I expect that people have written proper scholarly dissertations on this topic, but "someone's done a better job" has never stopped me from shooting my virtual mouth off before, so why stop now?

    I read a lot of science fiction, particularly from Baen these days thanks to their Free Library and their enlightened ebook stance. And in stories where there's action taking place off Earth, there's a pretty broad spectrum of answers to the question of alien races, from "Humans are all that's out there" all the way to "What's a hummin?" This essay is a stab at describing that spectrum in relatively large "bins", five main categories each with a few sub-categories. Keep in mind, many settings move along the spectrum, especially if the plot of the story involves the first contact with an alien race.

    Before I start, though, I should specify what I mean by "aliens", since several of the sub-categories hinge on that definition. As far as this essay is concerned, "aliens" means intelligent life forms whose place of evolutionary origin is different from that of humanity. There might have been some tinkering along the line to adjust aliens or humans or both (for instance, the bit in Star Trek where it's revealed that the reason everyone can interbreed is that an ur-species messed with the gene pools on hundreds of worlds), but the point is that they're autochthonous to different star systems. Being able to interbreed doesn't make them not alien, even if logic says a human would have a better chance breeding with a cactus than with an alien...sometimes you just accept the handwave (alien interference, "there's only three successful biological models in the entire galaxy," gene splicing test tube babies for mixed-species couples, whatever) and move on.

