History of Earth

The natural history of Earth concerns the development of planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to understanding of the main events of Earth's past, characterized by constant geological change and biological evolution.

Earth formed around 4.54 billion years ago, approximately one-third the age of the universe, by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere and then the ocean. Over time, Earth cooled, causing the formation of a solid crust, and allowing liquid water on the surface.

The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago. There are microbial mat fossils such as stromatolites found in very old rocks. Photosynthetic organisms appeared later and began changing the atmosphere. Complex multicellular life arose much more recently, leading to a great diversification of life forms. Earth's crust and its life continue to change through processes like plate tectonics.

Eons

In the study of Earth's past, time is measured in million 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.

Geologic time scale

Main article: Geologic time scale

The story of our planet Earth can be organized using something called the geologic time scale. This scale breaks down Earth's long history into different parts based on layers of rock.

There are five special timelines that show how we divide Earth's history. The first one shows all of Earth's time, but it squashes the most recent time a bit. The next four timelines zoom in closer, showing more detail on the latest parts of Earth's story. Each one gives us a better look at what happened more recently.

Solar System formation

Main article: Formation and evolution of the Solar System

See also: Planetary differentiation

The Solar System, including Earth, formed from a big cloud of dust and gas called the solar nebula. This cloud was mostly hydrogen and helium, with some heavier elements from old stars called supernovae. About 4.5 billion years ago, the cloud started to shrink, maybe because of a shock wave from a nearby supernova. As it shrank, it spun and flattened into a disk.

In the center of this disk, the material squeezed together so much that it became hot enough to start a fire, creating the Sun. Meanwhile, in the outer parts of the disk, tiny bits of dust stuck together to form bigger and bigger pieces, eventually making planets. Earth formed in this way about 4.54 billion years ago and was mostly done in just 10 to 20 million years. Scientists even think Earth might have formed in just three million years, which is much faster than we used to think. The wind from the young Sun blew away most of the leftover material, leaving behind the planets we see today.

Hadean and Archean Eons

Main articles: Hadean and Archean

The Hadean eon marks the beginning of Earth's history, starting with its formation and ending around 3.8 billion years ago. It was followed by the Archean eon. The oldest rocks on Earth date back to about 4.0 billion years ago, and tiny crystals called zircons are even older, dating to around 4.4 billion years. These findings suggest Earth had a solid crust and possibly oceans very early in its history.

One important theory explains how the Moon formed. According to the giant impact hypothesis, shortly after Earth formed, it was struck by a smaller planet. This collision threw material into space, which eventually came together to form the Moon. This event helped shape both Earth and its only natural satellite.

By the start of the Archean eon, Earth had cooled down significantly. Life as we know it could not survive on the surface because there was no protective layer of oxygen in the atmosphere. However, scientists believe the first simple life forms may have begun to develop during this time, possibly around 3.5 billion years ago. Some even think life might have started even earlier, during the Hadean eon, in environments deep beneath the ocean's surface.

Formation of the Moon

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

Earth's Moon is special because it is large compared to the planet it orbits. Rocks brought back 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, crashed into the early Earth. This collision threw material into space, which then clumped together to create the Moon.

First continents

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

Oceans and atmosphere

Earth may have had three different atmospheres over time. The first was made of light gases like hydrogen and helium, but these were blown away by solar winds. After the Moon-forming collision, Earth released gases from inside, creating a new atmosphere rich in gases that trap heat but poor in oxygen. Later, around 2.8 billion years ago, simple bacteria began producing oxygen, changing the atmosphere forever.

Water likely reached Earth through comets and asteroids. As Earth cooled, clouds formed, and rain created 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 conditions very different from today. Scientists have many ideas about how non-living chemicals could have turned into 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 various steps in life's beginning, such as RNA molecules that can copy themselves, protein molecules that help build and maintain cells, or lipid bubbles that could protect early cells. These ideas help scientists understand 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, landmasses grew into continents similar in size to those we have today. A major change was the increase of oxygen in the atmosphere, which was very important for life. Simple life forms evolved into more complex ones, and life began to live in many different ways. The Proterozoic also had some very cold periods called Snowball Earths, where much of the planet was covered in ice. After the last Snowball Earth, about 600 million years ago, life on Earth began to change and grow more quickly. 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

See also: Ozone layer

The earliest life forms got their energy from the environment around them. They used a process called fermentation, which works only without oxygen. Later, some life forms learned to use sunlight to make food, a process called photosynthesis. This was very important because it let life use the Sun’s energy. One kind of photosynthesis makes oxygen as a by-product. At first, this oxygen combined with minerals in the ground. But over time, oxygen built up in the air. This oxygen helped form a layer in the sky called the ozone layer, which protects life from harmful sunlight. However, oxygen was also dangerous to many early life forms, and many died when it appeared. Those that survived changed and learned to use oxygen to get even more energy from food.

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

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 current time period on Earth, which 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 of the 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.