  ISOCAM extragalactic mid-infrared deep surveys have detected a population of
strong IR emitters ten times more numerous than expected if there were no
evolution from z<0.2 (IRAS) up to the maximum redshift of these galaxies
(z=1.5). The mid-IR cosmic background produced by these galaxies (2.35+/-0.8 nW
m^-2 sr^-1, at 15 microns above 50 microJy) is larger than 30 per cent of the
energy radiated in the I band by the optical galaxies detected in the HDF,
which are two orders of magnitude more numerous. This fraction is much higher
than in the local universe (z<0.2) as probed by IRAS, where all integrated IR
emission from 8 to 1000 microns makes 30 per cent of the optical starlight.
Even assuming a conservative SED, they produce a major contribution to the 140
mu m DIRBE background measured by Hauser et al. (1998, 25.1+/-7 nW m^-2 sr^-1)
and Lagache et al. (1999, 15.3+/-9.5 nW m^ -2 sr^-1). This is both a
confirmation of the strong IR cosmic background that was first detected by
Puget et al. (1996) and the first identification of the galaxies responsible
for a large fraction of this background at 140 microns. We find that these
bright (several 10^11 Lsol) and massive (<M>= 1.5x10^11 Msol) IR galaxies have
a typical redshift of z=0.7 and optical colors similar to field galaxies. The
fraction of galaxies with morphological signs of interactions is larger at
higher redshift. This is consistent with the nature of local luminous IR
galaxies found by IRAS, which often exhibit evidence of galaxy interaction or
merging and indicates that interactions should play a crucial role in the
evolution of galaxies as expected from bottom-up scenarios.
