There is the more generic idea of leaving a track in history, making sure one's offspring survives. If one of my descendants make it off-planet and thrives ther

There is the more generic idea of leaving a track in history, making sure one's offspring survives. If one of my descendants make it off-planet and thrives there once the Zombies attack/the meteor strikes/the nukes go off then my line will continue. I have no logic explanation for wanting to continue my bloodline, but I guess it's just a natural instinct. The second reason to get off-world is the more far-fetched one, but assuming we can establish that other worlds are dead or sterile then we are free to customize it as we please without having to feel bad about it. In the long run we could leave Earth completely, allowing whomever comes next to develop freely without our intervention.