They are DNA-based organisms, like every other known life form, all of which evolved on Earth, so it seems reasonable to assume they evolved here too. Of course

They are DNA-based organisms, like every other known life form, all of which evolved on Earth, so it seems reasonable to assume they evolved here too. Of course you might hypothesize that DNA is a building block of life that is not unique to Earth, but that's a whole other can of worms...