First measurements by the Rosetta mission's ROSINA instrument show that the deuterium-to-hydrogen (D/H) ratio of the comet 67P/Churyumov-Gerasimenko is highly enriched in deuterium. These results contradict the theory that the water present in the Earth's atmosphere and oceans has a cometary origin. They also indicate that 67P comets do not all come from one region, the Kuiper Belt: some may have originated in the Oort Cloud. This international study, which involved laboratories from the CNRS, Université Toulouse III - Paul Sabatier, UPMC, UVSQ, Université d'Orléans, Université de Lorraine and Université de Franche-Comté, with support from CNES, is published in Science Express on 10 December 2014.