1. No aliens, period. Humanity is the only intelligent life in known space. Asimov's Foundation is probably the canonical "no aliens" setting, and the Battlestar Galactica remake falls into this category too (specifically, into 1.2).
   • 1.1. Nothing but baseline humans. Pretty self-explanatory.
   • 1.2. We Made "Aliens". Humanity turned to genetic modification to colonize harsher worlds, and over the generations this has led to some pretty alien-looking races, but they're all human in origin. If a period of collapse happened during the timeline, it's possible that when recontact occurs that no one knows that these races were once human. Alternately, we created robots or uplifted animals that eventually became self-aware and split off to form their own faction. Even those could end up diverging after a collapse, seeming to be totally alien constructs upon recontact.
   • 1.3. Seedlings. Humanity as we know it is actually part of a post-collapse starfaring race already, but natural evolution has taken us and other surviving colonies in different directions. Earth may be the cradle of humanity, or it could have been colonized millions of years ago and all the evidence of the once-mighty civilization decayed. All the aliens we encounter are really our genetic relatives, but we may not know it at first. The AT-43 wargaming setting appears to be like this, mixed in with some gene-modded gorillas and robots.
2. No currently technologically advanced aliens. This is the model favored by settings that try to echo the Age of Sail era (a LOT of Baen stuff, in other words).
   • 2.1. Low-tech aliens. Every species we run into either hasn't made it to spaceflight yet, or they've collapsed and regressed to barbarity. They may initially not even seem to be sapient, like the Treecats of Weber's Honor Harrington books. The "molts" in several of David Drake's settings fall into this category. Any aliens you may see on spaceships in this sort of setting got their tech from humans.
   • 2.2. Archeological remains only. There's signs of previous intelligent civilizations, either starfaring or not, but no one alive anymore who'd qualify as intelligent. Sometimes the stardrive humans use is derived from remains found on Earth, or from remains found on nearby planets colonized using our native-built crappy stardrives. In any case, there's no living aliens anywhere.
3. Humans are part of the mix. Humanity is on more or less equal footing with one or more other starfaring races. We may be the weak newcomer, or the decadent ancient civilization about to be replaced, but we're in the game.
   • 3.1. Contact made in space. We got to the stars on our own, then met one or more other races who made it there without our help. They may have visited us before we got spaceflight, but never openly, and our stardrive is either purely native-built or based on ancient wreckage of a dead civilization that visited us in the distant past. It's possible that our native stardrive is crap compared to commercial shipping that we run into, and we quickly upgrade. For instance, in Galaxy Rangers, humanity seems to have some sort of wimpy starfaring capacity before acquiring the Andorian Hyperdrive. Star Trek was originally in this category, with humanity making it to Alpha Centauri before making contact, but it was later retconned to humanity getting the notice (and sponsorship) of Vulcan before leaving Sol.
   • 3.2. Humanity sponsored everyone else. We started in category 2, but our client races eventually absorbed enough technological savvy to break away and become powers on a par with us. Or we actively encouraged it, on altruistic grounds (more species is better) or selfish ones (getting someone to do our dirty work).
   • 3.3. Humanity graduated. We started in category 4 as a subject race or a sponsored race and WE eventually absorbed enough technological savvy to break away and become powers on a par with everyone else. This seems to be how things happened in Schlock Mercenary, for instance.
   • 3.4. No one knows. The setting is so far in the future that no one really knows how humanity got to the stars, or even necessarily what planet we're originally from. Humans are just part of the setting, no one really considers how we got there except for academics. Star Wars is like this if all you see are the movies, although ISTR the books establish the cradle of their version of humanity.
4. Humanity is a client race. This may just be a nice way of saying we got conquered, but not necessarily. Humanity made contact with other races while still stuck in the Solar System, and any interstellar capacity is with the aid of or at the sufferance of those other races.
   • 4.1. Nice option. Friendly aliens brought us to the stars (although possibly with ulterior motives, like "Save us from the Queen of the Crown!") and gave us the tech or traded with us for it.
   • 4.2. Sinister option. It starts off looking like the Nice Option, but beyond any obvious ulterior motives ("Save us from the Posleen!") there's longer term plans to enslave us once we've done the immediate task. Usually, the dramatic tension here comes from the reader finding out pretty quickly that the aliens aren't nice, but the characters mostly starting off with the wool pulled over their eyes.
   • 4.3. Nasty option. We got conquered, and are a slave race within the interstellar community. Any humans you see are chattel for the overlords, or escapees on the run.
   • 4.4. Slightly nasty option. We got conquered, but managed to mount a successful resistance due to some plot device, or the simple fact that supply lines are a pain on the interstellar scale. Scavenging captured tech will let us eventually reach space on our own terms, and the setting will move to category 3 later on. The Ekhat/Jao books from Baen show this transition happening.
   • 4.5. Neutral option. Some alien race either deliberately seeds interstellar travel tech (Ancients et al in Stargate, the gatemakers in Schlock Mercenary, etc) or they've died off and we stumble across what they'd left on Earth back when there was nothing smarter than a vole here. Either way, we get to the stars thanks to someone else's tech, and we're very much at the mercy of anyone who knows what they're doing, so we need to get under someone's protection FAST. Note, if we're scavenging tech and there's no one alive out there to pose a threat, it's actually category 2.2.
   • 4.6. Superhero option. This is a weird special case. In most superhero universes, humanity hasn't colonized other worlds yet, and any stardrives are idiosyncratic supertech stuff that is mostly in the hands of heroes and villains. Alien races sometimes try to conquer Earth, but are usually beaten back. If humans need a lot of space transport capacity on short notice, they have to ask a friendly power for help, hence this going under category 4 and not 3. But humanity may be at least considered a category 3 sort of minor player despite being confined to one world.
5. No humans. There are sapient races, but nothing that is exactly Homo sapiens. This is pretty rare, though.
   • 5.1. TECHNICALLY no humans. In truth, there's a race that's functionally human but just not from Earth. They act like humans and think like humans and maybe even look a lot like humans, but they're not Terrans.
   • 5.2. Pre-human-emergence stories. Perhaps the tale is set 4 million years ago on a strange metallic planet inhabited by living machines, so any humans there are in the galaxy are simply irrelevant. Perhaps the story starts shortly before the arrival of humanity on the scene and will shift to another category later...but for now we're seeing the galaxy from the POV of an alien.
   • 5.3. Post-human stories. Maybe humanity got conquered so hard they were wiped out. Maybe we went all Singularity and vanished. There's signs of humans here and there, but none of the characters are human. If the aliens are actually gene-modded human descendants (for instance, in Man After Man, Dougal Dixon has post-human aliens invading Earth and conquering post-human natives...nothing resembling Homo sapiens exists anymore) then it's more of a 5.1.
   • 5.4. Irrelevant humans. Humans may be out there, but it doesn't matter. The story is so far away from human-explored space that no one has heard of them. Or, at best, humans might have been heard of, they just don't impact the story in any significant way. Matt Howarth's Konny and Czu comics seem to be in this category.