This question about alien species and planets brought up something I've been thinking about on and off for years.
We capitalize names of alien races like Vulcan, Timelord, Cylon (well, maybe not alien), Krell, Nox, Minbari, and so on, but we never capitalize human.
So how did we end up capitalizing names for sentient species when we don't do that for our own race?
Most of the names you give are derived from proper place names, or clan names, or such, so using "human" as a comparison is not accurate.
For instance, Vulcan and Minbari are named for their planets. We would capitalize Terran likewise (or Earthling or Martian).
Also Krell, Nox, and Timelord are groups of peoples (my apologies for not using a panxenic term, but "beings" was too confusing). We would likewise capitalize Irish, Passamaquoddy, Vandals, etc. (And naturally, Timelords are Gallifreyan, just as Mongols are Terran.) Or if you consider them more like ethnicities, you would still capitalize them, like you do with Jewish, Native American, Latina, etc.
Also, in response to Vulcans born off-world as still being Vulcans, I'd make the argument that Asians born in America (for instance) are still often called Asian, or Asian-American.
And finally, I think and I know I may well be dunned for it, that Cylon was a "brand" name for the original cybernetic organisms. And we would likewise capitalize Sunbeam, Keurig, General Electric, etc.
So then by example, human is not capitalized because it is not a proper noun, and not derived from a proper noun. Vulcans, Minbari, and Timelords are humanoid beings. There are also reptilian beings, silicon beings, and energy beings, but we don't capitalize any of those types of beings (human, humanoid, reptilian, silicon, energy, etc).
It is a good question, though. Here is a discussion that you may find interesting.
Actually, I think some writers in some contexts would capitalize "Human", especially in the context of interstellar sentient races, just as some writers capitalize Earth when referring to the planet in the context of other named planets.
I think the case for not capitalizing human is that we normally use it as a common noun, not a proper one. For example, we would write "there are people over there". We could just as well write "there are humans over there", but we wouldn't use "there are People over there".
I'm actually all for capitalizing Human and Earth in science fiction, though some writers will opt for different words instead, such as Terran (as mentioned in Kit's answer), so that the "properness" of the name is clearer.
Capitalization has nothing to do with conventions in any particular genre. In English, we capitalize proper nouns and adjectives derived from proper nouns. Vulcan, Earthling, and Venusian are all adjectives derived from the proper nouns Vulcan Earth, and Venus. Human is not a proper noun, any more the elf, dwarf, people, or purple-people-eater.
We capitalize Earth (or Venus, or Mars, or Terra, or Cygnus-XYJ) when referring to a planet, celestial body, or crazed celebrity brand pseudonym, because those would all be proper nouns. We do not capitalize earth when we mean dust or soil.
Incidentally, we capitalize Elf when we are referring to the movie, and I suppose if we were to start talking about the main character's dizzy, breathless wonder as "a thing" we might describe someone, somewhere, as being Elfian, but please don't.
I have a related post at http://wp.me/p1RPTJ-6x
I checked in in this because I am writing a scifi story and was running into the issue. I have concluded Earth is capitalized when discussing the place, but not the soil. And Human is capitalized when discussing the species as a specific people but not a classification. For example, the Human ambassador versus the ambassador's human nature.
