Earth History: A Brief Summary opens with a presentation of the origin of Earth, beginning about 5 billion years ago with an enormous cloud of interstellar material. Also examined are Earth's primitive atmosphere and how its composition changed in response to the evolution of life. Earth’s history is systematically detailed with a description of the geologic and biologic events that have occurred from the Precambrian, through the Paleozoic, Mesozoic, and Cenozoic Eras.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to:
•Describe the origin of Earth and the solar system in general.
•Discuss how Earth’s atmosphere has evolved and changed through time.
•List the principal geologic and biologic events for each era of geologic time.
Chapter Summary
•The nebular hypothesis describes the formation of the solar system. The planets and sun began forming about 5 billion years ago from a large cloud of dust and gases composed of hydrogen and helium, with only a small percentage of all the other heavier elements. As the cloud contracted, it began to rotate and assume a disk shape. Material that was gravitationally pulled toward the center became the protosun. Within the rotating disk, small centers, called protoplanets, swept up more and more of the cloud’s debris. Due to their high temperatures and weak gravitational fields, the inner planets (Mercury, Venus, Earth, and Mars) were unable to accumulate and retain many of the lighter components (hydrogen, helium, ammonia. methane, and water). However, because of the very cold temperatures existing far from the sun, the fragments from which the large outer planets (Jupiter, Saturn, Uranus, and Neptune) formed consisted of huge amounts of hydrogen and other light materials. These gaseous substances account for the comparatively large sizes and low densities of the outer planets.
•The decay of radioactive elements and heat released by colliding particles aided the melting of Earth's interior, allowing the denser elements, principally iron and nickel, to sink to its center. As a result of this differentiation, Earth's interior consists of shells or spheres of materials, each having rather distinct properties.
•Earth's primitive atmosphere probably consisted of the gases water vapor, carbon dioxide, nitrogen, and several trace gases that were released in volcanic emissions, a process called outgassing. The first life forms on Earth, probably anaerobic bacteria, did not need oxygen. As life evolved, plants, through the process of photosynthesis, used carbon dioxide and water and released oxygen into the atmosphere. Once the available iron on Earth was oxidized (combined with oxygen), substantial quantities of oxygen accumulated in the atmosphere. About 4 billion years into Earth’s existence, the fossil record reveals abundant ocean-dwelling organisms that require oxygen to live.
•The Precambrian spans about 87 percent of Earth’s history, beginning with the formation of Earth about 4.6 billion years ago and ending approximately 570 million years ago with the diversification of life that marks the start of the Paleozoic Era. It is the least understood span of Earth’s history because most Precambrian rocks are buried from view. However, on each continent there is a "core area" of Precambrian rocks called the shield. The iron ore deposits of Precambrian age represent the time when oxygen became abundant in the atmosphere and combined with iron to form iron oxide. The most common middle Precambrian fossils are stromatolites. Microfossils of bacteria and blue—green algae, both primitive prokaryotes whose cells lack organized nuclei, have been found in chert, a hard, dense, chemical sedimentary rock in southern Africa (5.1 billion years of age) and near Lake Superior (1.7 billion years of age). Eukaryotes, with cells containing organized nuclei, are among billion—year-old fossils discovered in Australia. Plant fossils date from the middle Precambrian, but animal fossils came a bit later, in the late Precambrian. Many of these fossils are trace fossils and not of the animals themselves.
•The Paleozoic Era extends from 570 million years ago to about 245 million years ago. The beginning of the Paleozoic is marked by the appearance of first life forms with hard parts such as shells.
Therefore, abundant Paleozoic fossils occur and a far more detailed record of Paleozoic events can be constructed. During the early Paleozoic (the Cambrian, Ordovician, and Silurian Periods). the vast
southern continent of Gondwanaland existed. Seas inundated and receded from North America several times, leaving thick evaporite beds of rock salt and gypsum. Life in the early Paleozoic was restricted to the seas and consisted of several invertebrate groups. During the late Paleozoic (the Devonian, Mississippian, Pennsylvanian, and Permian Periods), ancestral North America collided with Africa to produce the original northern Appalachian Mountains, and the northern continent of Laurasia formed. By the close of the Paleozoic, all the continents had fused into the supercontinent of Pangaea. During most of the late Paleozoic, organisms diversified dramatically, insects and plants moved onto the land, and amphibians evolved and diversified quickly. By the Pennsylvanian Period, large tropical swamps, which became the major coal deposits of today, extended across North America, Europe, and Siberia. At the close of the Paleozoic, altered climatic conditions caused one of the most dramatic biological declines in all of Earth’s history.
•The Mesozoic Era, often called the “age of dinosaurs,” begins about 245 million years ago and ends approximately 66 million years ago. Early in the Mesozoic much of the land was above sea level. However, by the middle Mesozoic, seas invaded western North America. As Pangaea began to break up, the westward-moving North American plate began to override the Pacific plate, causing crustal deformation along the entire western margin of the continent. Organisms that had survived extinction at the end of the Paleozoic began to diversity in spectacular ways. Gymnosperms (cycads, conifers, and ginkgoes) quickly became the dominant trees of the Mesozoic because they could adapt to the drier climates. Reptiles quickly became the dominant land animals, with one group eventually becoming the birds. The most awesome of the Mesozoic reptiles were the dinosaurs. At the close of the Mesozoic, many reptile groups, including the dinosaurs, became extinct.
•The Cenozoic Era, or “era of recent life," begins approximately 66 million years ago and continues today. It is the time of mammals, including humans. The widespread, less disturbed rock formations of the Cenozoic provide a rich geologic record. Most of North America was above sea level throughout the Cenozoic. Due to their different relations with tectonic plate boundaries, the eastern and western margins of the continent experienced contrasting events. The stable eastern margin was the site of abundant sedimentation as isostatic adjustment raised the Appalachians, causing the streams to erode with renewed vigor and deposit their sediment along the continental margin. In the West, building of the Rocky Mountains (the Laramide orogeny) was coming to an end, the Basin and Range Province was forming, and volcanic activity was extensive. The Cenozoic is often called “the age of mammals” because these animals replaced the reptiles as the dominant land life. Two groups of mammals, the marsupials and the placentals, evolved and expanded to dominate the era. One tendency was for some mammal groups to become very large. However, a wave of late Pleistocene extinctions rapidly eliminated these animals from the landscape. Some scientists believe that early humans hastened their decline by selectively hunting the larger animals. The Cenozoic could also be called the “age of flowering plants." As a source of food, flowering plants strongly influenced the evolution of both birds and herbivorous (plant-eating) mammals throughout the Cenozoic Era.