Over 4.5 billion years ago, our solar system formed from a giant molecular cloud that collapsed under its own tremendous gravity. The hot stew of hydrogen and helium gave birth to our sun and flung out a wide disc of gas and particles in the surrounding space. Over millions of years, this disc of gas coalesced into the planets of the solar system, including Earth. Over its history, Earth has gone through several distinct periods that are divided by shifts in the Earth’s geological composition or major paleontological changes, such as mass extinctions.
The history of Earth can be divided into 4 major eons, each of which are divided into their own eras, periods, epochs, and ages. The most recent age is the Meghalyan which began about 2250 BCE and the current epoch is the Holocene which began around 11,000 years ago after the end of the previous ice age. Several geologists have suggested that we are currently in a new epoch, dubbed the Anthropocene in regards to human impact on Earth’s geology and ecosystems. This nomenclature has not been universally accepted though.
Earth’s History: A Timeline
Hadean Eon (4.6 – 4.0 billion years ago)
The Hadean is the first geological eon of Earth’s history. Ranging from 4.6 billion to 4 billion years ago, the name “Hadean” is a reference to the Hades, the Greek god of the underworld, and describes the hellish conditions present after the Earth’s initial formation. The planet was still extremely hot and molten due to its recent accretion and was rocked by global seismic activity, radioactive materials, and collisions with other objects in the solar system. Constant barrage from planetesimals and meteors left the surface of the Earth a crater-ridden, volcanic and turbulent landscape.
Being so early in Earth’s history, much of what we know about the Hadean eon is tenuous and speculative. It is hypothesized that the moon formed during this time period, as a result of the collision of the Earth and a Mars-sized astronomical object. This event, dubbed the Theia Impact as a reference to the Greek titan Theia, is speculated to have occurred in the early Hadean eon, some 20 million to 100 million years after the formation of the solar system.
The first rocks were believed to have formed during this period also, and there is evidence that despite high surface temperatures of nearly 230 °C, liquid water still existed on the surface due to pressure from a dense and heavy atmosphere of carbon dioxide. There is also evidence that Earth’s tectonic plates began forming late in this period.
Archean Eon (4.0 – 2.5 billion years ago)
The next eon of Earth’s history, called the Archean eon, started with the close of the Hadean eon 4 billion years ago and lasted until about 2.5 billion years ago. The two most important events that occurred during the Archean eon were the formation of the continents and oldest rock formation, and the emergence of life on Earth.
Most of the rocks formed during the Archean were made under conditions of high heat and pressure, leading to high concentrations of granitic rock in the Archean crust. Sedimentary rocks also formed in the oceans It is thought that 5-40% of the currently existing continents were formed during the Archean period. The first full-fledged continent to be formed in the Archean is called Ur and existed about 3.1 billion years ago. The continent of Ur broke up a very long time ago, and pieces of it are now found in India, South Africa, and Australia.
Though the specifics are not known, the first kinds of living organisms developed during this period. The earliest evidence of life on Earth comes from graphite of biological origin discovered in Greenland that dates 3.7 billion years old. The earliest identifiable fossils have been dated to about 3.5 billion years old and consist of the remains of algal mats composed of bacteria and archaea. Several rocks from this era also show an abnormally high sulfur content, which is evidence for the activity of sulfidogenic bacteria. Organic analysis of sediment layers in the Earths crust indicate that land-dwelling microbes emerged approximately 3.22 billion years ago
At the time, life on Earth was entirely prokaryotic (either bacteria or archaea) and anaerobic due to the low oxygen-content of early Earth’s atmosphere. Currently, there are no known eukaryote fossils from the Archean eon, though it is possible eukaryotic life existed and just did not leave any fossils. However, one particular kind of bacteria in the Earth’s oceans called cyanobacteria was unique. Cyanobacteria produced oxygen and their activity in the Earth’s oceans over billions of years raised the Earth’s oxygen content, leading to a new geological age.
Proterozoic Eon (2,500 – 541 million years ago)
The oxygenation of the atmosphere by cyanobacteria had extreme consequences. One iron deposits in the oceans had been filled, produced oxygen began to accumulate in the atmosphere. Oxygen was poisonous to anaerobic life so the activity of cyanobacteria killed off many anaerobic species. This oxygenation also had the effect of allowing the development of multicellular eukaryotic life. Oxygen is very reactive so the oxygenation of the atmosphere allowed the development of organisms that could use oxygen. It is believed that the first eukaryotes emerged from the symbiosis of prokaryotic life.
