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?
Best Answer
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.