  We present measurements of carbon, oxygen, silicon, and iron in quasar
absorption systems existing when the universe was roughly one billion years
old. We measure column densities in nine low-ionization systems at 4.7 < z <
6.3 using Keck, Magellan, and VLT optical and near-infrared spectra with
moderate to high resolution. The column density ratios among C II, O I, Si II,
and Fe II are nearly identical to sub-DLAs and metal-poor ([M/H] < -1) DLAs at
lower redshifts, with no significant evolution over 2 < z < 6. The estimated
intrinsic scatter in the ratio of any two elements is also small, with a
typical r.m.s. deviation of <0.1 dex. These facts suggest that dust depletion
and ionization effects are minimal in our z > 4.7 systems, as in the
lower-redshift DLAs, and that the column density ratios are close to the
intrinsic relative element abundances. The abundances in our z > 4.7 systems
are therefore likely to represent the typical integrated yields from stellar
populations within the first gigayear of cosmic history. Due to the time limit
imposed by the age of the universe at these redshifts, our measurements thus
place direct constraints on the metal production of massive stars, including
iron yields of prompt supernovae. The lack of redshift evolution further
suggests that the metal inventories of most metal-poor absorption systems at z
> 2 are also dominated by massive stars, with minimal contributions from
delayed Type Ia supernovae or AGB winds. The relative abundances in our systems
broadly agree with those in very metal-poor, non-carbon-enhanced Galactic halo
stars. This is consistent with the picture in which present-day metal-poor
stars were potentially formed as early as one billion years after the Big Bang.
