How habitable is the Earth?

(Pay attention at the back: this is a trick question.)

We H. Sapiens Sapiens appear to be an infestation on this planet. After the slow-burning evolution of hominins in Africa, our ancestral populations erupted out into Eurasia in a geological eye-blink, spread into the Americas by way of the Bering land bridge (sea levels being somewhat lower during the ice ages) and finally reaching even the remotest islands of oceania around twelve thousand years ago. Today we're ubiquitous. Even our...