At the time, the atmosphere was made of mostly methane which is a very potent greenhouse gas. The accumulated oxygen reacted with the methane, making carbon dioxide. The lowered greenhouse effect caused a global drop in temperatures and plunged the earth into a 300,000,000 year long ice age. During this “snowball Earth” period, the entire surface of the Earth was frozen or nearly frozen.
During this time period, the Earth went through a high amount of tectonic activity resulting in the formation and fragmenting of a number of supercontinents, including ones named Columbia, Rodina, Laurentia, and Gondwana. Radiation from the sun still made land relatively inhospitable, so most life existed in the oceans. Over time, oxygen accumulation created the ozone layer which allowed life to diversify on land.
Paleozoic Era (541 – 245 million years ago)
The beginning of the Paleozoic era in the Cambrian period signified the largest diversification of life in Earth’s history. Starting with the development of hard-shelled invertebrates in the ocean, the diversity of life saw a huge uptick that resulted in the development of amphibians, reptiles, and mammals.
First came the development of fish and other invertebrates in the ocean. Eventually, certain fish species moved to the land. these first fish species to make their way onto the land are the ancestors of every land-dwelling eukaryote since, humans included. The first vertebrate land animals were amphibians who began to populate the continent of Gondwana. Around this time, the first arthropods and insects also emerged.
During this time, large plant species flourished on land and contributed to the accumulation of oxygen. Major shifts in the Earth’s climate contributed to the destruction of theses forests resulting in an event called the Carboniferous Rainforest Collapse. The compacting of the forest formed the majority of coal deposits in the world and the collapse of the forests dominating the land cleared the space for the development of reptiles.
The Paleozoic Era ended with the Permian-Triassic extinction event which killed off 96% of marine life and 70% of terrestrial life. The dominant theory on the nature of the Permian-Triassic extinction event is that a giant meteor struck Earth, which caused long term climate damage and volcanic eruptions.
Mesozoic Era (245 – 66 million years ago)
The destruction of massive land forests free ups pace to allow reptiles to become the dominant land species. Reptiles laid eggs in a hard shell so, unlike amphibians, they did not need to have any ties to the ocean to survive. Early in the Mesozoic era saw the emergence of dinosaurs (“terrible lizards”). These creatures dominated land for 160 million years and adapted to be both herbivores and carnivores. The first coniferous trees also developed around this time, making seed germination a common reproductive mechanism in plants.
The most recent supercontinent existed in the Mesozoic era as well. The supercontinent of Pangea was the most recent supercontinent to exist and the first one reconstructed by geologists. As Pangea slowly broke up over millions of years, each piece carried its population of dinosaurs with it. The continental breakup and transport explain why we find the same dinosaur bones on different present-day continents.
The Mesozoic era ended with the decline of the dinosaurs in the KT extinction event. 65 million years ago, a giant meteor struck into the present-day Yucatan peninsula. The global chaos caused the dinosaurs and most terrestrial species to go extinct, and only spared small mammals and organisms deep in the oceans. Some groups of dinosaurs survived and eventually developed into modern-day birds and avians. Evidence for the KT Extinction event includes the presence of a thin layer of iridium found globally in the Earth’s crust, a substance that is rare on Earth but more common in meteors.
Cenozoic Era (66 million – present day)
The extinction of the dinosaurs allowed that mammals could emerge and become the dominant land species. Most modern mammals can trace their lineage back to mammals that emerged after the KT extinction event. The continents began to take the current formation around this time as well. The first grasses evolved, and the first groups of mammals develop in the oceans and include primitive whales.
About 25 million years ago, most of the present day species on earth became recognizable. The first primate develops and begin living partially in the trees and partially in more plains-like areas. The first horses and the ancestors of modern elephants also emerge.
Fast forward to about 3 million years ago and we see the evolution of the earliest hominids including Sahelanthropus and Australopithecus. The development of hominids saw the first tools and the mastery of fire. Midway through the Quaternary period about 300,000 years ago, the first Homo sapiens in Africa. Over the next hundred thousand years, humans would migrate out of African and populate the entire world. The first behaviorally and anatomically modern humans emerged 50,000 years ago.
The timescale of Earth’s history is vast and there is still much we do not know. 4.5 billion years is an unfathomably long time and there are gaps in our understanding. The sheer longevity of Earth completely dwarfs human history; in fact, humans have existed for about 0.004% of the Earth’s total age.
The original atmosphere of earth, created nearly 4.5 billion years ago, contained an abundance of oxygen.
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