How did H2O begin in the Big Bang?

Shortly after the big bang (like, milliseconds or less), protons, neutrons and electrons swarmed in 10 billion degree heat. Within minutes, hydrogen and then helium, known as the lighter elements, had taken shape from these atomic building blocks in a process called nucleosynthesis. (lithium also occurred briefly)

The heavier elements didn't appear until much later, when the lighter elements underwent fusion inside of stars and were expelled into the Universe during supernovas. Over time, stars sent wave after wave of these heavier elements, including oxygen, out into space where they mixed with the lighter elements.

Of course, the formation of hydrogen and oxygen molecules and the subsequent formation of water are two different things. That's because even when hydrogen and oxygen molecules mix, they still need a spark of energy to form water. The process is a actually a violent one, and so far nobody has found a way to safely create water on Earth.

References

• Geologist
• www.howstuffworks.com?parent=water-come-from.htm&url=map.gsfc.nasa.gov%2Funiverse%2Fbb_tests_ele.html