  The origins of the ~10^9 Msun quasar supermassive black holes (BHs) at
redshifts z > 6 remain a theoretical puzzle. One possibility is that they grew
from ~10^5 Msun BHs formed in the 'direct collapse' of pristine, atomic-cooling
(temperatures T ~ 8000 K; PAC) gas that did not fragment to form ordinary stars
due to a lack of molecular hydrogen and metals. We propose that baryonic
streaming---the relic relative motion of gas with respect to dark matter from
cosmological recombination---provides a natural mechanism for establishing the
conditions necessary for direct collapse. This effect delays the formation of
the first stars by inhibiting the infall of gas into dark matter haloes;
streaming velocities more than twice the root-mean-square value could forestall
star formation until halo virial temperatures Tvir ~ 8000 K. The resulting PAC
gas can proceed to form massive BHs by any of the mechanisms proposed in the
literature to induce direct collapse in the absence of a ultraviolet
background. This scenario produces haloes containing PAC gas at a
characteristic redshift z ~ 30. It can explain the abundance of the most
luminous quasars at z = 6, regardless of whether direct collapse occurs in
nearly all or less than 1 per cent of PAC haloes.
