  Recent studies suggest that the initial mass function (IMF) of the first
stars was likely to be extremely top-heavy, unlike what is observed at present.
We propose a scenario to generate fragmentation to lower masses once the first
massive stars have formed and derive constraints on the primordial IMF. We
estimate the mass fraction of pair-unstable supernovae, shown to be the
dominant sources of the first heavy elements. These metals enrich the gas up to
about $10^{-5}$ solar metallicity, when a transition to efficient
cooling-driven fragmentation occurs producing 1 solar mass clumps. We argue
that the remaining fraction of the first stars ends up in 100 solar mass VMBHs
(Very Massive Black Holes). We obtain constraints on the fraction of first
stars that contribute to the initial metal enrichment and the transition
redshift for primordial IMF away from a top-heavy one, by making various
assumptions about the fate of these VMBHs at late times. We conclude with a
discussion of several hitherto unexplored implications of a high-mass dominated
star formation mode in the early Universe.
