How haven't they harmed the Earth? Humans, in the end, are animals, and they consume resources and occupy space. The difference between us and other animals is our ability to think and reason, so we have harmed the Earth with technology, drilling, mining, settlement, overharvesting, overfishing, large scale carbon emissions and pollution of all sorts, particularly water pollution and nuclear waste.
I think it is just now we are starting to realize the cumulative effects of humans' impact on Earth, especially in terms of species extinction and Global Warming. Human population growth is a major factor in the impact as well, and at some point, the number of humans on the planet will exceed its carrying capacity.