  Near-infrared VLT data of the GOODS-South area were used to look for galaxies
at z=7 down to a limiting magnitude of (J+Ks)AB=25.5. No high-redshift
candidates were detected, and this provides clear evidence for a strong
evolution of the luminosity function between z=6 and z=7, i.e. over a time
interval of only 170 Myr. Our constraints provide evidence of a significant
decline in the total star formation rate at z=7, which must be less than 40% of
that at z=3 and 40-80% of that at z=6. The resulting upper limit to the
ionizing flux at z=7 is only marginally consistent with what is required to
completely ionize the Universe.
