Were the 13th tribe human or cylon on Kobol?

I've seen some folk say they were always cylon, created by the 12 tribes on Kobol, later rebelling and fleeing to Earth 1.

Others say they were originally a human tribe, like the others on Kobol, who discovered resurrection and lost their ability to breed, basically becoming humans who are immortal.

Is the difference in opinion because some people have only watched the show, whilst others are taking into account spin offs, movies, comics, games, etc?

I like the idea that the 13th tribe were originally human because it had a nice symmetry to it and further blurs the line between humans and cylons. The cylons created by the 12 tribes/colonies eventually began to create humanoid models, so it's fitting that the 13th tribe were humans who "became cylons", with the two groups eventually forming a union when the final five of the 13th tribe join with the centurions.

Bit of a side track ramble but I love how the show poses questions about what it is to be human. Rewatching it now, just started S2, and noticing things I didn't the first time, when I was still quite young. Like Helo's love for Sharon on Caprica - when he finds out she's a cylon he denies his love for her but quickly realises it's not something he can just push away. Because what's the difference between her and a human born to other humans anyway? He experiences her as a human and she has the memories and emotions of living a human life. She is akin to a human who resurrects on death (or she would be without Cavil's meddling). Starbuck claims Helo is an idiot who has been tricked, but Sharon has not set out to trick him. She's been biologically programmed by Cavil, to follow a certain path, to fall for a certain person (and to later shoot another). We're all programmed in a way, just not with as much precision or with as much self-serving purpose, and instead by a more vague nature and nurture. Even though Sharon's feelings were purposely manipulated by a third party, they're as real as Helo's. Even with Cavil"s manipulation, Sharon isn't so different from traditional humans who also do some fucked up things because of the way they've been manipulated by others - and she's basically hypnotised when she shoots Adama! It can be really fun and enlightening to go back to shows many years later when you've grown and changed as a person, it can be like watching a different series. :)