I might have missed something here in your question but: A research group at the Natural History Museum of Denmark has discovered this by measuring how hydrogen isotope ratios in the oceans have changed over time.
"The water that covered the Earth at the dawn of time contained more of the lighter hydrogen isotope than the heavier hydrogen isotope, known as deuterium, than it does today,” says Emily Pope, a post doc, who has played a central role in the study.
Why isn't speed ice-making in the Winter Olympics?