  We compare the event rate density detected by LIGO to the comoving number
density of isolated stellar progenitors and find a range for their formation
redshift. Our limit depends on the threshold mass for making the black holes
(BHs) but only weakly on the metallicity of their progenitor. If $10 \%$ of all
BHs are in coalescing binaries, then enough progenitors have formed by $ 2< z_f
< 9 $.
