(From the New Book of Knowledge)

Earth History

When did the Earth form? How did it form? What was it like when it first formed? These questions have fascinated people for centuries.

Scientists trying to find answers to these questions think that the Earth is about 4.5 billion years old. This estimate is based on their study of rocks from three sources—the Earth itself, the moon, and meteorites, rocks that have fallen to Earth from space. All of these rocks contain radioactive atoms, which break down at a steady, known rate. They act like radioactive "clocks," indicating how much time has passed since the rocks were formed.

A great deal of geological activity, such as erosion, takes place on our planet. Thus, scientists think that none of the rocks that existed at the earliest stages of the Earth's formation have survived. The oldest rocks that have been studied and dated come from sites in Greenland, Canada, and Australia. These rocks are about 3.9 billion years old. Chemical tests on samples of such old rocks, however, suggest that the Earth itself formed much earlier. Furthermore, in studying meteorites and rocks from the moon, scientists have discovered that the meteorites and the moon probably formed about 4.5 billion years ago. All of these factors, taken together, indicate to scientists that all of the planets, including Earth, most likely formed at about the same time.

Since its formation, the Earth has undergone great changes. Mountains have formed and then worn away. Oceans have taken their place. Sediments in rivers and oceans have turned into rock. Some of this rock has been raised up to form new mountains. Many life-forms have evolved and then died out. They have left behind only a fossil record of their existence. The Earth's history is written in its rocks and in the fossils that the rocks contain. Rocks tell of the great changes in the geography, the climates, and the life-forms of planet Earth from prehistoric times to today.

How the Earth Formed

Geologists (scientists who study the Earth's physical changes), astronomers, and other scientists work together to study how the Earth formed. The ideas they develop are called hypotheses. A hypothesis is an interpretation based on evidence that has been observed and studied.

The most widely accepted hypothesis about how the Earth formed is the accretion hypothesis. Accretion refers to the formation of a body by the accumulation of many smaller bodies. Scientists think that after the sun formed, about 4.55 billion years ago, it was surrounded by a disk-shaped cloud of dust and gases. As these particles of dust and gas orbited the sun, they collided with one another and stuck together. As more and more particles stuck together, they began to form bodies called planetesimals. In time, these planetesimals grew in size and formed the Earth and other planets.

During its early history, the Earth was struck many times by nearby planetesimals. These collisions, together with radioactivity and other processes, heated the primitive planet. The heat caused most of the material in it to melt. As this material melted, the elements in it began to separate and flow to different parts of the planet. The heaviest element, iron, flowed to the center. Depending upon their weight and density, other elements floated to the surface or to other levels. As these materials separated and moved, they formed the Earth's three basic layers. In the center is an iron and nickel core. Surrounding the core is a thick layer of intermediate-density rock called the mantle. Around that is a thin surface layer of lighter rock called the crust. Scientists have detected these three zones by studying waves that pass through the Earth's interior during earthquakes.

The Earth's Early Environment

The Earth's early environment was very different from what it is today. There were no plants or animals. Landscapes consisted of barren plains, eroded valleys and hillsides, snowfields, and deserts. Fossil evidence shows that no substantial plants or animals existed before about 3.5 billion years ago.

The Atmosphere

Evidence suggests that the Earth's early atmosphere was also very different from what it is today. The atmosphere today consists mostly of nitrogen and oxygen gases. Scientists think that the early atmosphere consisted mostly of carbon dioxide gas and steam, with very little oxygen. They base this hypothesis on various kinds of evidence. Oxygen itself is a very reactive gas. This means that it reacts or mixes with other elements, such as iron, and is depleted or lost in the process. Oxygen would not last long in a planet's atmosphere unless it was constantly resupplied. Scientists also base their hypothesis on the fact that volcanoes, which must have been common on the early Earth, emitted large quantities of carbon dioxide and steam. For supporting evidence, they also look to Earth's neighbors, Mars and Venus. These planets have atmospheres of carbon dioxide. Scientists think that the early histories of the three planets were similar. The differences today are the result of differences in each planet's development. On Earth, plants developed. Plants absorb carbon dioxide from the air. They use it in the process of making food. Then they release oxygen back into the atmosphere. Plants thus spurred the growth of oxygen in our atmosphere and keep it in constant supply.

