  We discuss the constraints set on galaxy evolution by data from deep surveys
performed in the mid-IR and far-IR with ISO and with mm telescopes at longer
wavelengths. These reveal extremely high rates of evolution for IR galaxies.
According to our model, the deep ISO surveys at 15 micron may have already
resolved more than 50% of the CIRB intensity, hence allowing to investigate the
origin of the CIRB. From our fits to the observed optical-IR SEDs, these
objects appear to involve massive galaxies hosting luminous starbursts (SFR 100
Mo/yr). Our evolutionary scheme considers a bimodal star formation (SF),
including long-lived quiescent SF, and enhanced SF taking place during
transient events recurrently triggered by interactions and merging. We
interpret the strong observed evolution as an increase with z of the rate of
interactions between galaxies (density evolution) and an increase of their IR
luminosity due to the more abundant fuel available in the past (luminosity
evolution). Very schematically, we associate the origin of the bulk of the
optical/NIR background to the quiescent evolution, while the CIRB is
interpreted as mostly due the dusty starburst phase (the latter possibly
leading to the formation of galaxy spheroids). The large energy contents in the
CIRB and optical backgrounds are not easily explained, considering the moderate
efficiency of energy generation by stars: a top-heavy stellar IMF associated
with the starburst phase (and compared with a more standard IMF during the
quiescent SF) would alleviate the problem. IR data suggest that the strong
observed evolution to z~1 should turn over at higher z: scenarios in which a
dominant fraction of stellar formation occurs at very high-z are not supported
by our analysis.
