  Neutral hydrogen (HI) is the primary component of the cool interstellar
medium (ISM) and is the reservoir of fuel for star formation. Owing to the
sensitivity of existing radio telescopes, our understanding of the evolution of
the ISM in galaxies remains limited, as it is based on only a few hundred
galaxies detected in HI beyond the local Universe. With the high sensitivity of
the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we carried
out a blind HI search, the FAST Ultra-Deep Survey (FUDS), which extends to
redshifts up to 0.42 and a sensitivity of 50 $\rm \mu Jy \cdot beam^{-1}$.
Here, we report the first discovery of six galaxies in HI at $z>0.38$. For
these galaxies, the FAST angular resolution of $\sim\,4'$ corresponds to a mean
linear size of $\sim1.3\,h_{70}^{-1}\,$Mpc. These galaxies are among the most
distant HI emission detections known, with one having the most massive HI
content ($10^{10.93 \pm 0.04}~h_{70}^{-2}\, \rm M_\odot$). Using recent data
from the DESI survey, and new observations with the Hale, BTA, and Keck
telescopes, optical counterparts are detected for all galaxies within the
3-$\sigma$ positional uncertainty ($0.5\,h_{70}^{-1}\,$Mpc) and $\rm 200\,km
\cdot s^{-1}$ in recession velocity. Assuming that the dominant source of HI is
the identified optical counterpart, we find an evidence of evolution in the HI
content of galaxies over the last 4.2 Gyr. Our new high-redshift HI galaxy
sample provides the opportunity to better investigate the evolution of cool gas
in galaxies. A larger sample size in the future will allow us to refine our
knowledge of the formation and evolution of galaxies.
