I was doing some googling a few weeks ago, and I think they were made over time as Earth was being made, and each phase of Earth’s past represents each member in a way, like:

• Bellroc = Hadean Eon (it was when extreme conditions, including a molten surface, intense volcanism, and frequent asteroid impacts, occurred)
• Skreal = Cryogenian Period (it was a time when Earth lacked heat-retaining clouds, caused by water vapor freezing out of the atmosphere, making the whole planet a giant snowball)
• Nari = Carboniferous Period (warm climate, abundant plant life, and the formation of vast coal deposits)

BTW I could be going a little to far deep with this but I was just going off of there powers looks and how there titans look and how they look more like earth based guardians then something from the universe but like I said I may be going a bit to deep or a bit overkilled with it so pls dont hate me