If the species was meant to be in space, its fundamental bodily makeup would already be moving toward a form factor befitting a gravity-free, cosmic-ray rich environment, and as we've yet to see such creatures develop, it's plainly evident our species isn't ready for that.
Like rk said - This means that you don't understand how evolution by natural selection works. Evolution is conservative (not a political stab, conservative in the biological sense). There is no planning ahead. It seems you are misunderstanding evolution by natural selection on a fundamental level.
Humanity has a sort of extended phenotype- our technology. Beavers impact and shape their ecosystem, and there are many other "ecosystem engineers" in the animal kingdom. We Homo sapiens just do this on a bigger, more complex scale. We have evolved along with our tools, and will continue to do so. Reaching out into space is just occupying an otherwise vacant ecological niche. Natural selection is about gene propogation, and any species, given the "chance", will exploit everything possible to ensure their genes are more common than others'.
About the claim that humanity is not "worth more" than other species...You say we SHOULD fall victim to an extinction, but why? Sure, we've screwed with the planet and all the other species inhabiting it, but we see other species doing just that, all the time. Species (think ungulate) will literally eat themselves to death, by depleting all the available resources. But wait- one is then tempted to say that since we have a concept of morality, we should curb our destructive selfish ways! Doesn't that imply we are higher than the lowly moral-depauperate species beneath us?
Back to the issue at hand, however. I think space exploration is a good step in getting humanity to colonize other worlds. Sure, we've mucked up our own pretty well, and one lonely dome on Mars with do squat, but it is a small step in the right direction. Baby steps, we'll get there.