SPACE.com -- The Five Worst Extinctions in Earth's History
Each of these events, naturally caused (asteroids, earthquakes, global warming or cooling, volcanic eruptions, etc), occurred before the "age of mammals." In fact the last one made it possible for mammal to become the dominant
life forms. Each of these "mass extinctions" evidently wiped out about 70 to 75 percent of all the species that were in existence at the time. One of the species that becomes extinct is the previously dominant one.
In 1995, Richard Leakey and Roger Lewin (The Sixth Extinction: Biodiversity and its Survival) suggested that we are at the beginning of another mass extinction, this one caused by human behavior. The universe will no doubt, go on without us; but I am enough of a sentimentalist to grieve our passing and to hope that it can be avoided. Albert Nolan believes that by caring enough and by acting enough like we did care enough, we might be able to change the future. Caring and acting require the destruction of our false selves and a deep appreciation of the common good (common including all humans, all other life forms, the material universe, etc.)