  The detection of the amino acid glycine and its amine precursor methylamine
on the comet 67P/Churyumov-Gerasimenko by the Rosetta mission provides strong
evidence for a cosmic origin of prebiotics on Earth. How and when such complex
organic molecules form along the process of star- and planet-formation remains
debated. We report the first laboratory detection of glycine formed in the
solid phase through atom and radical-radical addition surface reactions under
cold dense interstellar cloud conditions. Our experiments, supported by
astrochemical models, suggest that glycine forms without the need for energetic
irradiation, such as UV photons and cosmic rays, in interstellar water-rich
ices, where it remains preserved, in a much earlier star-formation stage than
previously assumed. We also confirm that solid methylamine is an important
side-reaction product. A prestellar formation of glycine on ice grains provides
the basis for a complex and ubiquitous prebiotic chemistry in space enriching
the chemical content of planet-forming material.