The Continents

Over millions of years, forces deep within the Earth have slowly pushed huge sections of the planet's crust from one place to another. Such movements of the Earth's surface are called tectonic motions, and the pieces of crust are called plates. The process by which the plates are moved is called plate tectonics. This process sometimes moves entire continents from one place to another. Thus it is sometimes also known as continental drift.

In recent years, scientists have discovered many different plates and plate movements. They have studied deposits of ancient rocks and measured the properties of the deposits, such as their magnetism, age, and the types of minerals and fossils they contain. In this way, scientists have been able to construct maps showing the locations of continental landmasses hundreds of millions of years ago. They have also been able to show how the Earth's landmasses have changed during its long history.

Life on the Earth

People have always wondered how life originated on Earth. Scientists do not have a complete answer to this question. But they do have a general theory about when and how life began and how it has developed.

Paleontologists, the scientists who study fossils, have found fossil evidence of microscopic algae and bacteria in deposits of rocks dating from about 3.5 billion to 2 billion years ago. These early forms of life probably developed from even simpler organisms. Paleontologists thus think that life must have developed even earlier. Today scientists think that simple one-celled organisms first developed about 3.8 billion years ago, and perhaps even earlier.

The Building Blocks of Life

The development of the life-forms we know about depends on the ability of carbon atoms to join together to form very large complex molecules, or combinations of atoms. Two of the most important complex molecules are amino acids and proteins. These substances are the building blocks of life.

Scientists have learned that amino acids can form in many environments. They could have formed in the Earth's early oceans. Scientists have also found that carbon-based molecules in water can combine to make cell-shaped structures. Many scientists think that life formed on our planet as natural processes, many millions of years ago, built up concentrations of complex carbon-bearing molecules in the planet's oceans. Some of these molecules combined into cell-sized structures and reproduced to form simple microscopic living organisms such as blue-green algae.

Life then slowly evolved, or developed, into more complex life-forms. By about 2.5 billion years ago, there was abundant plant life in the oceans. This plant life had an important effect on the Earth's environment. As part of their life cycle, plants absorb carbon dioxide from the atmosphere and give off oxygen. By about 2.2 billion years ago, oxygen had become an important part of our planet's atmosphere.

Precambrian Time

The portion of Earth's history during which life first began to develop is called Precambrian time. It lasted from about 4.5 billion years ago, before any life existed, to about 600 million years ago. It is divided into two eons, the Proterozoic (between the Cambrian and 2.5 billion years ago) and the Archean (older than 2.5 billion years ago). During this time, life-forms slowly developed in the oceans. But no life could exist on land with the dense carbon dioxide-rich atmosphere.

After life began in the oceans of the Precambrian, it slowly evolved from the simplest to more complex organisms. Fossils of the oldest-known complex multicelled organisms have been found in Australia, the former Soviet Union, and Canada. These organisms included sponge-like creatures and other soft-bodied animals. They were assigned to a new period, the Ediacaran, in 2004.

Toward the end of the Precambrian, organisms began to develop hard protective shells. Such life-forms began appearing on the floors of Earth's shallow oceans about 542 to 488 million years ago. These fossils are important to scientists because they mark the beginning of an amazing proliferation of life on the floors of the Earth's oceans.

Fossils from the Precambrian are very rare. Those that do exist are tiny and poorly developed. This is not true for the millions of years that followed. During the "Cambrian Explosion," numerous life-forms evolved. Many fossils have been found from this time onward. The best-studied fossil sites include the Burgess shale beds near Banff, Canada. By studying fossils, scientists piece together the history of life on the planet.

Scientists have classified the last 600 million years of the Earth's history into three general eras. Each is characterized by the type of life that was dominant during that era—sea creatures, reptiles, and mammals. These three eras are further divided into different geologic periods according to the major life-forms found in the fossils of each period. The fossils provide clear evidence of the dramatic changes in life that occurred from one period to the next.

The Paleozoic Era

The era that began about 600 million years ago is called the Paleozoic era. (The word "paleozoic" comes from two ancient Greek words meaning "early" and "life.") The Paleozoic era spans about 360 million years. During this time, life emerged from the oceans and began to develop on the land. By the end of the Paleozoic era, life was teeming on both land and in the seas.

