Investigating the History of Earth (Introduction to Earth Science)
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It may be hard to believe that the Earth, with all the complexity and biodiversity we observe today, originated in a cloud of gas and dust. Yet much of the plant and animal life that seems so common now evolved relatively recently on the timeline of Earth's long history. The Earth's remarkable origins are chronicled in this insightful volume, which also examines the prehistoric organisms-from bacteria to dinosaurs-that populated the planet long before humans arrived.
professional development activities, publications, and communication networks. For More Information American Astronomical Society 2000 Florida Avenue NW Suite 400 Washington, DC 20009-1231 (202) 328-2010 Web site: http://aas.org The American Astronomical Society is a major organization of professional astronomers in North America. Its Web site provides a variety of publications and educational resources for those interested in contemporary astronomy. 79 Investigating the History of Earth
evolution to have resulted in the great diversity of life seen today. There were dissenting opinions, though. Physicists such as Lord Kelvin calculated that Earth, apparently still molten inside, should have cooled to a solid throughout if it were more than about 40 million years old. Similarly, astronomers figured that the Sun could not be more than a few tens of millions of years old. They believed that the Sun shone because of energy from gravitational contraction, so it could not have
dioxide in the air, releasing oxygen in the process. These bacteria are thought to have been quite similar to the cyanobacteria that are still abundant today. It is likely that cyanobacteria were not the only type, and perhaps not the first type, of organism during this period. Bacteria called chemoautotrophs likely existed in the depths of the sea and even in rock. Some early organisms may have been archaeans, rather than true bacteria. Archaeans are a group of microorganisms that can live in
organisms in the ocean continued to take carbon dioxide from the air and to bury the carbon as they died and sank to the bottom. With less of this greenhouse gas present, and the Sun at only about 94 percent of its present brightness, temperatures began to drop. Once snow and ice began to form, they reflected more of the Sun’s light away. This vicious circle is an example of “positive feedback,” in which an effect itself causes more of the same effect to occur. It allowed 27 Investigating the
the world became colder. This led to the extinction of many species. Silurian Period During the Silurian period, the continents began to approach each other once again. Most of A fossilized Halysites catenularia, an extinct type of coral, dates to the middle of the Silurian period. It was collected from the Lockport Dolomite in western New York. Courtesy of the Buffalo Museum of Science, Buffalo, N.Y. 37 Investigating the History of Earth the late Ordovician ice melted, and the world became