Earth Science
Earth Time - Precambrian Time and the Paleozoic Era

Earth Time - Precambrian Time and the Paleozoic Era
Reading Level
     edHelper's suggested reading level:   grades 9 to 10
     Flesch-Kincaid grade level:   9.72

     challenging words:    clam-like, eurypterid, Eurypterids, graptolites, life-forms, reef-like, rhipidistian, rhipidistians, scorpion-like, stromatolites, tribolite, tribolites, tuhr, metamorphism, paleontologists, preexisting
     content words:    Charles Darwin, Paleozoic Era, Cambrian Period, Ordovician Period, Silurian Period, New York, Devonian Period, Carboniferous Period, Permian Period, West Virginia

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Earth Time - Precambrian Time and the Paleozoic Era
By Trista L. Pollard

1     The grass leaves, and twigs you step on today will be studied by a paleontologist thousands, maybe millions of years from now. These specimens in our yards and on our sidewalks provide the clue to the environmental changes that occur on our planet. They tell about the changes in climate, the organisms that lived on the planet, and the areas of water that existed during a specific time. When scientists study fossils, they are interested in the evolution of organisms.
2     Evolution occurs when new organisms develop from preexisting organisms. Fossils and rock layers have given scientists information about the changes in species of animals gradually over time. After observing fossils and rock layers, scientists believe that evolution occurs through natural selection. Scientists noticed that animal skeletons had similar features, and therefore supported the theory of evolution. Who introduced this theory to the world? In 1859, Charles Darwin, an English naturalist, presented his theory that evolution occurred through natural selection.
3     Why is evolution important to the study of Earth's history? Well, changes in the environment, geology, and climate of Earth can affect how well organisms survive on the planet. Any change to our planet's environment, no matter how small, can have a huge impact on how animals survive. For example, when the sea level changes the habitat of organisms in coastal areas may be threatened. Some organisms will thrive while others will die. Fossils help scientists to study how changes to our past environment affected organisms on the planet. They answer the "why" questions: why some plants and animals survived and other plants and animals became extinct. Paleontologists also look at fossils to determine why certain organisms did not change for long periods of time. There were also organisms that did evolve; however, after evolving, they became extinct. As you can see, scientists have many questions about evolution that they hope fossils can answer.
4     Before we discuss the organisms, let's discuss the birth of our planet. Most paleontologists say our planet began 4.6 billion years ago. They believe our birth came in the form of a nebula or large cloud. This nebula spun around the sun which was also new. As our nebula spun around the sun, it picked up particles of matter that clumped together. Our planet, as well as other planets, was born. Precambrian time represents this time and ended about 542 million years ago. Precambrian time includes about 88 percent of our planet's history. Unfortunately, we know very little about this time because of the lack of fossils. The rocks from this time are extremely damaged due to earthquake and volcanic activity. Scientists have been unable to identify the original order of rock layers from this time period. Without a specific order, dating Precambrian rocks is difficult.
5     Every continent contains Precambrian rocks called shields. After many hundred millions of years, mountain building, volcanoes, sedimentation, and metamorphism have produced these shields. The intense heat and pressure from tectonic activity have also deformed these rocks and uplifted some of them to the surface. These rocks are part of North America's Precambrian shield. Scientists have found half of our most valuable minerals are located in rocks within the Precambrian shield. What about life during this time?

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