  The growth of luminous structures and the building blocks of life in the
Universe began as primordial gas was processed in stars and mixed at galactic
scales. The mechanisms responsible for this development are not well understood
and have changed over the intervening 13 billion years. To follow the evolution
of matter over cosmic time, it is necessary to study the strongest (resonance)
transitions of the most abundant species in the Universe. Most of them are in
the ultraviolet (UV; 950A-3000A) spectral range that is unobservable from the
ground. A versatile space observatory with UV sensitivity a factor of 50-100
greater than existing facilities will revolutionize our understanding of the
Universe.
  Habitable planets grow in protostellar discs under ultraviolet irradiation, a
by-product of the star-disk interaction that drives the physical and chemical
evolution of discs and young planetary systems. The electronic transitions of
the most abundant molecules are pumped by the UV field, providing unique
diagnostics of the planet-forming environment that cannot be accessed from the
ground. Earth's atmosphere is in constant interaction with the interplanetary
medium and the solar UV radiation field. A 50-100 times improvement in
sensitivity would enable the observation of the key atmospheric ingredients of
Earth-like exoplanets (carbon, oxygen, ozone), provide crucial input for models
of biologically active worlds outside the solar system, and provide the
phenomenological baseline to understand the Earth atmosphere in context.
  In this white paper, we outline the key science that such a facility would
make possible and outline the instrumentation to be implemented.
