History Of Earth: Timeline, Process & Facts

On a clock of planet Earth's entire life spanning 4.6 billion years, the time that human beings have been here accounts for about one minute. In other words, humans have only inhabited the planet a mere 0.13 million years. Have you ever wondered what happened before people arrived on the scene?

Scientists estimate Earth's age and history using a geologic time scale that analyzes fossils imbedded in alternating rock layers called strata.

For example, an exposed sedimentary rock formation may show a horizontal layer of limestone with snail fossils, a layer of conglomerate rock and a layer of shale and fish fossils. Rock strata reveal valuable clues about when and how changes occurred during Earth's formation.

The history of the Earth is broken into incrementally smaller stretches of time: eons, eras, periods and epochs. The Precambrian Eon (not to be confused with the Cambrian Era) extends from the formation of the Earth to the emergence of multicellular organisms, and includes the Hadean, Archaean and Proterozoic eons. The Phanerozoic Eon encompasses everything from that point forward: the Paleozoic, Mesozoic and Cenozoic eras.

Although there are no eye witnesses, of course, scientists are convinced that the Earth formed billions of years ago from space dust that clumped together during the formation of the solar system. Around 4.5 billion years ago, molten iron and nickel sank and formed the Earth's core. A hot, rocky mantel formed in middle Earth, and the outermost crust cooled and hardened.

Oceans formed from condensed water vapor falling as rain, and aquatic cyanobacteria (blue-green algae) released oxygen into the sea after making food through photosynthesis. Oxygen reacted with iron in the water and sank to the ocean floor. When the supply of iron was exhausted about 1.5 billion years ago, lots of oxygen was released into the air, and that is when everything changed.

Plants and animals evolved and moved from sea to land; amphibians and reptiles were first to adapt. Dinosaurs ruled the Earth from 225 to 65 million years ago. After dinosaurs became extinct, mammals rapidly evolved and diversified. Homo sapiens (humans) evolved about 130,000 years ago and migrated out of Africa around 35,000 years ago.

According to NASA, Earth's inner core is made up of iron and nickel and heats to 9,800 degrees Fahrenheit. The middle of the Earth is comprised of molten rock.

The Earth's surface consists of a much cooler layer that is about 19 miles deep in most spots, with the exception of the ocean floor where the active mantel is within 3 miles.

Temperature is a key determinant in whether a species survives or faces extinction. The Earth has experienced dramatic climate changes such several ice ages and mass extinctions. Although the possibility of another meteorite strike exists, a more immediate threat is the proliferation of greenhouse gasses.

According to NASA, ice cores extracted from Greenland and Antarctica indicate that pollutants have significantly increased global warming. The history of Earth's temperature has shown that even slight changes in the Earth's rotation can affect climate. NASA further reports that the temperature of the Earth has increased 1.62 degrees Fahrenheit since the end of the 19th century.

The history of Earth's name goes back approximately 1,000 years, according to astronomers at Cal Tech. The name Earth derives from an English and German word for ground. The other planets were named for Greek and Roman deities. For instance, large planet Jupiter is named after the head Roman god.

Colloquial names for the Earth such as "Terra" are not recognized by the scientific community. Names for celestial bodies are determined by the International Astronomical Union. Earth is the name approved for use in English-speaking countries.

The giant-impact hypothesis is the generally accepted explanation for how the Earth ended up with an orbiting moon. Astrophysicists suggest that a Mars-size terrestrial body named Theia hit the Earth with great force, and the particles that bounced into space were pulled together by gravity forming an orbiting moon.

Other theories focus on co-accretion, which means the Earth and Moon formed the same time from a solar nebula. Another theory is that primitive Earth's gravitational field ensnared a large object that became the Moon.

In the late Paleozoic Era, a fissure in the tectonic plates – below the supercontinent Pangea – widened. Volcanic activity underground spewed ash and magma through weak spots in the Earth's crust. Continued movements of the tectonic plates along with volcanic rifts led to the separation of Pangea into the smaller continents.

Pangaea split into Gondwanaland and Laurasia. Gondwanaland became Africa, Antarctica, Africa, Australia, India and South America. Laurasia divided into the North American continent and Eurasia. Today, the continents are identified as Africa, Antarctica, Asia, Australia, Europe, North America and South America.

As strange as it sounds, evidence of tropical forests and dinosaurs can be found in the ice sheets of Antarctica. About 200 million years ago, Antarctica was part of supercontinent Pangaea, and the temperature was balmy. The climate cooled substantially after Antarctica split from Pangaea and moved towards the South Pole.

The Hadean Eon occurred 4.6 to 4.0 billion years ago when the Earth was first formed. The name comes from the word Hades, an unbearably hot, hellish place. Much earlier, about 13.7 billion years ago, and for reasons not altogether understood by scientists, an expansive explosion occurred known as the Big Bang. A massive cloud of gasses and interstellar dust gave rise to the sun and solar system.

