system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar] what were the civilizations before the main 4

In Avatar, the last airbender,  we get to learn about the sun warriors who were one or the precursors to the fire nation. What might have been the precursors to the other nations?
gpt
Keep in mind these aren’t necessarily direct predecessors culturally, but parallel or adjacent developers of bending techniques.

The Sun Warriors for the Fire Nation. The Earth Kingdom has ruins in a cavern beneath Ba Sing Se suggesting a subterranean civilization perhaps closer to badgermoles.

I’d expect there were several more waterbending tribes similar to the Swampbenders across the Earth Kindgom. Such ad near the large inland sea where the Serpent’s Pass is, or just along the coasts.

And of course we see that all the benders once lived on lion turtles on LOK S2.