Although the present is the key to the past, not all the keys fit to
unravel our geologic past. Some conditions in the past will never be the
same. For example, the atmosphere is always changing. When life was
first noted in the fossil record, the atmosphere was not like it was
today. It wasn’t until the beginning of the Cambrian that methane,
hydrogen, and ammonia were not the common gases in our atmosphere.
Organisms, like plants, have changed the chemical composition of the
atmosphere. Humans could not have survived in the atmosphere when
trilobites were king of the oceans.
The level of the water has also evolved. Only about a billion years
after the Earth was formed, was there evidence of water. Erosion of the
rocks and the addition of gases from the atmosphere, have all changed
the composition of the oceans.
Environments through time have changed. There are lessons in the
present, but all geologists realize the past is a difficult subject to
Atmosphere of today