Now that I think about it, there's some possibilites with Immortals and the Asguard. I find it interesting that neither race can have children. I don't know all the details, but I think that Asguard can't have children or even clone themselves any more. It's possible that Immortals are all clones of the originals, wherever the originals came from. Maybe the originals used to work with the Asguard on their cloning programs and in other areas. When they figured out that they had genetically engineered away the ability to have children, the originals stopped trying to "improve" their DNA and asked the Asguard to figure out a way to correct this while they waited in stasis of some sort so that their knowledge wouldn't be lost. The Asguard were perfectly happy cloning themselves for reproduction and went on genetically engineering themselves. They did honor their friends' request though, and tried to genetically engineer clones so that the Immortals would be able to reproduce again. They decided that Earth would be a good place for the failed experiments to go (assuming that they could tell early on whether or not they had succeeded). It's probably a bit difficult to clone an Immortal, as evidenced by how few new Immortals appear each year/decade.

I should probably mention that some of these ideas came from "Cover Story" by Magda1 and possibly from other stories that I can't remember off the top of my head. "Cover Story" had a pretty good Methos- it's too bad the author doesn't plan on finishing it.