It seems to me that our evolutionary line has pretty much reached its end. We have become so good at adapting to our environments through technology that there is no motivation for us to evolve, except in our way of thinking. Any disaster so cataclysmic that it would force us to evolve, instead of just causing our brains to adapt, would likely wipe us out completely. If that didn't happen, it would depend entirely on what sort of disaster it was: if most of the world floods, perhaps we would become amphibians. If it was a massive species extinction, we would need to change our diets, and probably shrink so we wouldn't need as much energy. It's impossible to say what we would become. This isn't to say that humanity is perfect, or that there won't be evolution in other creatures, perhaps producing beings that will take our place of dominance, but I believe the human race is the end of our line. Another branch on the tree of life may grow and surpass us, but we're stuck the way we are.