  Massive present-day early-type (elliptical and lenticular) galaxies probably
gained the bulk of their stellar mass and heavy elements through intense,
dust-enshrouded starbursts - that is, increased rates of star formation - in
the most massive dark matter halos at early epochs. However, it remains unknown
how soon after the Big Bang such massive starburst progenitors exist. The
measured redshift distribution of dusty, massive starbursts has long been
suspected to be biased low in redshift owing to selection effects, as confirmed
by recent findings of systems out to redshift z~5. Here we report the
identification of a massive starburst galaxy at redshift 6.34 through a
submillimeter color-selection technique. We unambiguously determined the
redshift from a suite of molecular and atomic fine structure cooling lines.
These measurements reveal a hundred billion solar masses of highly excited,
chemically evolved interstellar medium in this galaxy, which constitutes at
least 40% of the baryonic mass. A "maximum starburst" converts the gas into
stars at a rate more than 2,000 times that of the Milky Way, a rate among the
highest observed at any epoch. Despite the overall downturn of cosmic star
formation towards the highest redshifts, it seems that environments mature
enough to form the most massive, intense starbursts existed at least as early
as 880 million years after the Big Bang.
