  We report the discovery and analysis of the most metal-poor damped
Lyman-alpha (DLA) system currently known, based on observations made with the
Keck HIRES spectrograph. The metal paucity of this system has only permitted
the determination of three element abundances: [C/H] = -3.43 +/- 0.06, [O/H] =
-3.05 +/- 0.05, and [Si/H] = -3.21 +/- 0.05, as well as an upper limit on the
abundance of iron: [Fe/H] < -2.81. This DLA is among the most carbon-poor
environment currently known with detectable metals. By comparing the abundance
pattern of this DLA to detailed models of metal-free nucleosynthesis, we find
that the chemistry of the gas is consistent with the yields of a 20.5 M_sun
metal-free star that ended its life as a core-collapse supernova; the
abundances we measure are inconsistent with the yields of pair-instability
supernovae. Such a tight constraint on the mass of the progenitor Population
III star is afforded by the well-determined C/O ratio, which we show depends
almost monotonically on the progenitor mass when the kinetic energy of the
supernova explosion is E_exp > 1.5x10^51 erg. We find that the DLA presented
here has just crossed the critical 'transition discriminant' threshold,
rendering the DLA gas now suitable for low mass star formation. We also discuss
the chemistry of this system in the context of recent models that suggest some
of the most metal-poor DLAs are the precursors of the 'first galaxies', and are
the antecedents of the ultra-faint dwarf galaxies.
