History of Earth

The natural history of Earth concerns the development of planet Earth from its formation to the present day. Many areas of natural science help us understand the big events in Earth's past, marked by always-changing geology and evolution.

Earth formed around 4.54 billion years ago, about one-third the age of the universe, by coming together from the solar nebula. Volcanoes likely made the first atmosphere and oceans. As Earth cooled, a solid crust formed, allowing liquid water on its surface.

The earliest undisputed evidence of life on Earth dates back at least 3.5 billion years. There are microbial mat fossils such as stromatolites in very old rocks. Photosynthetic organisms came later and changed the air. Complex multicellular life appeared much later, leading to many different kinds of living things. Earth’s surface and its life keep changing through plate tectonics.

Eons

In the study of Earth's past, time is measured in millions of years ago (Ma), with each unit meaning about 1,000,000 years. The history of our planet is split into four big parts called eons. These eons began about 4,540 million years ago when Earth was formed. During each eon, Earth changed a lot in its makeup, weather, and the living things on it. Each eon is then split into smaller parts called eras, periods, and epochs.

Eon	Time (mya)	Description
Hadean	4,540–4,000	Earth is formed out of debris around the solar protoplanetary disk. There is no life. Temperatures are extremely hot, with frequent volcanic activity and hellish-looking environments (hence the eon's name, which comes from Hades). The atmosphere is nebular. Possible early oceans or bodies of liquid water. The Moon is formed around this time probably due to a protoplanet's collision into Earth.
Archean	4,000–2,500	Prokaryote life, the first form of life, emerges at the very beginning of this eon, in a process known as abiogenesis. The continents of Ur, Vaalbara and Kenorland may have existed around this time. The atmosphere is composed of volcanic and greenhouse gases.
Proterozoic	2,500–538.8	The name of this eon means "early life". Eukaryotes, a more complex form of life, emerge, including some forms of multicellular organisms. Bacteria begin producing oxygen, shaping the third and current of Earth's atmospheres. Plants, later animals and possibly earlier forms of fungi form around this time. The early and late phases of this eon may have undergone "Snowball Earth" periods, in which all of the planet suffered below-zero temperatures. The early continents of Columbia, Rodinia and Pannotia, in that order, may have existed in this eon.
Phanerozoic	538.8–present	Complex life, including vertebrates, begin to dominate Earth's ocean in a process known as the Cambrian explosion. Pangaea forms and later dissolves into Laurasia and Gondwana, which in turn dissolve into the current continents. Gradually, life expands to land and familiar forms of plants, animals and fungi begin appearing, including annelids, insects and reptiles, hence the eon's name, which means "visible life". Several mass extinctions occur, among which birds, the descendants of non-avian dinosaurs, and more recently mammals emerge. Modern animals—including humans—evolve at the most recent phases of this eon.

Geologic time scale

Main article: Geologic time scale

The story of our planet Earth can be organized using the geologic time scale. This helps us understand Earth's long history by looking at layers of rock.

We divide Earth's history into five special timelines. The first timeline shows all of Earth's time, but it makes the most recent time look smaller. The next four timelines look more closely at the later parts of Earth's story. Each one helps us see what happened more clearly in the recent past.

Solar System formation

Main article: Formation and evolution of the Solar System

Hadean and Archean Eons

Main articles: Hadean and Archean

The Hadean eon is when Earth began, starting with its formation and ending around 3.8 billion years ago. After this came the Archean eon. The oldest rocks on Earth are about 4.0 billion years old. Tiny crystals called zircons are even older, dating to around 4.4 billion years. These show Earth had a solid surface and maybe oceans very early.

Artist's conception of Hadean Eon Earth, when it was much hotter and inhospitable to all forms of life.

One theory tells us how the Moon formed. The giant impact hypothesis says that soon after Earth formed, it was hit by a smaller planet. This crash sent material into space, which came together to make the Moon. This event helped shape both Earth and its only natural satellite.

By the start of the Archean eon, Earth had cooled a lot. Life as we know it could not live on the surface because there was no oxygen in the air. But scientists think the first simple life may have started during this time, maybe around 3.5 billion years ago. Some believe life might have begun even earlier, deep under the ocean.

Formation of the Moon

Artist's impression of the enormous collision that probably formed the Moon

Main articles: Moon, Origin of the Moon, and Giant-impact hypothesis

Earth's Moon is big for a moon. Rocks from the Moon show it is about 4.53 billion years old. Scientists think the Moon formed when a large object, about the size of the planet Mars, hit early Earth. This crash sent material into space, which then came together to make the Moon.

First continents

Artist's impression of a Hadean landscape with the relatively newly formed Moon still looming closely over Earth and both bodies sustaining strong volcanism.

