  We show that the source population of long gamma-ray bursts (GRBs) has
declined by at least a factor of 12 (at the 90% confidence level) since the
early stages of the Universe ($z \sim 2 - 3$). This result has been obtained
using the combined BATSE and \it Ulysses \rm GRB brightness distribution and
the detection of four GRBs with known redshifts brighter than 10$^{52}$ erg
s$^{-1}$ in the 50 - 300 keV range at their peak. The data indicate that the
decline of the GRB source population is as fast as, or even faster than, the
measured decline of the star formation rate. Models for the evolution of
neutron star binaries predict a significantly larger number of apparently
bright GRBs than observed. Thus our results give independent support to the
hypernova model, which naturally explains the fast decline in the progenitor
population.

Astrophysics