Cosmic rays may have been important for the origin of life on Earth by driving the formation of
prebiotic molecules. We calculate the intensity of stellar and Galactic cosmic rays reaching Earth
at the time when life is thought to have begun (∼ 3.8Gyr ago), using a combined 1.5D stellar wind
model and 1D cosmic ray model. We formulate the evolution of a stellar cosmic ray spectrum
with stellar age, based on the Hillas criterion. We find that stellar cosmic ray fluxes are larger than
Galactic cosmic ray fluxes up to ∼4 GeV cosmic ray energies ∼ 3.8Gyr ago. However, the effect
of stellar cosmic rays may not be continuous. We apply our model to HR 2562b, a young warm
Jupiter-like planet orbiting at 20 au from its host star where the effect of Galactic cosmic rays may
be observable in its atmosphere. Even at 20 au, stellar cosmic rays dominate over Galactic cosmic
rays.