  The James Webb Space Telescope (JWST) has discovered a surprising population
of bright galaxies in the very early universe (< 500 Myrs after the Big Bang)
that is hard to explain with conventional galaxy formation models and whose
physical properties remain to be fully understood. Insight into the internal
physics of galaxies is captured best via observations of excited-state atomic
transitions of ionized gas, but beyond z~7-9, the brightest spectral signatures
are redshifted into the mid-infrared regime, where observations are
increasingly more difficult. Here, we present the first detection of a hydrogen
recombination line (H{\alpha}) and doubly-ionized oxygen ([OIII]4959,5007{\AA})
at z>10 using the JWST Mid-Infrared Instrument, MIRI. These detections place
the bright galaxy GHZ2/GLASS-z12 at z=12.33+/-0.02, making it the most distant
astronomical object with direct spectroscopic detection of these lines and the
brightest confirmed object at this epoch. These observations provide key
insights into the conditions of this primeval galaxy, which shows hard ionizing
conditions rarely seen in the local Universe and likely driven by compact,
young (<30 Myr) star formation. Its oxygen-to-hydrogen abundance is close to a
tenth of the solar value, indicating a rapid metal enrichment during the
earliest phases of galaxy formation. This study confirms the unique conditions
of the brightest and most distant galaxies recently discovered by JWST and the
huge potential of mid-IR observations to characterize these systems, opening a
range of new possibilities in the study of the very early Universe.
