If you were going to put the entire timespan of the Earth's existence (around 4.6 billion years) on a clock, the time that humans have been here only accounts for about a minute. We've existed for about 0.004 percent of the Earth’s total age.
That's billions of years of time before we even came on the scene. What happened the rest of the time when we weren't here? When did life and living things first arise on Earth?
Let's go over the history of life on Earth including when it first arose, early theories of how living things evolved, the origin of life through the eons and how we got to where we are today.
History of Life on Earth: Earth's Timeline
The timeline of the Earth is broken up into chunks of time called "eons." Each of these eons marks important events in the life of the planet and the history of life on Earth.
The Hadean Eon is named after the Greek god Hades. At the time of its formation 4.6 billion years ago, the Earth was essentially a large, extremely hot (above the boiling point of water, hot) ball of toxic gas, lava, explosions, asteroids and metals. In other words, it was a toxic hellscape.
Not only that, but no rocks, continents, or oceans had formed yet. Terrestrial and marine environments that exist on Earth now are crucial for the evolution of life because they provide space, materials, climate and other features that organisms need to survive and thrive.
Knowing that, it's understandable that this eon, which lasted for 6 million years, could not sustain any life.
However, this early Earth did have one significant event that's thought to have sparked one of the crucial elements of life. The heavy bombardment stage was a period during the Hadean Eon when the Earth was bombarded with space debris, asteroids and other matter.
Scientists believe that these asteroids may have helped spark the formation of DNA, liquid water and important geologic formations.
Archean Eon: The True Origin of Life
After the Hadean Eon came the Archean Eon, which lasted from 4.0 billion to 2.5 billion years ago.
The first major event for the evolution of life was the Theia impact, or the formation of the moon. During the Hadean Eon, Earth was spinning significantly faster than it does now. This made the Earth unstable and produced extreme weather/climate patterns.
In what's known as the Theia impact, a Mars-sized object collided with the Earth, which resulted in large pieces of debris breaking apart. It's believed that Earth's gravitational force kept the larger pieces in its orbit, and they came together to form one large body that we now know as the moon.
After this large impact, the rotation slowed and stabilized, which may have resulted in the tilt of the Earth and led to seasonal changes that we now know are an important factor in creating ecosystems, biomes and organism adaptations.
Besides that, three very important events occurred during this time period:
- Oceans formed.
- The first evidence of life appeared.
- Continents and rocks began to form (an estimated 40 percent of continents formed during this period).
As the Earth cooled and the layers of the Earth formed, large amounts of water vapor were released. The temperature continued to drop, which allowed that water vapor to cool to liquid water and form the oceans around 3.8 billion years ago.
What does this mean? It means that life most likely first emerged in the oceans because the oceans formed first, and they are where the first fossil evidence of life was discovered. Also during this time period, there was no useable oxygen in the atmosphere, which means the first life forms were anaerobic.
Theories of How Life Emerged
The main theory of how life emerged is known as the "primordial soup" theory or abiogenesis.
Primordial soup: Scientists theorized that once the oceans formed, all of the components, elements and matter that are necessary for the creation of life and life's complex molecules (proteins, DNA and so on) were floating around in a kind of "primordial soup."
They believe that all that this needed was a spark of energy (like a lightning strike or an explosion, both of which were common in the early Earth's environment) in order to create essential molecules for lifelike amino acids/proteins and nucleic acids (genetic material). The Miller-Urey experiment replicated the conditions of early Earth to show that chemical reactions could occur this way to create simple amino acids.
Once those molecules were created, scientists believe that things arose gradually, slowly creating more and more complex molecules via simple chemical reactions. Once the building blocks were created, they eventually all came together to form living organisms. This gradual formation of life from inorganic molecules is also known as the Oparin-Haldane hypothesis.
Asteroids: Another theory has to do with the heavy bombardment stage. Early Earth was constantly bombarded with asteroids and space matter. Some scientists theorize that molecules for life, or even life forms themselves, were transported to Earth via these asteroids.
