Earth History refers to the set of major events that have taken place on our Planet since its formation, including the main evolutionary phases and stages of development. Over the past two centuries, scientists and scholars have recovered the fossilized remains of animals and plants and carefully observed the rocks of the Earth, with the goal of reconstructing its history. The Earth actually began to exist in the Solar System about 4560 million years ago, when it was just a rocky body with a very high temperature.
For the first forms of life, we have to go back as far as 4000 million years ago, exploring the ocean depths. Since then, living things have continued to spread and diversify in an evolutionary process that is far from linear. The same has happened to natural environments, which have inevitably undergone continuous changes due to the fervent volcanic activity, the impact of meteorites and climatic changes with catastrophic potential.
How the Earth was born
The history of the Earth and its evolution is particularly rich in events that have changed its appearance over time. And so it will continue in the future. A history that has a fundamental relevance to understand the environment that surrounds us and that, as we said, began about 4560 billion years starting from what is known as the solar nebula.
Initially, there was only a spinning disk composed of gas and dust: after the formation of our Sun, the excess material began to gather in different areas, contributing to the creation of the planets that we know today. In its first "vagaries" of life, the so-called neo Earth presented a mass made mainly of rock, metals, gas and radioactive elements in a state of fusion. The Planet appeared then as a homogeneous and undifferentiated molten sphere.
Subsequently, with the beginning of a long process of cooling of the entire agglomerate, the heavier compound materials concentrated towards the center begin to separate. Materials, these, as can be for example the iron, of which is constituted for great part the terrestrial nucleus. When then the water began to accumulate, it helped to cool the outer part of what is the Planet today, forming a real crust, rich in elements that create light compounds.
Currently, despite technological evolution, we are not able to know if the water went to form by the reaction between hydrogen and oxygen or if it arrived from space already as such. This fundamental phase has allowed the Earth to separate into three shells, completely concentric. These are the crust, the mantle and the core: the first, as we said, is richer in light elements and not very dense; the second prefers iron and magnesium; and the last instead is rich in siderophiles and heavy compounds.
Very important in the History of the Earth has been the compound effect of heavy and light materials, which have contributed to the construction of its current physical structure. As for the atmosphere, at least in its primordial phase, it is due to the Planet's intense volcanic activity. The resulting water vapor, condensing and increasing by taking advantage of the ice carried by comets, then led to the formation of the oceans.
At that time, however, the Earth's motions must have been faster than they are today. It wasn't until the formation of the Moon, most likely from an impact with a wandering celestial body, that the Moon-Earth system we know was established. Thanks to gravity, the Earth's motions began to slow down and stabilize, thus reducing the variations of inclination of the axis. It is not an exaggeration to think that, without this stabilizing action, life as we know it today could not exist.
History of planet Earth: the first continents
Thanks to scientists who have been able to reconstruct the Earth's history over time, it is now known that the initial crust that formed after the first cooling of the Earth's crust practically all disappeared. This happened because of frequent tectonic movements and, at the same time, because of intense meteoritic bombardment.
The first huge pieces of continental crust, the product of a differentiation of light elements during a partial melting of the lower crust, appeared about 4 billion years ago, and formed what at present are the cores around which the various continents developed. Even today, it is possible to observe what remains of them thanks to what are called cratons, in which are found some of the oldest rocks on Earth, dating back as far as 4 billion years ago.
Thanks to their study, we discovered that on the surface are present, among other things, many sedimentary grains smoothed by erosion during transport in the water. This shows that there were already rivers and seas at the time.
Thus, all the prerequisites for the origin of life are created, although there are two different theories about it, both valid. The first school of thought argues that organic components have arrived on Earth directly from space, it is the so-called Panspermia, while the second one suggests on the contrary that these have originated directly on our Planet.
In any case, the general mechanisms proposed by both theories are quite similar, although it is not possible to establish the precise moment in which life has actually developed. It is hypothesized that it happened about 4 billion years ago, when the first molecule went to duplicate itself laying what are the fundamental foundations to the birth of the first common ancestor of all life.
The origin of life on Earth
In the primordial Earth, some molecule was then able to reproduce copies of itself, becoming a replicator. Similarly, the creation of more or less suitable variants for survival, contributed to the beginning of evolution among inanimate matter. One of the oldest and most credited theories explains this complex process from the high volcanic energy, lightning and ultraviolet radiation, which would have induced the production of more complex molecules.
Among these, there would have been even relatively simple organic compounds, such as nucleotides and amino acids, which are the necessary constituents for there to be life. This primordial organic soup began to grow in quantity and concentration, causing different molecules to react with each other. Until one replicator molecule managed to emerge and impose itself on the others, only to be replaced in its function almost completely by DNA.
It seems possible then that RNA is the primitive replicator, considering that it contains the genetic information of living beings. As explained in the so-called Bubble Theory, at some point DNA took the place of RNA as the genetic repository, and "proteins known as enzymes consequently went to fill the role of catalysts of reactions, leaving RNA with the role of transferring information, synthesizing proteins and modulating the whole process."
Recent studies, indicate that the last universal common ancestor for all life forms present on Earth lived around 3.5 billion years. This cell is commonly referred to as LUCA - an acronym for Last Universal Common Ancestor - and is in fact the progenitor of all life known today on our planet.
As do virtually all modern cells, it used DNA to store the genetic code, RNA to transfer information, and finally protein synthesis and enzymes to catalyze its reactions. However, it is still debated whether life formed in the sea or in a subaerial environment. What is certain, is that at a later stage the well-known process of photosynthesis has allowed all life forms to accumulate energy directly from the Sun.
Photosynthesis appeared with what scholars indicate as cyanobacteria, microorganisms that for a long time dominated the waters of our primordial Planet, and that have helped to produce a progressive decrease of carbon dioxide, favoring at the same time an increase in the concentration of oxygen. In turn, the oxygen saturated the waters and diffused into the atmosphere, creating the ozone layer.
Life, aided by the ozone present in the higher parts of the atmosphere to absorb harmful ultraviolet rays, was thus able to colonize the Planet. Only 490 million years ago, and after five mass extinctions - the last one led to the extinction of the dinosaurs due to the impact of a meteorite - mammals were able to establish themselves. Leading, with the evolution that we all know a bit, to the late birth of the human race.