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 interpret.

Atmosphere of today