Eventually we as humans will consume and or destroy all the natural resources on Earth. Earth will be livable, but the prime real estate will be on Mars. I also think our planet kind of self cleanses itself, so that as too many people are on the planet, disese will reduce our numbers. So the reasons to leave this planet will be compelling. Our current science has plans to do "terra forming" on Mars. The idea is that we are pretty sure there is water there, and we have bacteria that we can introduce that produce oxygen at 10 times the rate of any plant. Terra forming basically says that it may be possible to create a sustainable atmospher for humans on Mars. The one issue we probably can't change is the gravity, so we would actually evolve differently into a different species, our bodies will change in shape and function, certain muscles grow weaker and certain bones lose density, even the heart changes. The heart would actually get smaller and thinner. I can't wait.