Human degradation of the environment has the potential to stall an ongoing process of planetary evolution, and even rewind the evolutionary clock to leave the planet habitable only by the bacteria that dominated billions of years of Earth’s history

planets may proceed through a natural series of evolutionary steps that transform them from lifeless balls into the home of bacteria and other microscopic life and then into a place that supports more complex life

Along the way, life interacts with the nonliving parts of the planet in planet-changing ways

there’s no guarantee that a planet will proceed from one to the next. Each step represents a moment of both crisis and opportunity

The Earth today may be at the brink of another step, Langmuir said. Complex life has evolved into intelligent life that dominates the planet — ecosystems, food webs, and energy flow — as no species ever has before

Whether the planet takes the next step or not may depend on us