The sun took the helium and large masses of elements came together to form orbiting planets, including the lava covered Earth. Heavy materials like molten iron and nickel sunk to the Earth's core. Layers of lighter materials formed the mantel and a thin crust covered with rock and basalt.

Temperature gradients in the core and mantel caused convection currents that moved the tectonic plates of the Earth's surface, a phenomenon still occurring today.

Magnetic fields and a primordial atmosphere of toxic gases formed. Also during this stage, the Earth was pummeled by asteroids that created geologic formations. Comets containing ice, ammonia, carbon dioxide and methane repeatedly hit the Earth.

Scientists hypothesize that the unrelenting force of asteroid impacts along with the presence of water and the building blocks of amino acids may have sparked the formation of DNA, the essence of life.

Between 4.0 billion and 2.5 billion years ago, Earth cooled and ancient life appeared. Earth's rotation slowed after colliding with a large planetary size body and acquiring the Moon. The mishap stabilized the Earth's rotation and may have tilted the Earth, resulting in the four seasons of the year. During this time, evidence of life first emerged, and the continents started to form.

An estimated 40 percent of the continents formed during this period. The Earth started to cool and oceans formed from water vapor condensation. Continents formed from granite about 3.1 billion years ago. Researchers proposed that the first large landmass Ur, was located near modern day India, Australia and South Africa.

From 2500 million to 541 million years ago, the Great Oxygenation Event (sometimes also known as the Great Oxidation Event) ushered in major climate change. Anaerobic organisms died out from the toxicity of high oxygen levels and were replaced by multicellular, aerobic eukaryotic organisms.

Atmospheric oxygen reacted with high levels of methane to create carbon dioxide. Because methane is better at retaining heat, the greenhouse effect was reduced, triggering a 300-million-year-long ice age called snowball Earth.

The tectonic plates formed supercontinents. Increasing levels of oxygen thickened the ozone layer and provided protection from ultraviolet radiation. The presence of oxygen and a UV shield allowed terrestrial life to appear and diversify.

The current eon, which started around 541 million years ago, is the Phanerozoic. The first era of the Phanerozoic Eon was the Paleozoic Era. The so-called Cambrian explosion and diversification of life occurred around 541 million to 245 million years ago during that era.

Fossil evidence indicates that the Cambrian explosion occurred when hard-shelled invertebrates evolved in the ocean. Fish came next, followed by the evolution of fish to land-animals and amphibians that shared similar anatomical features like backbones, jaws and mouths.

Lush plants in the rain forest flourished until the carboniferous rain forest collapse that some scientists believe was brought on by global warming. Masses of decaying organic matter were buried, pressurized and compacted into coal deposits. Large deserts replaced vegetation and created a habitat for reptiles.

The eon ended with another mass extinction, the Permian-Triassic Extinction. A large asteroid strike is generally thought to be the culprit. An estimated 96 percent of marine animals and 70 percent of land animals died.

Dinosaurs ruled the Earth 252 million to 66 million years ago. After the loss of forests in the Paleozoic, these creatures evolved to lay hard-shelled eggs on land instead of water. Dinosaurs dominated for approximately 160 million years. Next, birds evolved from a type of dinosaur.

The first coniferous trees appeared when plants evolved to use seed germination. Plentiful food and increased levels of oxygen from the conifers allowed for very large living organisms like dinosaurs to thrive on Pangea.

The end of the Mesozoic era and the start of the Cenozoic era was the time of another catastrophic extinction when a 6-mile-wide asteroid stuck the Earth's surface causing a thick dust cloud that blocked the sun. The asteroid strike and resulting climate changed are believed to have caused the extinction of dinosaurs.

From 66 million years ago to the present-day, mammals and Homo sapiens (humans) proliferated. With the demise of the dinosaur, mammals became the dominant species, including large creatures like whales and mammoths. Savannah grasses developed, providing food and habitat in areas where there were no trees.

The first primate originated about 25 million years ago, and the first hominid around 3 million years ago. Apes left the trees and walked upright to spot predators in the African grasslands. Homo sapiens dates back to Africa some 300,000 years ago. Early humans showed ingenuity in making tools, creating art, gathering food and hunting.

Geographical changes brought about by movement of tectonic plates included expansion of the Atlantic Ocean. Building pressure formed the Rocky Mountains in the western part of the continent as the eastern part moved closer to the Pacific. The Earth's temperature dropped slightly in the Cenozoic Era.

Changes in the Earth are forever occurring as the tectonic plates move slowly under the Earth's thin crust. Earthquakes happen when tectonic plates either slip by each other or one slips under another, causing trembling of the Earth above the fault plane.

For instance, the San Andreas fault in California is a crack between two tectonic plates that bump into each other, causing not only the big quakes that make the news, but also little rumbles that often go unnoticed. Major weather events also claim lives and cause mass destruction.