History of the earth

Just how was the Earth created? In some fiery furnace atop a great mountain?On some divine forge with the hammer of the gods shaping out of pure ether? How about of a great ocean known as chaos? These all sound familiar? Interestingly enough, these ancient creation stories contain an element of scientific fact to them. When it comes to how the earth was formed, forces that can only be described as fiery, chaotic and indeed, godlike, were indeed involved, though the timeline and buildings materials might have been somewhat different and more complex.

The history of the Earth describes the most important events and fundamental stages in the development of the planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of the age of the universe. An immense amount of biological and geological change has occurred in that time span.

Scientists believe that about one hundred billion years ago the Earth, the Sun, and all the planets of the Solar System were nothing but a cloud of cold dust particles swirling through empty space. Gradually, these particles were attracted to each other and came together to form a huge spinning disk. As it spun, the disk separated into rings and the furious motion made the particles white-hot. The center of the disk became the sun, and the particles in the outer rings turned into large fiery balls of gas and molten-liquid that cooled and condensed to take on solid form. Four or five billion years ago, they turned into the planets that we know today as Earth, Mars, Venus, and the outer planets.

The Earth was created approximately 4.54 billion years ago when part of the Sun's accretion disc agglomerated into a spherical body. At that time, shortly after the birth of the Sun itself, the solar system's matter was much more scattered around, in the form of asteroids and dust rather than planets. This "matter cloud" has been called a Bok globule, and these globules have been observed in other parts of the Galaxy. Scientists have determined the age of the Earth relatively precisely using isotope dating of the world's oldest rocks.

It is not known precisely what mechanism caused the precursor of the solar system, a gaseous nebulae, to form into the Sun and its attendant accretion disc. It may have been shockwaves from a nearby supernova, or simple gravitational collapse due to a threshold density. Whatever the cause, when enough density gathered in the center of the gas cloud, it ignited to become the Sun. The resulting heat banished volatiles (materials with low melting points) to the outer solar system, while leaving rocky bodies, like the Earth, Mercury, Venus, and Mars, in the inner solar system, where they could grow.

This first eon in which the Earth existed is what is known as the Hadean period, named after the Greek word “Hades” (underworld) which refers to the condition of the planet at the time. During this time, the Earth’s surface was under a continuous bombardment by meteorites, and volcanism is believed to be severe due to the large heat flow and geothermal gradient. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, accumulated in the atmosphere and cooled the molten exterior of the planet to form a solid crust and produced the oceans. This period ended roughly 3.8 years ago with the onset of the Archean age, by which time, the Earth had cooled significantly and primordial life began to evolve.

As the surface continually reshaped itself over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago, the earliest-known supercontinent of Rodinia began to break apart, then recombined 600 – 540 million years ago to form Pannotia, then finally Pangaea, which broke apart 180 million years ago, eventually settling on the configuration that we know today.

Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted in these elements compared to cosmic abundances. After the impact, the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen. Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.

In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would therefore have started to form even as the Earth formed. The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.

Planetesimals at a distance of 1 astronomical unit (AU), the distance of the Earth from the Sun, probably did not contribute any water to the Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long. The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU. Comets may also have contributed. Though most comets are today in orbits farther away from the Sun than Neptune, computer simulations show they were originally far more common in the inner parts of the solar system.

As the planet cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga. By the start of the Archean eon they already covered the Earth. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox. Stars are known to get brighter as they age, and at the time of its formation the Sun would have been emitting only 70% of its current power. Many models predict that the Earth would have been covered in ice. A likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. Another greenhouse gas, ammonium would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago. No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years. The eukaryotic cells emerged between 1.6 – 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis. The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either mitochondria or hydrogenosomes. Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.

The history of life was that of the unicellular eukaryotes, prokaryotes and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period. The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria.

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct. Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis.

About 500 million years ago, plants and fungi colonised the land and were soon followed by arthropods and other animals. Insects were particularly successful and even today make up the majority of animal species. Amphibians first appeared around 364 million years ago, followed by early amniotes and birds around 155 million years ago (both from "reptile"-like lineages), mammals around 129 million years ago, homininae around 10 million years ago and modern humans around 250,000 years ago. However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both biomass and species being prokaryotes.