The "ensurement of mans survival" will be a side effect of space colonization and not really a reason to do it. We might be able to subisidize space colonization some, for the marketing reason of "survival" but not really so much that we would accelerate expansion into space much. We must find economically viable reasons to be in space before a massive "enough" (and self sufficient) presence off planet is attained.

Personally, I think 2 major advancements need to be had before this occurs. The first is extraordinary reduction in cost to orbit. Currently, I think the only (hopefully) viable method on the horizon is the space elevator if it can be made to work. The other is self replicating robotic systems that can exploit massive space resources in an exponential manner. While both are probably a few decades away, I see no show stoppers for either advancement at this point.

With those advancements, economic exploitation of space will follow and I would look for the eventual surpassment of space settlement to exceed earth settlement. This will take hundreds of years, but just like the opening of the "New World" to European colonization, it will be an exciting time that could go in many directions with many experiments.