Section 2: Precambrian Era
The first era is called the Precambrian (pre-cam-bre- an) Era. Besides being the oldest era, the Precambrian also lasted the longest. It lasted from about 4.5 billion years ago to about 540 million years ago, which means it included almost 4 billion years of Earth’s history.
During this era, layer after layer of granite and other igneous rock was formed from the magma deep inside the earth. (Any rock formed from magma is called igneous rock.) The oldest of these rock layers is now found about three miles below the surface in western North Dakota but only a few hundred feet below the surface in the eastern part of the state.
The only plants that lived during the Precambrian Era were microscopic (my-kro-skop-ik), which means visible only under a microscope. No animals lived yet during this era.
The length of the Precambrian Era from the beginning of the Geologic Calendar:
4.5 billion years minus 540 million years equals 3 billion 960 million years, or 4,500,000,000 – 540,000,000 = 3,960,000,000 (almost 4 billion).