Established Theory:
Fossils, which are known to have existed during certain geological periods, can be used to assign dates to the layer they are found in. Fossils found in lower layers are generally older organisms than those found in higher layers.
Alternative Theory:
The age of fossils was originally calculated based on the theory of evolution. Smaller and simpler organisms are considered older while larger more complex organisms are considered more recent. By placing all of the known organisms found in the fossil record the evolutionary tree of life was created. The assumed age of the fossil is used to determine the age of the layer the fossil was found in. Many times the fossils of a species that has existed during one geological period, according to the evolutionary tree of life, are found in lower layers containing fossils of more recent organism. At other times the fossils are found throughout all the different geologic layers. At other times one fossil is found to intersect many different geologic layers itself.
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