Of course, part of the reason that humans exist is because we evolved to adapt to this environment, so it’s not all a matter of blind luck. That notwithstanding, we still can only survive within a very precise elemental environment—comprised, for example, of specific ratios of potassium and sodium. All this means that if anything were to change in the chemistry of Earth’s crust or atmosphere (say, by our burning through the ozone layer or by pumping our atmosphere full of toxic substances), the planet would no longer be habitable for humans.