Different Ages of the World

How has life on Earth developed over its nearly 5 billion years to what it is today?

Introduction:

The origin of life on Earth is one of science's greatest mysteries. Earth is roughly 4.5 billion years old and has transformed from a molten sphere to the continental, diverse, and life-sustaining planet it is today. This process has occurred slowly and gradually over billions of years; scientists have divided up Earth’s history into roughly four different eons (an eon is a period lasting approximately one billion years): (1) the Hadean Eon lasting from around 4.5 billion years ago (also written as 4.5 Ga) to 4.1 Ga, (2) the Archean Eon going from 4.0-2.8 Ga, (3) the Proterozoic Eon occurring from 2.5-0.63 Ga, (4) and the Phanerozoic Eon lasting from 541 Ma through today.

Knowledge of the Hadean and Archean eons is inherently limited due to the nature of the planet at this time, which was still in the process of forming its surface, and geological structures that are now buried far below the surface. Nevertheless, the Phanerozoic, meaning visible life, has abundant and complex fossilized remains—most of our knowledge of the fossil record comes from this eon. Together, these four eons tell the story of Earth’s beginning, life’s origin, and the rise of complex life (LabXchange, 2019).

Early Life:

There was no life for the first 700 million years of Earth’s history. Scientists have traced the first signs of life back to the early 3.85 Ga. However, this life did not resemble what humans would consider to be life today. Instead, there were only microscopic single-celled organisms, such as bacteria, and it took another 200 million years for the first oxygen-producing life to evolve (NCSE, 2016).

The first method that archaeologists can use to detect early signatures of life is examining carbon isotopes (an isotope is an atom with a different amount of neutrons than protons). Life on Earth requires carbon, tending to incorporate more C12 over C13. The reason behind this preference is that C12 is lighter than C13, evident through having one less neutron, and this lighter weight is more readily accessible to the given organism. Therefore, carbon with a higher ratio of C12 compared to C13 is an indicator of life. This depletion in C13 compared to C12 was found in rocks from Greenland that date back to 3.85 Ga (NCSE, 2016).

While scientists predict that life on Earth first originated 3.85 billion years ago by comparing isotopes, there is physical evidence of life just 200 million years later, in the form of stromatolites, microfossils created by photosynthetic bacteria. These photosynthetic bacteria usually live in shallow water and over time become covered in clay and other particles, causing them to move towards the surface to receive the sunlight they need to survive. As the bacteria moves upwards, it would leave behind dead layers. Scientists discovered stromatolites in Western Australia dated at 3.45 Ga by analyzing the number of dead layers left behind  (NCSE, 2016).

Formation of Oxygen:

Humans take for granted the presence of oxygen and the diverse ecosystems of animals. The ecosystems today are structured by feeding relationships through the extraction of oxygen from their food. However, that is not how life on Earth used to be. For much of its early history, Earth was not an inhabitable environment for animals because it lacked oxygen and was high in methane. Our planet had a reducing atmosphere, consisting of carbon dioxide, methane, and water vapor as opposed to the present-day atmosphere, primarily composed of nitrogen and oxygen. The little oxygen that was present reacted with methane and was sealed in the Earth’s crust. However, cyanobacteria, a microbe requiring low levels of oxygen, worked to release oxygen until the atmosphere changed (National Museum of Natural History, n.d.).

As time passed, enough oxygen accumulated in Earth’s atmosphere to allow for the evolution of oxygen-metabolizing organisms. This increase in oxygen, now a major component of the atmosphere, made the environment uninhabitable for microbes that could not bear it. Evidence for this Great Oxidation/Oxygenation Event is visible in changes in seafloor rocks called Branded Iron Formations, through a reaction between oxygen and iron dissolved in seawater. This formed iron oxide minerals on the ocean floor. Scientists believe this marked the beginning of oxygen’s permanent presence in the atmosphere (American Society for Microbiology, 2022).

