Explain how the geologic time scale is used to organize Earth’s history. Think about how the geologic time scale was created and how it is divided. Then answer the following questions.

How did scientists from the geologic time scale?
How is this scale organized?
What processes changed the Earth's environment during Precambrian time?

The first principle you need to understand about geologic time is that the laws of nature are always the same. This means that the laws describing how things work are the same today as they were billions of years ago. For example, water freezes at 0°C. This law has always been true and always will be true. Knowing the natural laws helps you think about Earth’s past, because it gives you clues about how things happened very long ago. It means that we can use present-day processes to interpret the past. Imagine you find fossils of sea animals in a rock. The laws of nature say that sea animals must live in the sea. That law has never changed, so the rock must have formed near the sea. The rock may be millions of years old, but the fossils in it are a clue for us today about how it formed.

Now imagine that you find that same rock with fossils of a sea animal in a place that is very dry and nowhere near the sea. How could that be? Remember that the laws of nature never change. Therefore, the fossil means that the rock definitely formed by the sea. This tells you that even though the area is now dry, it must have once been underwater. Clues like this have helped scientists learn that Earth’s surface features have changed many times. Spots that were once covered by warm seas may now be cool and dry. Places that now have tall mountains may have once been low, flat ground. These kinds of changes take place over many millions of years, but they are still slowly going on today. The place where you live right now may look very different in the far future.

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