A Summary of Earth's History

The Precambrian. The vast unit of time known as the Precambrian started with the origin of the earth about 4.5 billion years ago and ended 570 million years ago. Largely thought to be a hot, steaming, and forbidding landscape, the primitive crust of the newly condensed planet continued to cool. The crust consisted largely of igneous intrusions and volcanic rocks, and sediments that were eroded from this irregular surface. Geologic remnants from this time are the highly deformed and metamorphosed cratons of the continents. The Precambrian is subdivided, from oldest to youngest, into three eons, the Hadean (4600−3900 million years ago), Archean (3900−2500 million years ago), and Proterozoic (2500−570 million years ago). Little is known about the Hadean because there are so few rocks of that age, and those that do exist are intensely deformed and metamorphosed. The Archean was dominated by crustal building and the development of extensive volcanic belts, arcs, and sedimentary basins that were probably related to plate tectonic activity. Marine rocks including chert contain the fossil remains of microscopic algae and bacteria. The Proterozoic is known for large‐scale rifting of continental crust across the world and the filling of these rifts with huge amounts of sedimentary and volcanic rocks. Extensive iron deposits formed in shallow Proterozoic seas, indicating there was enough free oxygen to precipitate iron oxide minerals (for example, hematite [Fe 2O 3]) from the iron in the water. The increase in the amount of free oxygen is thought to be a result of photosynthetic action by primitive life forms in the sea. The fossil record has preserved layered algal mounds called stromatolites, an abundance of microscopic species, and trails and burrows from wormlike organisms.

The Paleozoic era. The Paleozoic era (570−245 million years ago) was long believed by geologists to mark the beginning of life, because of the sudden abundance of complex organisms with hard parts in the fossil record. These organisms included trilobites and shelled animals called cephalopods (cephalopods were the ancestors of modern squids and octopi). Life was restricted to the sea and included graptolites, brachiopods, bryozoans, and mollusks.

A single southern landmass consisted of what is today South America, Africa, India, Antarctica, and Australia. In the northern hemisphere, land masses that represent North America, Siberia, northern Europe, western Asia, and China had not yet joined the southern landmass. North America was essentially a lowland that was periodically flooded by the ocean, forming extensive deposits of sandstone, limestone, and barrier reefs.

By the end of the Paleozoic, all of the continents had come together to form Pangaea. This formation resulted in extreme seasonal weather conditions and one of the greatest periods of extinction in the earth's history—up to 75 percent of amphibian species and 80 percent of marine species disappeared. This time was also marked by the rapid development of land plants, forests of short trees, armor‐plated fishes, sharks, and bony fishes. The Devonian period, the fourth period in the Paleozoic era, is known as the “Age of the Fishes.” Air‐breathing amphibians began to move from the ocean to land. Large tropical swamps dominated much of the landscape.

The Mesozoic era. The Mesozoic era occurred from about 245 million to 66 million years ago. The fossil record from this era (the “Age of the Dinosaurs”) is dominated by a multitude of dinosaur species. Common sedimentary deposits are red sandstones and mudstones. The low‐lying areas were frequently flooded by shallow marine transgressions. Tropical conditions resulted in extensive swamps that later became coal beds. By the mid‐Mesozoic, Pangaea rifted into northern Laurasia and southern Gondwanaland. Igneous and volcanic activity formed the mountain ranges in western North America.

In the Mesozoic era, new trees such as conifers and ginkgoes appeared. Reptiles laid eggs on land. Dinosaur species included meateaters, herbivores, winged reptiles, and marine reptiles. Mammals were just beginning to emerge during this time. The end of the Mesozoic is marked by more mass extinctions, especially of the dinosaurs. Surviving species included turtles, snakes, crocodiles, and various lizards.

The Cenozoic era. The Cenozoic era, also called the “Age of Recent Life” or “Age of Mammals,” encompasses the last 66 million years of the earth's history. Life forms continued to become more complex. The Cenozoic has the most complete geologic record of any era because it is so recent. The continents were fully separated. Plate tectonic activity created many orogenic and volcanic events in North America, including the western fault‐block mountains and huge lava flows. Eastern North America was tectonically stable, and the Appalachians eroded to lower elevations. Valleys in the western part of the continent were filled with great thicknesses of sediments from the mountain ranges.

The fossil record indicates a diverse array of mammals (including marsupials and placentals), flowering plants, grasses, and microscopic foraminifera. New birds and mammals evolved that were adapted to the new vegetation species. Prehistoric humans also began to emerge. Waves of mass extinctions occurred toward the end of Pleistocene epoch, including those of mammoths, mastodons, sabertoothed cats, ground sloths, and camels. North America underwent multiple glaciations in the last 20,000 years, which helped mold the landscapes we see today.