
include spatially-resolved kinematics, detailed morphologies and photometry
from UV to mid-IR. Six billion years ago, half of the present-day spiral
progenitors had anomalous kinematics and morphologies, as well as relatively
high gas fractions. We argue that gas-rich major mergers, i.e., fusions between
gas-rich disk galaxies of similar mass, can be the likeliest driver for such
strong peculiarities. This suggests a new channel of disk formation, e.g. many
disks could be reformed after gas-rich mergers. This is found to be in perfect
agreement with predictions from the state-of-the-art LCDM semi-empirical
models: due to our sensitivity in detecting mergers at all phases, from pairs
to relaxed post-mergers, we find a more accurate merger rate. The scenario can
be finally confronted to properties of nearby galaxies, including M31 and
galaxies showing ultra-faint, gigantic structures in their haloes.
