To understand evolution, humans must think in much larger units of time than those we use to define our lives. After all, evolutionary change isn't apparent in days, months, or years. Instead, it's documented in layers and layers of rock deposited over 4.6 billion years. The stretch of geologic history is commonly referred to as "deep time," and it's a concept perhaps as difficult to conceive as deep space. Can humans measure deep time? Yes. Will we ever truly comprehend such immensity of time? Probably not. But to develop a better understanding of evolutionary change in its proper historical context, we must try. This timeline provides a framework for doing so.

Geology Introduction: The Changing Planet Earth has been significantly altered over its 4.6-billion-year history by climate swings, volcanism, drifting continents, and more. These dynamic conditions, in turn, have influenced every living thing that has inhabited the planet. Clearly, Earth is more than just an inanimate, unchanging ball of rock. As much as evolution is about life and its many forms, biology alone cannot fully explain it. By integrating the physical sciences, which include geology, chemistry, and physics, into our study of life on Earth, we can better understand the conditions in which life has evolved.

Extinction Introduction: End of the Line The story of life is told primarily by its ghosts -- the victims of extinction. Scientists say that only one in a thousand species that have ever lived survives today. The other 99.9 percent are extinct, gone forever. With few exceptions, the life span of individual species is short by geological standards, on average between 2 and 10 million years. No matter how well adapted a creature has been to its environment, history has shown that even the most dominant can be wiped away. Ironically, extinction is a springboard to other life. Even in the most catastrophic events, death is not complete. Surviving species continue to evolve, often filling niches left by the victims. Extinction is by and large a natural process in which species, groups, and even whole families of organisms disappear. Background extinctions, which are ongoing at all times through the history of life, eliminate one family every million years or so. The more destructive and relatively sudden kind of extinction, the mass extinction event, is caused by environmental influences and has a global impact on diversity. All extinctions identified in this timeline are mass extinction events.