Amazingly the past of Sonic's world is possibly great and complex. I mean think about what happened back then.
3000 yrs ago: The creation of Emerl, the destruction of the Echidna civilization, Angel Island begans to form with the Chaos Emeralds gone
2000 yrs ago: Black Doom manages to somehow transport his temples, which was somehow a flying fortress, to Earth. Makes you wonder how much he had intervene in Earth's history back then
???? yrs ago: Eggman mentions that a boy recieved an angel wing as a gift from the Babylonians. And the boy was able to rule the world. Also, the Babylonians great technogoly (sp) was something.
I'm pretty sure Emerl and the Echidnas are two separated events.
If memory serves well, Emerl and the destruction of the 4th civilization was 4000 years ago, while the Echidnas and Perfect Chaos were 2000 years ago.
There are also many unknown ones:
Both Sonic 1 and Sonic 2 had ruin levels, but we don't know which civilization they belonged to.
Btw, that's one of the things that attracts me in the series. The misterious ancient ruins bits, mixed into the sci-fi element-littered current world.
It IS quite fascinating when you think about it! I wonder if all of this will be left as vague as Sonic's backstory... Probably, but one can dream.
Would be pretty cool if Sonic and Tails went on a adventure/quest around the world to discover more on this past historic events. Levels could be reminiscent to Lost World in Sonic Adventure, and the quest could be similar to the Ixis Naugus saga in the comics. Sonic the Archeologist. lol.
Never leaves his hat behind.
Actually I think it'd be cool if all these ancient civilizations' stories never got really cleared up, but were continuously referenced. Perhaps giving us a little more information here and there, but never clearing it up completely; thus leaving plenty of space for imagination. 
It would be really cool to learn more about Sonic's world. Maybe the civilizations are connected. Maybe they could tie it in with Sonic's past. It would be cool to go back to the ancient civilization plots, instead of the unoriginal sci-fi storylines they keep using.
Ever notice that in Sonic R (Saturn version) and in Sonic Adventure's Emerald Coast (Dreamcast version) there are TWO suns?
Yea probably just a mistake (to lazy to make complete 360 degree bg) but still Radical City in Sonic R only had one moon.
It isn't a mistake. Sonic's homeplanet is really Tatooine. The manuals never mention it cuz Mos Espa and all the other towns were wiped out a long, long time ago.
in a galaxy far, far away?
And a long, long time ago.