The first period of the Paleozoic era is the Cambrian period. The best-known creatures from this period are trilobites, small hard-shelled creatures that resemble horseshoe crabs. Abundant fossils of trilobites were originally found in an area of Great Britain called Cambria, for which this geologic period is named.

During the next period, the Ordovician period, life was still limited to the seas. But different types of marine life began to develop. Among them were shellfish, corals, and fishlike vertebrates (animals with backbones). The Silurian period followed. It saw the first appearance of substantial plant and animal life on land. The earliest living things on land were probably simple plants, such as mosses. The earliest land animals seem to have been centipedes and spiderlike creatures. As the first forms of life began appearing on land, life was blossoming in the oceans, too.

The Devonian period, which began about 405 million years ago, has been called the Age of Fishes because the Earth's seas were dominated by fish. Among them were fearsome 30-foot (9-meter) sharklike fish and fish with massive bony plates. The seas were also filled with invertebrates (animals lacking a backbone) and seaweed. The Devonian period also marks the proliferation of plant life on land. Toward the end of this period, about 360 million years ago, forests of ferns and asparaguslike plants covered much of the Earth's surface. This abundance of plant life led to an important development. In the seas, there was intense competition for food among the numerous types of fish. On land, however, there was no competition for food because there were no land animals. Scientists think that mutations, or genetic changes, gradually allowed certain types of fishlike creatures to spend short periods of time out of water, foraging for plant food on land. This led to the development of the first amphibians about 360 million years ago. (Amphibians are vertebrates that live part of their lives on land and part of their lives in water.)

The Mississippian and Pennsylvanian periods of Paleozoic time have been called the Coal Ages, or the Carboniferous periods. During these periods, the Earth's climate was hot and moist. Most of its landmasses were low, heavily forested, swampy areas. As trees and plants died, they became buried in swampy sediments. Over long periods of time, these plant remains turned into coal, forming massive coal deposits in many regions of the Earth. It is still not uncommon to find fossil imprints of leaves and tree stems in coal.

An important milestone in animal evolution began around 300 million years ago. Animals developed that could survive entirely on land without returning to the oceans. These were the early reptiles. Unlike fish and insects, who lay eggs with soft outer layers, reptiles lay eggs with hard shells. This characteristic made it possible for reptiles to lay their eggs on land, where the shells created protected, moist environments in which babies could grow.

During the Permian period, which ended about 250 million years ago, conifers, or cone-bearing trees, appeared. By the end of the period, large areas of the Earth were covered with forests of conifers and ferns. The forests were filled with insects, amphibians, and small reptiles. At this time, the Earth's land and climate were changing as well. Tectonic forces were creating new mountains and oceans. In some parts of the world, glaciers covered the land.

The Great Dying

About 240 million years ago, a great disaster occurred. For some reason, about 90 percent of all species of life on Earth died out within a few million years. This event is sometimes called the Great Dying. It abruptly ended the Permian period and the whole Paleozoic era. The cause of this disaster is one of Earth's great mysteries. Some scientists think that animals may have died out because of drastic climate changes. Some think that intense volcanic activity, caused by continental drift, may have released large quantities of deadly volcanic gases. Still others think that a large asteroid may have struck the Earth. The collision would have blasted debris around the planet, blocking the light of the sun and affecting the planet's climate. Whatever happened, it led to a whole new era of geologic history, one in which huge animals dominated the Earth.

The Mesozoic Era

This new era of geologic history is called the Mesozoic era. (The word "mesozoic" comes from two ancient Greek words meaning "middle" and "life.") Many life-forms got a new start in the Mesozoic era. Reptiles, which had been small and obscure in the Paleozoic era, dominated the Earth's landmasses. The Mesozoic era is often called the Age of Dinosaurs. These reptiles were the largest, most powerful, and most fearsome animals of the time. Although dinosaurs were the dominant animals, small mammals also began to appear. These early mammals were timid, mouselike creatures. They were dwarfed by the huge reptiles that roamed the Earth.

