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.