Those who study the natural history of our planet have identified five mass extinctions since the dawn of the Paleozoic Era (see Earth's Mass Extinctions). Worldwide fossil records confirm these events, all of which reset the structure of life's evolutionary tree.
More controversial is the conviction of many biologists that we have entered the Sixth Mass Extinction on Planet Earth, one triggered by the activities of humans. While life has colonized our planet for 3.6 billion years, humans (Homo sapiens) have roamed its surface for less than 140,000 years; for almost half that time, our species was confined to the African Continent and, for more than 95% of our history, we had little impact on the natural ecosystems of Earth.
Within the last 10,000 years, however, urbanization, cultivation, domestication, exploration and industrialization have had dramatic effects on our home planet and on the other life forms that inhabit its varied marine and terrestrial landscapes. Spurred on by our uncontrolled population growth, we have destroyed natural habitats, polluted land, air and water, extirpated species via overhunting and overfishing and have significantly augmented global warming with all of its potential ramifications. Of course, unlike other species, we also threaten our planet and our own welfare with global pandemics and nuclear conflict. While we have the means to derail the Sixth Mass Extinction, one wonders if we humans, fueled by tribalism, greed and religious zealotry have the will to do so.
