A bit like Feynmans description of burning logs as a way to release stored sun energy.<p>Yes, the energy for splitting the molecule comes from the sun. This makes it possible to store the energy for later release when the captured hydrogen is made to combine with oxygen.<p>So energy conversion into a more useful form. And if one wants to pull this all the way, it really started with other hydrogen atoms that due to gravity was forced close enough to form helium and in the process release photons. Which happened thousands of years and eight light minutes away. So it really is gravity. ;-)