SGPflughaupt
Gee,Padders, you certainly brought up alot of questions there. Did it rain while on holiday and you were stuck inside with nothing else to do but think about StarGate?
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Let me put on my thinking cap while you pull on some hip-waders......
Okay,the Movie showed Ra arriving in his Galactic RV so I assume he did not arrive by gate but by travelling through space.
Humans on other planets may have been put there by other aliens (like the Asgard as you suggest).
I Can see it now.....(dream fade).
Alien 1 - "Well,here we are,a strange new planet. What do you want to do first?"
Alien 2 - "LETS GET SOUVENIRS..those funny looking creatures inventing fire would make good ones."
or...
Alien Child - "Mommy,it followed me back to the spaceship...can I keep it, huh, huh, pleeease".
Gou'ald arrival before 3000BC is one I couldnt say but the show has never indirectly suggested that IMO. Would humans had been there to greet them and say "Beam me up,Sco'tty"...well I read some where that some historians think at humans have had villages going back to over 5000BC.
At least these sound like okay replys but then I am only trying to rationalize those questions you brought up.