  The fraction of distant disks and mergers is still debated, while
3D-spectroscopy is revolutionizing the field. However its limited spatial
resolution imposes a complimentary HST imagery and a robust analysis procedure.
When applied to observations of IMAGES galaxies at z=0.4-0.8, it reveals that
half of the spiral progenitors were in a merger phase, 6 billion year ago. The
excellent correspondence between methodologically-based classifications of
morphologies and kinematics definitively probes a violent origin of disk
galaxies as proposed by Hammer et al. (2005). Examination of nearby galaxy
outskirts reveals fossil imprints of such ancient merger events, under the form
of well organized stellar streams. Perhaps our neighbor, M31, is the best
illustration of an ancient merger, which modeling in 2010 leads to predict the
gigantic plane of satellites discovered by Ibata et al. (2013). There are still
a lot of discoveries to be done until the ELT era, which will open an avenue
for detailed and accurate 3D-spectroscopy of galaxies from the earliest epochs
to the present.
