To understand evolution, humans must think in much larger unitsof time than those we use to define our lives. After all, evolutionary change isn'tapparent in days, months, or years. Instead, it's documented in layers and layers ofrock deposited over 4.6 billion years.
The stretch of geologic history is commonly referred to as "deeptime," and it's a concept perhaps as difficult to conceive as deep space. Can humansmeasure deep time? Yes. Will we ever truly comprehend such immensity of time? Probablynot. But to develop a better understanding of evolutionary change in its proper historicalcontext, 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-yearhistory by climate swings, volcanism, drifting continents, and more. These dynamicconditions, 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, biologyalone cannot fully explain it. By integrating the physical sciences, which includegeology, chemistry, and physics, into our study of life on Earth, we can betterunderstand the conditions in which life has evolved.
Extinction Introduction: End of the Line
The story of life is told primarily by its ghosts -- the victimsof extinction. Scientists say that only one in a thousand species that have ever livedsurvives today. The other 99.9 percent are extinct, gone forever.
With few exceptions, the life span of individual species is shortby geological standards, on average between 2 and 10 million years. No matter how welladapted a creature has been to its environment, history has shown that even the mostdominant can be wiped away. Ironically, extinction is a springboard to other life.Even in the most catastrophic events, death is not complete. Surviving species continueto 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. Backgroundextinctions, which are ongoing at all times through the history of life, eliminateone family every million years or so. The more destructive and relatively suddenkind of extinction, the mass extinction event, is caused by environmental influencesand has a global impact on diversity. All extinctions identified in this timelineare mass extinction events.
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