Meghan Hodges
Earth has undergone remarkable changes over billions of years. Scientists have studied the history of Earth, from its beginning to the present day, and have discovered that the planet has gone through significant transformations. The early Earth was uninhabitable, with a toxic atmosphere of hydrogen and helium. As the planet cooled, volcanic eruptions released water vapour into the atmosphere, which condensed and returned to Earth as rainfall, forming the oceans. The continents moved around, split apart, and collided until they reached their present locations. The atmosphere became oxygen-rich, and the first complex organisms developed around 2 billion years ago. Human activities have since had an unprecedented impact on the planet, with pollution from industrial processes, automobiles, and the burning of coal causing serious environmental issues. NASA has tracked these transformations via satellite, providing a visual record of the changes on Earth.
| Characteristics | Values |
|---|---|
| First atmosphere | Hydrogen and helium |
| Second atmosphere | Water vapour, carbon monoxide, methane, ammonia, nitrogen, carbon dioxide, hydrochloric acid, and sulfur |
| First life forms | Anaerobic simple cells |
| First complex life forms | 2 billion years ago |
| Tectonic plates | Moving, splitting, and colliding |
| Oceans | Opened up and moved around the globe |
| Weathering and erosion | Caused by rainfall and storms |
| Pollution | Caused by bacteria, human and animal waste, and garbage |
| Industrial Revolution | Air pollution from factories and homes |
| 20th Century | Increase in water and air pollution |
| 21st Century | Shifting to cleaner energy sources |
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What You'll Learn
The Earth's formation
The Earth formed around 4.5 billion years ago, alongside the rest of the Solar System. The standard model for the formation of the Solar System is the solar nebula hypothesis. According to this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. This cloud was composed of hydrogen and helium, created shortly after the Big Bang, and heavier elements ejected by supernovae.
As the cloud of dust and gas that would become the Solar System cooled, hydrogen atoms began to move more rapidly and violently. Charged hydrogen atoms, or protons, started fusing, forming helium and releasing massive amounts of energy. This led to the formation of the Sun, which consumed more than 99% of the matter in the nebula. The remaining material began to clump together into various masses, which eventually grew large enough to have a gravitational pull, shaping them into the planets and dwarf planets that make up our Solar System today.
The proto-Earth grew by accretion until its interior was hot enough to melt heavy, siderophile metals. These metals, such as iron and nickel, had higher densities than the silicates, so they sank towards the centre of the planet. This process, known as the iron catastrophe, resulted in the separation of a primitive mantle and a metallic core, producing the layered structure of the Earth and setting up the formation of its magnetic field.
The Moon is thought to have formed during a giant impact between the proto-Earth and another protoplanet, possibly called Theia, around 4.5 billion years ago. This collision may have resulted in the formation of the Earth's moon from the impact debris, a mixture of molten rock and hot gas, flung out into space.
Over time, the Earth cooled, forming a solid crust and allowing liquid water on the surface. The Hadean eon, the first eon in Earth's history, represents the time before a reliable fossil record of life. It began with the formation of the planet and ended 4 billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution.
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Pre-Cambrian period
The Precambrian is an informal geologic time unit that spans from the formation of Earth about 4.6 billion years ago to the beginning of the Cambrian Period around 540 million years ago. It is divided into three eons: Hadean, Archean, and Proterozoic. The Precambrian is the earliest part of Earth's history, and relatively little is known about it.
During the earliest Precambrian (Hadean), the Earth was cooling down from an initially liquid state, resembling a giant planetary lava lake. The Earth would have looked like a water world with live oceans and dead desertic continents. The colour of the ocean and atmosphere would have looked familiar.
In the Archean eon, cyanobacteria were the dominant life forms, living in reef-like structures known as stromatolites. Extensive evaporites dating to 3.5 billion years ago from Western Australia suggest that the Earth had warm, tropical waters.
During the Proterozoic eon, the first multicellular organisms evolved, and sexual division developed. There may have been a super glaciation in the late Proterozoic, known as "Snowball Earth", where much of the planet was covered in ice. When the ice melted, conditions were set for the explosion of life that took place at the start of the Cambrian.
The Precambrian was a time of significant tectonic activity, with the North American craton colliding with microcontinents and oceanic island arcs, adding to the continents. It is believed that most of Earth's landmasses collected into a single supercontinent called Rodinia around 1130 million years ago.
