The Earth we know looks very different from what it had shortly after its birth, about 4,470 million years ago. Then it was a mass of conglomerate rocks whose interior was heated and eventually melted the entire planet.
Over time the bark dried and became solid. In the lower parts water accumulated while, above the earth's crust, a layer of gases formed, the atmosphere.
Water, earth and air began to interact quite violently since, in the meantime, lava flowed in abundance through multiple cracks in the crust, which was enriched and transformed thanks to all this activity ...
But let's go to the beginning to understand how we got here.
The origin of the world
According to scientists, about 13.8 billion years ago in "nowhere" there was a big explosion, the Big Bang. The unleashed force drove the matter, extraordinarily dense, in all directions, at an enormous speed, like that of light. Over time, and as they moved away from the center and reduced their speed, huge masses of that matter were grouped and condensed to form, later, galaxies.
We do not know what happened in the place of the Universe that we now occupy during the first 9,000 million years; if there were other suns, other planets, empty space or simply nothing. About halfway through this period, or perhaps before, a galaxy must have formed.
The formation of the Sun and the planets
Near the limit of this galaxy, which we now call the Milky Way, a portion of matter was concentrated in a denser cloud some 5,000 million years ago. This happened in many places, but we are especially interested.
It is believed that a nearby star exploded about 4.6 billion years ago becoming a supernova. The shock wave of that explosion set in motion the materials of our protosolar nebula.
The cloud began to spin faster and flattened into a disk. Gravitational forces gathered most of the mass in a central sphere and, around it, turned much smaller ones.
The central mass became an incandescent sphere, a star, our Sun. Small masses also condensed while describing orbits around the Sun, forming the planets and some of their satellites. Among them, at least one was at the right distance and with the right size to have water in a liquid state and retain an important gaseous envelope. Naturally, this planet is ours, the Earth.
Solid, liquid and gas
After an initial period when the Earth was an incandescent mass, the outer layers began to solidify, but the heat coming from the interior melted them again. Finally, the temperature dropped enough to allow the formation of a stable earth's crust.
At first the Earth had no atmosphere, and therefore received many meteor impacts. The volcanic activity was intense, which motivated large masses of hot lava to go outside and gradually increase the thickness of the crust when it cooled and solidified.
This activity of the volcanoes generated a large amount of gases that eventually formed a layer on the crust. Its composition was very different from the current one, but it was the first protective layer and allowed the appearance of liquid water. Some authors call that atmosphere "I" Earth's primary atmosphere formed by hydrogen and helium, with some methane, ammonia, noble gases and little, very little, oxygen.
In the eruptions, from the oxygen and hydrogen, water vapor was generated which, when ascending through the atmosphere, condensed, giving rise to the first rains. Over time, with the coldest crust, the water from the rainfall could be kept liquid in the deepest areas of the earth's crust, forming seas and oceans, that is, a hydrosphere.
From here the paleontology is responsible for studying geological history and paleobiology specializes in the biological history of the Earth. In the following pages we discuss the most significant facts of these stories.
• Big Bang Theory. How was this Universe born?
• Why and where to study Paleontology
• What is Paleobiology?
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