The movement of Earth's outer layers, called plate tectonics, began shaping the planet early. The first pieces of land, called continents, started forming near the end of the Hadean eon, around 4.0 billion years ago. These early lands were made of lighter rocks that floated on heavier rocks below. The oldest pieces of these first continents are found in Canada.

Oceans and atmosphere

Earth may have had three different atmospheres. The first was made of light gases like hydrogen and helium, but these were blown away. After the Moon-forming crash, Earth released gases, making a new atmosphere that trapped heat but had little oxygen. Later, around 2.8 billion years ago, simple bacteria began making oxygen, changing the atmosphere forever.

Water probably came to Earth from comets and asteroids. As Earth cooled, clouds formed, and rain made the oceans. Evidence suggests oceans may have formed as early as 4.4 billion years ago.

Origin of life

Main article: Abiogenesis

Life on Earth began in the oceans under very different conditions from today. Scientists have many ideas about how non-living chemicals could have become the first living cells. One famous experiment showed that simple building blocks of life, like amino acids, could form under early Earth conditions.

Different theories suggest steps in life's beginning, such as RNA molecules that can copy themselves, protein molecules that help build cells, or lipid bubbles that could protect early cells. These ideas help scientists learn how life might have started on our planet.

Proterozoic Eon

Main article: Proterozoic

The Proterozoic eon lasted from 2.5 billion to 538.8 million years ago. During this time, land grew into continents like the ones we have today. A big change was more oxygen in the air, which was very important for life. Simple life became more complex and lived in many ways. The Proterozoic had very cold times called Snowball Earths, when much of Earth was covered in ice. After the last Snowball Earth, about 600 million years ago, life on Earth began to change and grow faster. Around 580 million years ago, new life forms appeared, setting the stage for a big burst of life called the Cambrian Explosion.

Oxygen revolution

Main article: Great Oxidation Event

Lithified stromatolites on the shores of Lake Thetis, Western Australia. Archean stromatolites are the first direct fossil traces of life on Earth.

Snowball Earth

Main article: Snowball Earth

The Sun was slowly getting brighter during the Proterozoic, which should have made Earth warmer. But instead, Earth grew very cold at times. Evidence shows that about 2.2 billion years ago, Earth may have frozen over, an event called the Huronian glaciation. Later, between 750 and 580 million years ago, Earth went through four very cold periods where much of the planet was covered in ice. Scientists think these icy times might have been caused by the way the continents were arranged or by changes in gases in the air. When the ice melted, life began to grow and change quickly again.

Emergence of eukaryotes

Further information: Eukaryote § Origin of eukaryotes

Scientists group all living things into three big groups. Over time, these groups changed and split apart. Around 2 billion years ago, a simple cell was joined by a smaller cell that could use oxygen. The smaller cell lived inside the larger one and helped it get more energy. This partnership became very important, and the two cells worked together to survive. Later, other cells brought in green, oxygen-making organisms, which also became part of the larger cells. These changes helped life become more complex and led to many new kinds of organisms. Around 1.1 billion years ago, the first groups that would become plants, animals, and fungi began to appear, and some of them started living together in groups.

Supercontinents in the Proterozoic

Main article: Supercontinent cycle

Artist's rendition of an oxinated fully-frozen Snowball Earth with no remaining liquid surface water.

We can learn about Earth’s past by looking at how continents moved. Sometimes, the continents came together to form a huge landmass called a supercontinent, and later they split apart again. About 1 billion years ago, most land was joined in a supercontinent called Rodinia. Rodinia later broke apart, and the continents may have joined again for a short time around 550 million years ago. By the end of the Proterozoic, most land was gathered near the South Pole.

Late Proterozoic climate and life

Near the end of the Proterozoic, Earth went through two very cold periods where the oceans may have frozen over completely. These happened about 716.5 and 635 million years ago. Scientists are still learning why these icy times happened. After these cold periods, life on Earth changed quickly. New kinds of life appeared, and they were larger and more different than before. These new life forms were part of the Ediacaran period, and they helped set the stage for even more changes in life on Earth.

Phanerozoic Eon

Main article: Phanerozoic

The Phanerozoic is the time period on Earth that started about 538.8 million years ago. It has three parts: the Paleozoic, Mesozoic, and Cenozoic. This is when many different kinds of life forms appeared, including most animals we know today.

The Paleozoic era was the first and longest part of the Phanerozoic. During this time, many types of life began to exist, and plants and animals first appeared on land. Two big events happened that caused many living things to die out. The landmasses of Earth moved together to form a huge continent called Pangaea.

The Mesozoic era lasted from 251.9 million years ago to 66 million years ago. It had three smaller time periods: the Triassic, Jurassic, and Cretaceous. This era began with a big event that caused many species to die out, and it ended with another event that caused the dinosaurs to disappear.

The Cenozoic era started 66 million years ago. It includes the Paleogene, Neogene, and Quaternary periods. During this time, animals like mammals and birds survived and changed to fit their environments. This era continues today.