First Life Forms
Scientists theorize that RNA-based unicellular organisms formed at hydrothermal vents deep in the ocean around 3.8 billion years ago.
Scientists discovered fossil evidence of algal mats and used radiometric dating techniques to date them at about 3.7 billion years old. Cyanobacteria fossils were also found and dated at approximately 3.5 billion years old.
Not only was this pivotal in the sense that these are the first known living organisms on Earth, but they also set up the foundation for the emergence of life as we know it today. These organisms were producers/autotrophs, meaning they created their own food and energy using light from the sun using photosynthesis.
Photosynthesis uses the sun's light plus carbon dioxide to yield sugar and oxygen. These examples of early life and early organisms were responsible for creating almost all of Earth's oxygen, which allowed for more life moving forward. The creation of the Earth's oxygen by these organisms is called the Great Oxygenation Event. (You may also see the term "Great Oxidation Event.")
At this point, it's hypothesized that all life was anaerobic and prokaryotic. Evidence of terrestrial life didn't emerge until 3.2 billion years ago, after the formation of continents. And since the ozone layer hadn't formed yet, UV radiation from the sun made most all land life on Earth's crust impossible, keeping almost all life in the ocean.
The Proterozoic Eon followed the Archean, lasting from 2500 million to 541 million years ago.
After the Great Oxygenation Event, all of those original anaerobic organisms died off because oxygen was toxic to them. Ironically, their own life and their increase in Earth's oxygen levels led to their extinction.
Life was about to be tested yet again, however. All of the new oxygen reacted with the high levels of methane in the atmosphere to create carbon dioxide. This rapidly decreased the Earth's temperature, plunging it into "snowball Earth," which was an ice age that lasted about 300 million years.
Also occurring during this eon was the formation of the tectonic plates and the full formation of the continents on the Earth's crust.
Increasing oxygen levels also allowed for the formation and thickening of the ozone layer, which protects the Earth from dangerous radiation from the sun. This allowed for life to emerge on land.
It was also during this eon that eukaryotic cells arose, including the first multicellular organisms and multicellular life. Eukaryotic cells emerged when simple cells engulfed other cells, including mitochondrial and chloroplast-like cells, forming one larger and complex cell. This is called the endosymbiotic theory.
Life from here diverged and evolved from just prokaryotic and single-celled organisms like bacteria and archaea into eukaryotic and multicellular life like fungi, plants and animals.
After the Proterozoic Eon came the Phanerozoic Eon. This is the current eon, and it's divided into eras, periods, epochs and ages.
Perhaps the next biggest event in the evolution of life is the called the Cambrian explosion. It took place in the Paleozoic Era, which lasted from 541 million to 245-252 million years ago. (Era years can change slightly depending on the source you find.)
Before the Cambrian explosion, most life was small and very simple. The Cambrian explosion was the explosion and diversification of life on Earth, specifically the sudden emergence and complexity of animals and plants.
Scientists believe this is because of the rise in oxygen levels in the atmosphere, the end of snowball Earth and the development of favorable environmental conditions for life to increase in complexity.
First came the "age of invertebrates." Hard-shelled invertebrates evolved from soft-shelled ones. Next came fish and marine vertebrates, and from there, those fish evolved into amphibians and land- and water-dwelling animals.
Almost all land animals evolved from these marine and fish common ancestors. They evolved to have spines, vertebrates, jaws and limbs. Vertebrates first appeared in the fossil record around 530 million years ago.
There was also a huge explosion of plants and forests, including rainforests, around the world. This led to another huge increase in the oxygen levels in the atmosphere because of these plants' photosynthesis byproducts. Insects emerged, and they were gigantic because of the large amount of available oxygen.
Mass extinction events: All of this new life came to a crashing halt with the Carboniferous Rainforest Collapse. Because of rapid climate change, it led to the first mass extinction of many of these new forests and plants.
