From The Life and Death of Planet Earth:
There's a difference between a human's life and the life of our planet. Ruth Ward, born in 1916, aged gracefully but never resembled her youth again. Hers was a one-way trip. Planets have a different trajectory—the Earth, for instance, appears to be on a round trip of sorts. If you fire a cannon straight up, the projectile climbs to a certain height, slows, stops, and then falls back to the ground. Our planet's trajectory is similar. It started as a very hot, oxygen-free world. Water, air, plants, solar energy, and plate tectonics created the conditions for natural evolution, and many people assume that the cannonball of biological complexity is still arcing upward. We believe that the cannonball has already begun to drop, and that the Earth has already started a return to a hot world where life becomes less diverse, less complicated, and less abundant through time. The last life on Earth may look much like the first life—a single-celled bacterium, survivor and descendant of all that came before.