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The Hadean period
During this period, the Earth's surface was molten with superheated lava due to the abundance of short-lived radioactive elements and frequent impacts with other bodies in the Solar System. The Earth's atmosphere was very thick and rich in hydride, water vapour, methane, and ammonia. As the Earth's surface cooled, water vapour condensed into liquid water, and a superocean covering nearly the entire planet was formed.
The Earth's early atmosphere slowly became more oxygen-rich as solar radiation split water molecules and cyanobacteria began the process of photosynthesis. The formation of the Earth's continents and oceans was also underway, with the continents in very different locations than they are today. The Hadean eon was followed by the Archean eon, with the Late Heavy Bombardment hypothesized to have occurred at the boundary between the two periods.
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Earth's early atmosphere
Initially, the atmosphere was not able to support life. It was rich in methane, ammonia, water vapour, and the noble gas neon, but lacked free oxygen. Early volcanic eruptions released large amounts of water vapour into the atmosphere, which condensed and returned to Earth in rainfall, forming the oceans.
Over time, solar radiation split water molecules, and cyanobacteria began the process of photosynthesis, slowly enriching the atmosphere with oxygen. This process was aided by photosynthesising organisms in the oceans, which further increased oxygen levels. The first complex organisms on Earth developed around 2 billion years ago.
Humans have had a significant impact on the Earth's atmosphere, particularly since the industrial period. Air pollution and climate change have led to the shrinking of the ozone layer. NASA has tracked these transformations through satellite images, showing the impact of human activities on the planet.
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The Anthropocene epoch
The term Anthropocene, meaning the "recent age of man", was coined by American biologist Eugene Stoermer in the late 1980s and popularised by Dutch chemist and Nobelist Paul Crutzen in 2000. Crutzen drew attention to the significant influence of human behaviour on Earth's atmosphere in recent centuries, suggesting that we have entered a new geological epoch. The Anthropocene is also referred to as the "Age of Humans", underscoring the unprecedented impact of human activities on the planet.
The concept of the Anthropocene has garnered significant attention in both scientific and public spheres. In 2008, British geologist Jan Zalasiewicz and his colleagues put forth the first proposal to formally adopt the Anthropocene Epoch as a geologic interval. In 2016, the Anthropocene Working Group (AWG) of the International Union of Geologic Sciences (IUGS) voted in favour of recommending the Anthropocene as a formal geologic epoch. However, in 2024, the AWG's proposal was voted down by the Subcommission on Quaternary Stratigraphy (SQS) due to its shallow sedimentary record and recent proposed start date. Despite this rejection, the term Anthropocene continues to be widely used and recognised as a descriptor of human impact on the Earth system.
The Anthropocene is marked by significant transformations of the Earth's natural systems. Humans have reshaped the planet by clearing forests, damming rivers, constructing vast roads, and transporting species across the globe. NASA's "Images of Change" series visually captures these changes, showcasing before-and-after images of rainforests, glaciers, and cities over time. The loss of biodiversity, climate change, and ozone layer depletion are among the pressing issues that define this epoch. The recognition of the Anthropocene invites cross-disciplinary scientific collaboration and underscores the importance of human agency in shaping the future of our planet.
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Frequently asked questions
The Earth has undergone remarkable changes over billions of years. Here's what it might have looked like before human-induced pollution:
- The atmosphere was predominantly made up of hydrogen and helium, which was toxic to all life.
- The Earth was forming, and it was hot due to continuous bombardment by asteroids, meteors, and comets.
- The continents were in different locations, and the oceans were moving around the globe.
- There were no complex life forms, only simple cells that did not require oxygen to live.
Human activities have had a significant impact on Earth, with pollution being a problem since the earliest ancestors. Here are some examples of how human activities have contributed to pollution:
- Industrial processes and the burning of coal in factories have released pollutants into the air.
- Automobiles and motor vehicles have contributed to air pollution.
- Unsanitary conditions during the Middle Ages caused by human and animal waste led to diseases such as cholera and typhoid fever.
- Industrial waste and chemicals contaminated waters, with rivers like the Cuyahoga in the United States catching fire due to the concentration of debris and oil on the surface.
NASA has tracked major transformations of the Earth's surface through its "Images of Change" series, providing before-and-after images of the same locations. Here are some examples:
- The Paraguay-Parana River: The construction of the Yacyreta Dam in 1985 displaced 15,000 residents and altered the natural flow of the river.
- The Aral Sea: Once the fourth-largest lake in the world, it has nearly disappeared due to irrigation and agricultural practices.
- Dubai, United Arab Emirates: The rapid growth of the city between 2000 and 2011, with the construction of artificial islands and tourism developments, has resulted in a loss of biodiversity and polluted water.