A revolutionary event occurred when microbes, single-celled organisms that lack organelles, began living inside other microbes, serving as organelles. Cells began living together because of the direct benefits of more efficient feeding and protection. These groups of specialized cells working together became the first animals around 800 million years ago. As time continued and the planet advanced during the Ediacaran Period (635 to 541 million years ago), there was an accumulation of other organisms. By the end of this period, oxygen levels rose, which brought them closer to levels sufficient to sustain oxygen-based life (National Museum of Natural History, n.d.). With an oxygen-rich atmosphere, life was able to rapidly evolve from organisms to full-fledged animals, such as dinosaurs.

Dinosaur Extinction:

Dinosaurs were a group of reptiles that dominated the land for over 140 million years taking on diverse shapes and sizes, symbolizing Earth’s history and evolution. Due to the lack of details, there is much discussion among paleontologists regarding what killed Earth’s Cretaceous inhabitants and how long it took them to become extinct. However, scientists are sure it was a massive event that impacted all types of life on Earth, from the smallest organisms to the largest dinosaurs. A common misconception portrayed today is that all life on Earth suddenly went extinct at the end of the Cretaceous Period (about 66 million years ago), when in fact it was only 75% of life. Furthermore, evidence depicts that dinosaurs were already in decline in the latest portion of this period (Britannica, 2024).

Earth was enduring a period of climate change before the asteroid crashed with notable shifts in the climate spanning over millions of years. For a few million years there was a great amount of volcanic activity occurring which emitted gasses into the atmosphere, resulting in a major impact on global climate. Some long-term changes during this period included the continents drifting around and splitting apart from each other. This created bigger oceans, impacting the climate and vegetation because of the change in the ocean-atmosphere patterns around the world. (Natural History Museum, n.d.).

The instant devastation by the asteroid, near the impact site on the coast of Mexico’s Yucatan Peninsula in Chicxulub crater, and the secondary effect of the asteroid impact are considered to be why the dinosaurs went extinct so suddenly. The asteroid is believed to have been between 10-15 kilometers wide. The velocity of the asteroid caused a much larger crater of 150 kilometers in diameter to form, increasing the total devastation in the vicinity. This brought massive tidal waves, heat waves, and huge amounts of debris into the atmosphere (Natural History Museum, n.d.).

When about 75% of Earth’s animals suddenly went extinct, there was a domino effect on the food chain leading the ecosystems of the land and the ocean to collapse. The initial impact reduced plant growth, leading to herbivores having trouble surviving, and causing carnivores to have less food. This brought shorter breeding seasons with harsher conditions. Overall, however, plants were less affected than animals because their seed and pollen allowed them to survive longer harsh periods. Flowering plants dominated Earth and what little life was able to survive formed the basis of the life known today. While life suffered some level of extinction, the ancestral lines that led to modern animals survived (Natural History Museum, n.d.).

Ice Ages:

Unknown to many, planet Earth has had at least five major ice age events, the most recent having begun about 2.6 million years ago and the glacial period ending just 11,000 years ago. In general, ice ages come to an end or shift to interglacial periods (periods in which glaciers melt) based on Earth’s orientation to the sun (Dubey, 2024).

The most recent ice age has come to be known as simply the “Ice Age.” During this ice age, early humans were in the Stone Age and roamed the planet with wooly mammoths; it was only after this ice age that humans emerged as the most dominant land animals. Even though this ice age lasted hundreds of thousands of years, it only reached its peak, or its glacial maximum, 20,000 years ago (History, 2015). Life on Earth was vastly colder than it is today: The average temperatures around the world during this peak were 5 degrees Celsius colder; 8% of the planet's surface was covered in ice, and sea levels were approximately 125 meters lower (Dubey, 2024).

Even though the glacier period of the Ice Age ended, technically Earth is still considered to be in an ice age. Ice ages last millions of years, and throughout every ice age, there are glacial periods, when glaciers spread, and interglacial periods, when glaciers melt (Dubey, 2024).

Conclusion:

The four eons of Earth’s history depict the planet’s evolution from a molten to a habitable environment. From the Hadean Eon with the formation of Earth to the first emergence of life in the Archeon to the Great Oxidation Event during the Proterozoic, and finally, the Phanerozoic taking us through history to the world we know today, these four eons tell the story of Earth’s history and the evolution of life.

References

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