In the place of these forests came large deserts, which lead to the evolution and domination of reptiles.
However, they weren't safe. Another mass extinction ended this era, called the Permian-Triassic extinction. The fossil record and fossil evidence suggest that an asteroid strike killed 96 percent of life in the ocean and 70 percent of terrestrial vertebrates.
After that extinction event killed most life on Earth, reptiles and dinosaurs emerged to dominate the deserts left behind.
Dinosaurs dominated as the main life on Earth for about 160 million years. And from dinosaurs came the later evolution of birds.
Plant life took a turn during the Mesozoic; the era is sometimes called the Age of Conifers. Plants evolved a new way to reproduce with the evolution of the first coniferous trees (they use seed germination).
As more plants came back after the previous extinction event, oxygen levels increased again, which allowed for very large organisms. Remember how big Tyrannosaurus Rexes were? That's because there was so much oxygen in the atmosphere to support such huge organisms.
The Mesozoic also ended with a mass extinction event called the K-T extinction (also known as the Cretaceous-Paleogene extinction event) as a result of another asteroid impact.
Almost all species became extinct except for marine life and very small mammals.
The Cenozoic Era began right after the K-T extinction 66 million years ago, and it is the era we're in right now.
After the extinction event, life diversified again with mammals emerging as the dominant animal species. This included the emergence of large marine mammals like whales and large terrestrial mammals like mammoths.
Plants diversified and grasses developed as the continents drifted to their present-day formations instead of remaining as one of the many supercontinents that emerged over the history of the Earth.
In terms of our own lives, our common ancestor and the first primate emerged about 25 million years ago. The first hominid emerged around 3 million years ago, with the first Homo sapiens in Africa 300,000 years ago.
Currently, we are in the Phanerozoic Eon, Cenozoic Era, Quaternary Period. Most sources list the Holocene Epoch as the current epoch (if you'd really like to be specific, the last age of the Holocene Epoch is the Meghalayan Age), but in the 2000s, scientists became more convinced that humans had begun another epoch called the Anthropocene Epoch.
In May 2019, the Anthropocene Working Group, a group that is part of the International Commission on Stratigraphy, voted to make the Anthropocene Epoch a part of the Geological Time Scale, with the mid-20th century as an approximate starting point.
This does not yet mean that the Anthropocene is totally official as the group still needs to get approval from both the International Commission on Stratigraphy and the International Union of Geological Sciences. However, it is a substantial step in the process of delineating a new epoch.
Holocene extinction: The planet could very well be on its way to another drastic life change as we've seen happen in many eras of the Earth's history. Scientists say that because of human impact on the Earth's environment and climate, there is a mass extinction happening in the present day called the "Holocene extinction."
Unless we change our impacts on the environment, specifically those affecting climate change, we could be looking at another huge shift and extinction of life (including ourselves) in the near future.
- Khan Academy: Hypotheses About the Origins of life
- Encyclopaedia Britannica: K-T Extinction
- Royal Ontario Museum: The Burgess Shale
- The Guardian: Earth's Sixth Mass Extinction Event Under Way, Scientists Warn
- LiveScience: Have There Always Been Continents?
- Earth How: Earth Timeline: A Guide to Earth’s Geological History and Events
- Cosmos Magazine: The Big Five Mass Extinctions
- NewScientist: Timeline: The Evolution of Life
- Arizona State University: Ask a Biologist: Cells Living in Cells
- Nature: Anthropocene Now: Influential Panel Votes to Recognize Earth’s New Epoch
- Subcommission on Quaternary Stratigraphy: Results of Binding Vote by AWG
- The University of Texas: The Origin and Evolution of Life on Earth
About the Author
Elliot Walsh holds a B.S in Cell and Developmental Biology and a B.A in English Literature from the University of Rochester. He's worked in multiple academic research labs, at a pharmaceutical company, as a TA for chemistry, and as a tutor in STEM subjects. He's currently working full-time as a content writer and editor.