1.2.3 Scientific points of view
About 5bn years ago a star exploded in our galaxy, the Milky Way, and become a Supernova. The debris of the explosion crashed in a nearby cloud of gas bringing together the ingredient required to form the solar system. As a direct result of the explosion, the mixture -gas and dust- became very hot. Dust particles stuck together that grew bigger and bigger with time until their own gravity became strong enough to pull the mixture together. Later again, the central lump became so hot and dense that fusion reactions started generating its own energy. The sun was born this way while the remaining dust and cloud formed a disc around it. With time the sun grew in size, the disk cooled down but over millions of years the dust started to stick together forming first grains, then lumps, boulders and finally planetesimals. These were big enough to have their own gravitational field. As more time went by, some of the planetesimals grew in size, became the planets of the sun as we know them today, and settled in their own orbits. Big blocks of rocks were still flying in space. Some of these planetesimals merged with the earth but, as we will, a powerful one hit the planet earth, and the moon was born.

Most scientists now agree that the earth is about 4.6 billion years old and moreover, it was not born as it appears now. On the contrary the earth, like all stars, planets and other celestial objects, is the result of evolution. It is the result of a long time evolution that is the outcome of the coalescence of a cloud of dust and gas. It was formed at the same time as the whole of the solar system.

A cloud of hydrogen and helium, left over from the Big Bang about 13.7 billion years ago, mixed with heavier elements created by other stars floated for a long time in space. This cloud was compressed possibly by the explosion of a nearby supernova and it started to spin. The cloud was flattened into a disk that broke down into the sun and the well-known planets, the earth being the third one closest to the sun.

At that time the earth was bombarded by interplanetary debris and heated by the pressure resulting from its contraction under the effect of gravity. Initially the earth did not have a crust, mantle or core and, on the contrary all the elements were evenly mixed. There were no continent, no oceans and no atmosphere. Meteorite collisions, radioactive decay and planetary compression heated the earth that became very hot. The earth was completely molten and heavier chemical elements such as iron drifted toward the centre to form the earth core. At that point a magnetic field was created that protected the planet until now. This magnetic field was also an important factor in allowing life first to appear followed by evolution as described by Darwin.

The earth cooled slowly over a period of 100 to 150 million years the temperature decreased to about 2,000C the melting point of iron and the core was formed followed by the formation of a solid crust and a mantle. Internal gas escaped in the atmosphere through volcanic eruptions about 4bn years ago, and comet bombardment brought water leading to the formation of the oceans. This period covers the first 500 years of the earth's history.

When the earth was about 750 million years old the atmosphere contained ammonia, methane, carbon dioxide and nitrogen with no free oxygen. Life as we know it today would have been impossible since, in addition, as there was no ozone layer, the surface of the earth was bombarded by deadly ultraviolet radiation from the sun. Slow changes and chemical coalescence improved the environment and about 3.5 billion years ago the first simple living organisms appeared. At the same time the first continents were formed and later on they were regrouped in supercontinents. These initial continents were different from those we have now as they formed and broke again due to the movement and collision of the tectonic plates on which the earth's crust rests. Even now the continents are not stable and they still move continually even if their speed is luckily rather low.

The earth was hit by a big piece of rock, a large planetesimal the size of Mars, about 4.5bn years ago. A piece of the earth was cut off and blown into space. This object found an orbit around the earth and became our moon.

The direct hit knocked the earth sideways, changing its angle of tilt to the sun from 0 to 23.5 degrees and the earth started to have seasons.

The earth's position in the solar system allowed life to appear. If its orbit had been similar to those of Mercury and Venus that are closer to the sun, it would have been to warm for the oceans to form. If its orbit had been close to Mars', then it would have been too cold and the oceans would have frozen. In other words, the earth's position was just right for life. There was not much oxygen in the atmosphere but some sort of living organisms appeared early on, about 4bn years ago. Little by little the atmosphere changed as algae released oxygen and about 2.5bn years ago the conditions were met for complex life to evolve.