The Mesozoic era lasted from about 240 million to 65 million years ago. It is divided into three periods—the Triassic, Jurassic, and Cretaceous. These three periods are marked by somewhat different species of dinosaurs and other creatures.

The Triassic period saw the appearance of the first true dinosaurs—the "terrible lizards." The period also saw the development of new types of invertebrates in the sea. There were also huge seagoing reptiles. And the first tiny mammals appeared on the land. In the Jurassic period, new types of reptiles evolved. Among them were large flying reptiles and huge plant-eating dinosaurs, some of which were almost 100 feet (30 meters) long. Meat-eating dinosaurs also lived in this period. They fed on plant-eating dinosaurs and other animals. The first birds also appeared in the Jurassic period. The Cretaceous period was the peak of dinosaur development. Many different dinosaurs, including some with horns and bony armor, roamed the Earth. New plants, including fruit trees and flowering plants, also appeared at this time. These plants provided food for small mammals and other plant-eating animals.

The End of the Dinosaurs

Like the Paleozoic era before it, the Mesozoic era ended abruptly. About 65 million years ago, 75 percent of the animal species on Earth, including all dinosaurs, died out within about a million years or less. Some scientists think they know what may have happened. In the 1980's, geologists found a unique layer of soil about an inch or so thick at many sites throughout the world. This thin layer of soil contains dust from asteroids, small pieces of once-melted rock, mineral crystals subjected to violent pressure, and amounts of soot that could only be the result of fires. This evidence suggests that about 65 million years ago a large asteroid, about 6 miles (10 kilometers) in diameter, hit the Earth. It blasted dust into the upper atmosphere and hot debris around the planet. The hot debris touched off worldwide forest fires. And the dust blocked sunlight for months, killing many plants that served as food for animals. Without food, many animals, especially the larger ones, could not survive.

Scientists found support for this theory in the late 1980's when they discovered the remains of an asteroid impact crater hidden under sediments on Mexico's Yucatan Peninsula. The crater is about 110 to 190 miles (180 to 300 kilometers) in diameter. Studies indicate that it was formed about 65 million years ago—the same time that many animal species died out. Scientists think that this crater may be the site of the impact that led to the end of the Mesozoic era. More evidence for this theory came in 1998, when researchers studying ocean mud samples found a small piece from what they believe was the asteroid that struck Mexico.

The Cenozoic Era

With the end of the dinosaurs, the Cenozoic era began. (The word "cenozoic" comes from two ancient Greek words meaning "recent" and "life.") This era has lasted for the past 65 million years, and it continues today. The most important development during the Cenozoic era was the emergence of mammals. It has often been called the Age of Mammals.

Scientists have divided the Cenozoic era into two periods, the Tertiary and the Quaternary. And these periods have been divided into smaller divisions of time called epochs. Each epoch is characterized by different developments in mammal evolution.

In the first epoch, the Paleocene, tiny mammals were common. Gradually, genetic mutations produced many new and larger types of mammals, each adapted to its own particular environment. During the Eocene and Oligocene epochs, early horses, camels, elephants, whales, and monkeys first appeared. The Miocene epoch saw the spread of apes throughout Asia and Africa. Other mammals such as bears, seals, and raccoons also emerged.

About 2 to 4 million years ago, during the Pliocene epoch, apelike creatures in Africa developed larger brains. They began making crude tools out of wood and stone. Primitive humans had appeared by the end of the Pliocene epoch. Modern humans did not develop until the Pleistocene epoch. The epoch we live in now is the Holocene epoch. It has been characterized by the growth and spread of human civilization.

Earth's History Continues

Most of Earth's changes have occurred very slowly. In fact, when change is rapid, such as at the end of the Paleozoic and Mesozoic eras, the results can be catastrophic to life. While the Earth itself continues to change slowly, human beings are beginning to realize that their activities can cause the environment of the Earth to change very rapidly. Just since the early 1900's, for example, the burning of fossil fuels has added large amounts of carbon dioxide to the Earth's atmosphere. Scientists are trying to determine just how significant such events can be to the planet. Taking steps to preserve the heritage of the Earth's long, eventful history may be necessary to ensure that the planet continues to be a safe habitat for human beings.

William K. Hartmann
Author, The History of Earth