Geologic Time. Early Earth History

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1 chapter 10 3 Geologic Time section 2 Early Earth History Before You Read Think of a picture of a volcano you have seen. Describe what Earth would be like if the land were almost completely covered with volcanoes. What You ll Learn what Earth was like during its early history how species adapted to changes on Earth Read to Learn Precambrian Time It may seem strange, but during the first billion years of Earth s history, the land was covered with volcanoes. Over the next 3 billion years, simple life-forms began to live in the oceans. Precambrian (pree KAM bree un) time is the longest part of Earth s history. Precambrian time lasted from about 4.5 billion years ago to about 544 million years ago. What is known about early life forms? Little is known about the organisms that lived during Precambrian time. Most Precambrian rocks are buried deep within Earth where they have been changed by heat and pressure. Few fossils can survive these conditions. Most Precambrian organisms had soft bodies. These organisms did not have hard body parts that leave fossil imprints in rock. One clue to early history of life is found in ancient stromatolites (stroh MA tuh lites). Stromatolites are layered mats formed by colonies, or groups, of cyanobacteria. Cyanobacteria are blue-green algae thought to be one of the earliest life-forms on Earth. Highlight As you read, highlight the key terms and main ideas in each paragraph. B Compare and Contrast Make the following Foldable to compare the Precambrian and the Paleozoic Eras. Reading Essentials 143

2 1. Infer How might Earth be different today if cyanobacteria had not lived in ancient seas? 2. Explain why scientists have been able to learn more about life in the Paleozoic Era than about life in Precambrian time. How did early life affect the atmosphere? Cyanobacteria first appeared on Earth about 3.5 billion years ago. Cyanobacteria contained chlorophyll and used photosynthesis. This is important because during photosynthesis, they produced oxygen, which helped change Earth s atmosphere. Following the appearance of cyanobacteria, oxygen became a major gas in the atmosphere. Also of importance was that the ozone layer in the atmosphere began to develop, shielding Earth from ultraviolet rays. It is hypothesized that these changes allowed species of single-celled organisms to evolve into more complex organisms. Near the end of Precambrian time, invertebrates (ihn VUR tuh brayts) appeared. Invertebrates are animals without backbones. Early invertebrates had soft bodies, so few were preserved as fossils. Because of this, many Precambrian fossils are trace fossils. Examples of trace fossils are tracks, trails, or burrows. Trace fossils provide information about how organisms lived and behaved. What were other unusual life-forms? In the late Precambrian, a group of animals lived that were similar to some animals today. These soft-bodied animals looked like modern jellyfish and worms. The first fossils of these earliest invertebrates were found in the Ediacara Hills of Australia. This group of organisms became known as the Ediacaran (ee dee uh KAR un) animals. They have been found on every continent, except Antarctica. Ediacaran animals lived on the bottom of Precambrian seas. Some scientists think these animals may have had tough outer coverings on their bodies. Trilobites may have outcompeted the Ediacaran animals and caused their extinction. However, no one knows for sure why the Ediacaran animals disappeared. The Paleozoic Era Beginning in the Paleozoic (pay lee uh ZOH ihk) Era, animals with shells and other hard body parts began to appear. Because hard body parts are well preserved in fossils, it is easier to find traces of life in this era. The Paleozoic Era, or era of ancient life, began about 544 million years ago and ended about 248 million years ago. 144 Geologic Time

What organisms lived during this era? During most of the Paleozoic Era, warm, shallow seas covered much of the planet. Many life-forms lived in the oceans.

3 What organisms lived during this era? During most of the Paleozoic Era, warm, shallow seas covered much of the planet. Many life-forms lived in the oceans. Trilobites were common, especially early in the Paleozoic. Other organisms developed shells. As a result, the fossil record of this time contains many shells. However, invertebrates weren t the only animals in Paleozoic seas. Animals with backbones, called vertebrates, evolved during this time. The earliest vertebrates were fishlike creatures without jaws. During the Devonian Period, fish with strong jaws evolved. These huge fish, armed with heavy protective coverings on their bodies, could eat large sharks. By the Devonian Period, forests began to grow on land. Some vertebrates adapted to the land environment. How did early life forms move onto land? Most fish, both ancient and modern, breathe through gills. But in the Devonian Period, many fish also had lungs. Because of their lungs, they could live in water that had low levels of oxygen and swim to the surface to breathe air. One kind of ancient fish with lungs also developed fins that were like legs, as shown in the figure below. These leglike fins were used to swim and to crawl around on the ocean floor. Paleontologists hypothesize that today s amphibians might have evolved from these fish. Modern amphibians, such as frogs, live both in water and on land. All amphibians have one thing in common they all lay their eggs in water or in a moist place. 3. Describe What did the earliest vertebrates look like? Picture This 4. Circle Draw a circle around the leglike fins on the fish shown in the figure. Reading Essentials 145

4 5. Identify What was one adaptation that allowed reptiles to live out of water? 6. Identify What collided with the North American Plate and created high mountains? What adaptations allowed reptiles to remain on land? By the Pennsylvanian Period, some amphibians evolved eggs that were covered by a protective coating. The coating helped prevent the eggs from drying out. As a result, these animals, called reptiles, did not need to lay their eggs in water. Reptiles also have skin covered with hard scales. The scales prevent water loss from their bodies. These adaptations allow reptiles to live farther from water and in dry climates where many amphibians cannot live. How were mountains formed? During the Paleozoic Era, there were great changes on Earth s surface. Several mountain ranges formed during this time. One example is the Appalachian Mountains in the Eastern United States. Mountain building occurred in several stages. First, North America moved closer to Europe and Africa. This closed the ocean that had separated them. Several volcanic island chains that had been in the ocean collided with the North American Plate. The collision of the plate and the island chains created high mountains. The next mountain-building event occurred when the African Plate crashed into the North American Plate. This collision formed mountains on both North America and Africa. Rock layers were folded and faulted. Some rocks that were near the eastern coast of the North American Plate were pushed west along faults as far as 65 km. Sediments were uplifted to form an immense mountain belt. Then, about 200 million years ago, the North American and African plates began to separate. The ocean between them began to open up again. What caused the end of the Paleozoic Era? At the end of the Paleozoic Era, more than 90 percent of all ocean species and 70 percent of all land species died. Perhaps as the supercontinent Pangaea formed, changes in the ocean and land caused species to die. Another hypothesis is that erupting volcanoes changed Earth so much that organisms could not survive. Perhaps an asteroid hit the planet and destroyed its environments. 146 Geologic Time

5 After You Read Mini Glossary cyanobacteria: blue-green algae thought to be one of the earliest life-forms on Earth Paleozoic Era: era that began about 544 million years ago and ended about 248 million years ago; has abundant fossils Precambrian time: longest part of Earth s history, lasting from about 4.5 billion years ago to about 544 million years ago 1. Review the terms and their definitions in the Mini Glossary. Then write two sentences, one about each time period covered in this section. Use the three vocabulary words in your sentences. 2. Fill in the blanks in the boxes below. Blue-green algae, called arose during the Animals without backbones, called arose during the when animals with backbones, called, evolved, also evolved. 3. Did highlighting help you understand the information in this section? Visit blue.msscience.com to access your textbook, interactive games, and projects to help you learn more about the early history of Earth. End of Section Reading Essentials 147

Geologic time. 8 th Grade

Geologic time. 8 th Grade + Geologic time 8 th Grade + Trilobites n Trilobites are small, hard-shelled animals that lived in ancient seas. Trilobites are considered to be index fossils. n Index fossils lived over vast regions of