This should be obvious from the title card. We’re a long time ago, in a galaxy far, far away. Human beings evolved on this planet, Sol 3, over the last sixty million years or so depending on how you count. If we don’t want to go all “Chariots of the Gods?” we have to throw out the notion that the people represented by human actors in Star Wars movies are in fact human. They’re something else.
Why represent them as human? Let’s assume that the Star Wars movies are dramatizations of real history: that Luke, Leia, Han et. al. actually existed in a galaxy long, long ago (etc.), and that George Lucas accessed this history via the Force and wanted to represent it on film. Star Wars tells the story of a dominant-species empire arising from a pluralistic society, then being overthrown by courageous rebels and warrior monks. Lucas had to cast this drama with human actors, and the obvious choice was to use unmodified humans to represent the most common species.
While convenient, this approach does present one problem: watching the Original Trilogy, we assume that the ‘humans’ of the GFFA (Galaxy Far Far Away) are biologically and sociologically identical to Sol 3 humans. When obviously they’re not! In fact, I think a few important context clues present a very different picture of the dominant race of the Original Trilogy.
Read the rest of Max Gladstone’s theory for what he thinks is the most likely answer. From 2013, but I just came across this link today.