Hadean, Archaean and Proterozoic Eons (4,500 - 542 m.y.a.)

Formation of a Planet

Scientists believe that earlier than 4.5 billion years ago, Earth was a lifeless ball of swirling dust, rock, and ice revolving around our young sun. The gravity generated by this ball of material attracted other space debris, causing our planet to grow. As Earth grew in size, so did its gravity.

Scientists hypothesize that everything changed when a giant cosmic body collided with Earth. The heat generated from this massive collision melted the entire planet, and the blast caused chunks of molten rock to catapult into space, forming our moon.

Earth’s Surface Forms

After this collision, Earth was hot molten rock. As the planet orbited the sun, dense material sank toward the center of the planet, forming Earth’s dense metal core. Earth’s surface cooled and became solid.

Even though Earth was now solid, the next 700 million years were not hospitable to life. The fumes generated from the cooling rocks on Earth filled the early atmosphere with water vapor and noxious carbon dioxide and nitrogen gases.

Eventually, temperatures dropped and atmospheric gases condensed into droplets of water. Storms followed, raining down and filling empty depressions and basins on our planet’s surface to form lakes and oceans.

Life Develops

Approximately 3.8 billion years ago, the impact from comets began to subside. This enabled water to settle on Earth’s surface, and the oceans were formed.

At that time, chemical interactions may have produced molecules complex enough to reproduce. These molecules would soon evolve to become more and more complex, similar to certain types of bacteria we see today. These organisms flourished in the sunlit portions of the ocean and, by the trillions, they transformed Earth by filling the atmosphere with breathable air.

Paleozoic Era (542 - 251 m.y.a.)

Cambrian Period (542 - 488.3 m.y.a.)

During the Cambrian period, shallow seas covered much of the land and ancient continents were situated south of the Equator. There was a great explosion of invertebrate life developing in these early seas, including trilobites and mollusks.

Ordovician Period (488.3 - 443.7 m.y.a.)

A warm, shallow sea covered much of Earth. Life was dominated by invertebrates, or animals without internal skeletons. Fossil records indicate that early vertebrates started to appear, including jawless fish.

Silurian Period (443.7 - 416 m.y.a.)

In the Silurian period, coral reefs began to develop. The long, drawn out collisions between moving continents began to form mountains. Fish developed jaws, and plants and arthropods appeared on land.

Devonian Period (416 - 359.2 m.y.a.)

This period marks the “Age of Fishes,” as the fossil record shows an abundance of shark and fish fossils. Trilobites and corals were common in the oceans and the first amphibians set foot on land.

Carboniferous Period (359.2 - 299 m.y.a.)

Plants dominated the landscape in great swamp forests filled with massive woody trees. Most of the coal we burn today is the fossilized remnants of trees from this period. True reptiles appeared, as did winged insects, including dragonflies and cockroaches. Pangaea, an ancient super-continent that included all of the major landmasses on Earth, began its formation at the end of the Carboniferous period.

Permian Period (299 - 251 m.y.a.)

The land was dominated by reptiles. Fossil records indicate the largest mass extinction in Earth’s history occurred in this period—an estimated 90 percent of marine species and 70 percent of land animals went extinct. Scientists speculate that either large scale climate changes or a series of massive volcanic eruptions caused this mass extinction.

Mesozoic Era (251 - 65.5 m.y.a.)

Triassic Period (251 - 199.6 m.y.a.)

The Mesozoic era is divided into three periods. The early Mesozoic era is called the “Age of Reptiles” and begins the Triassic period. During this period, the central region of Pangaea was dominated by a hot, dry climate. The Triassic period marks the appearance of the first dinosaurs. Early ancestors of present day turtles and crocodiles, as well as the first mammals, also started to appear in the fossil record. Fossils dating to this period indicate that forests consisted mainly of conifers, palm-like trees, and ginkgo trees.

Jurassic Period (199.6 - 145.5 m.y.a.)

Pangaea started its separation during the late Triassic and early Jurassic periods. North America began to separate from Africa and South America. Large dinosaur fossils have been found dating back to this period, including Stegosaurus, Diplodocus, and Apatosaurus. The fossil record also indicates the appearance of early birds.

Cretaceous Period (145.5 - 65.5 m.y.a.)

During this period, continents moved closer to their current positions, with South America splitting from Africa. Geologic evidence dating back to this period also indicates that there was widespread volcanic activity. Flowering plants and snakes appeared. Mass extinctions appear to have been prevalent near the end of this period. Fossil records indicate the sudden disappearance of many land and marine life forms, including the dinosaurs.

Cenozoic Era (65.5 m.y.a. - present)

Paleogene Period (65.5 - 23.03 m.y.a.)

The Cenozoic era is divided up between the Paleogene period and the Neogene period. The Himalayas formed in the Paleogene period. A continental glacier covered much of Antarctica, and continents continued to slowly move to their present day positions. The beginning of this era also marks the “Age of Mammals,” when horses, elephants, bears, rodents, primates, and whales began to appear.

Neogene Period (23.03 m.y.a. - present)

The Neogene period began with much of the land covered by a thick sheet of ice. During the last two million years, mammals, flowering plants, and insects have dominated the land. Most scientists speculate that modern humans developed approximately 190,000 years ago in Africa.

Today

Looking at images of Earth from space, it is hard to imagine that this blue and green planet was once a lifeless ball of molten rock. It has taken over 4.5 billion years for Earth to become what it is today.

Dr. George Guthrie is working to apply our current understanding of Earth’s history and his own stratigraphy research to provide a means for storing carbon dioxide within Earth. As technology improves and new fossils are unearthed, our understanding of Earth’s past will continue to deepen.

Armed with knowledge, skills, and technology, it will be up to young scientists, like you, to further develop this understanding. This will ultimately help us better prepare ourselves for environmental challenges that lie ahead for this ever-changing planet.