They are all cylons.
Humans died off 3600 or so years ago after we nuked each other on Earth in a war with cylons(Terminator anyone?). The thriving races on the 12 Colony planets are all higher forms of cylons who all want to become like their creators (other sci-fi movies with this theme) and then destroy their creators (even more sci-fi themes like Stargate SG1/Atlantis)
Really awesome series. I have loved it from the beginning and will be sad when it goes away. Looking forward to the next series...Caprica, which goes more political/down to earth and takes place during the 40 years of peace.