On the Earth, the slowly hardening crust breaks over and over again, allowing lava and incandescent gas to rise to the surface. These gases formed the primordial atmosphere, an atmosphere that is much different from the one we know about today. It consisted of nitrogen, hydrogen, carbon dioxide and probably small amounts of methane and ammonia. It was also rich in water vapor which, reaching the surface, cooled and was transformed into rain. But the young rocks were so hot that the rain evaporated immediately and was again transformed into steam in a never-ending cycle. In the meantime, the planet slowly began to cool and the rain no longer completely evaporated but began to collect in basins forming larger and larger expanses of water: the first seas. This was followed by a period of hundreds of millions of years for which it is difficult to reconstruct a precise history. Certainly, however, we know that our planet probably changed its "face" many times. Then, three and a half billion years ago, life began in the primordial seas: first simple bacteria and microscopic algae, followed by more complex organisms that developed a fundamental process for the future of the Earth: photosynthesis. It is thanks to this process that the first water plants, containing a green substance called chlorophyll, rely on sunlight for their survival: they absorb carbon dioxide and water, transforming them into sugars that are used as an energy source, but above all they emit oxygen as a waste product. This system was so successful that the first plants completely changed Earth's atmosphere. In fact, the oxygen produced first combined with the iron dissolved in the sea water, causing it to oxidize and making deposits on the bottom. Then, when the iron was "depleted", the oxygen became so abundant that it began to "evaporate" from the water, accumulating in the atmosphere and forming the air that we breathe today. Over hundreds of millions of years, life continued to evolve and then even reached the land: first the Fish age, 225 000 000 years ago, followed by the Reptile age, 65 000 000 years ago and finally, the Mammal age, which our species belongs to, and which still exists today. While plants and animals spread over the Earth, reproducing in millions of species, the current continents did not exist, or in any case were quite different from what we know today. In fact, as far back as we can measure, a mere 220 000 000 years ago, the continental masses were grouped into a single "supercontinent" that geologists called Pangea (a Greek word that means "the entire Earth"). Pangea slowly began to split up until, 135 000 000 years ago, the emerging land was divided into two large masses, one in the north, called Laurasia, and one in the south, called Gondwana. These masses continued to break apart and 65 000 000 years ago they began to assume their present-day shape and position. And here is how our planet was seen by the astronauts from the "Apollo" lunar mission and was immortalized in what is now a famous photograph taken from the Moon: a blue-green globe that clearly shows Africa, the Mediterranean basin, a part of Asia and the frozen cap of the Antarctic. Who would have ever said that these continents and these oceans which are now so familiar are not permanent and unchangeable features of our world, but instead have changed profoundly in the past and will change just as much in the future, making the Earth unrecognizable for present-day man?
On the Earth, the slowly hardening crust breaks over and over again, allowing lava and incandescent gas to rise to the surface. These gases formed the primordial atmosphere, an atmosphere that is much different from the one we know about today. It consisted of nitrogen, hydrogen, carbon dioxide and probably small amounts of methane and ammonia. It was also rich in water vapor which, reaching the surface, cooled and was transformed into rain. But the young rocks were so hot that the rain evaporated immediately and was again transformed into steam in a never-ending cycle. In the meantime, the planet slowly began to cool and the rain no longer completely evaporated but began to collect in basins forming larger and larger expanses of water: the first seas. This was followed by a period of hundreds of millions of years for which it is difficult to reconstruct a precise history. Certainly, however, we know that our planet probably changed its "face" many times. Then, three and a half billion years ago, life began in the primordial seas: first simple bacteria and microscopic algae, followed by more complex organisms that developed a fundamental process for the future of the Earth: photosynthesis. It is thanks to this process that the first water plants, containing a green substance called chlorophyll, rely on sunlight for their survival: they absorb carbon dioxide and water, transforming them into sugars that are used as an energy source, but above all they emit oxygen as a waste product. This system was so successful that the first plants completely changed Earth's atmosphere. In fact, the oxygen produced first combined with the iron dissolved in the sea water, causing it to oxidize and making deposits on the bottom. Then, when the iron was "depleted", the oxygen became so abundant that it began to "evaporate" from the water, accumulating in the atmosphere and forming the air that we breathe today. Over hundreds of millions of years, life continued to evolve and then even reached the land: first the Fish age, 225 000 000 years ago, followed by the Reptile age, 65 000 000 years ago and finally, the Mammal age, which our species belongs to, and which still exists today. While plants and animals spread over the Earth, reproducing in millions of species, the current continents did not exist, or in any case were quite different from what we know today. In fact, as far back as we can measure, a mere 220 000 000 years ago, the continental masses were grouped into a single "supercontinent" that geologists called Pangea (a Greek word that means "the entire Earth"). Pangea slowly began to split up until, 135 000 000 years ago, the emerging land was divided into two large masses, one in the north, called Laurasia, and one in the south, called Gondwana. These masses continued to break apart and 65 000 000 years ago they began to assume their present-day shape and position. And here is how our planet was seen by the astronauts from the "Apollo" lunar mission and was immortalized in what is now a famous photograph taken from the Moon: a blue-green globe that clearly shows Africa, the Mediterranean basin, a part of Asia and the frozen cap of the Antarctic. Who would have ever said that these continents and these oceans which are now so familiar are not permanent and unchangeable features of our world, but instead have changed profoundly in the past and will change just as much in the future, making the Earth unrecognizable for present-day man?
