Origin of life
Violent storms, grandiose and terrifying scenery: this is how the earth was four billion and six hundred million years ago. Then, very slowly, this scorching young planet began to cool. Immense columns of steam rose toward the sky and then returned to the earth in the form of rain: and at the end of the deluge the sea was born.
Meteorites continued to fall on our planet, the air was rich in methane, ammonia and carbon dioxide; the sun hidden by very thick clouds, the sky orange-colored, the rays of light managed with difficulty to reach the earth. The water in the oceans was very hot, exceeding 100 degrees.
Then a giant asteroid the size of Mars crashed into the earth. Part of our planet literally turned to dust, and the fragments were thrown into space where they came together by the effect of gravity-thus the moon was born.
At that time the moon was much closer to the earth than it is today, and its gravitational force was felt with great vigor, causing violent surges in sea levels.
But it was in these ever-moving oceans that the chemical reactions essential for the formation of the first organisms occurred.
The primordial sea contained substances that are toxic to humans today, such as hydrogen cyanide and potassium cyanide. Thousands of volcanoes opened at the bottom, incessantly emitting fumes filled with hydrogen sulfide, a substance incompatible with life as we know it today. These underwater craters had been created as a result of the asteroid impact that led to the formation of the moon, and from their mouths substances from the center of the earth poured into the water.
From the very beginning, however, the sea contained some ingredients necessary for life; other substances may have come from the cosmos on the walls of meteorites falling to earth, and lightning strikes provided very high doses of energy, instrumental in building new chemical compounds that in turn recombined together to produce the fundamental one: DNA. This molecule was able to duplicate itself and contained a long string of information capable of making an organism function: the genetic code, the basis for all evolution up to man, was born.
DNA consists of 4 elements that are repeated in ever-changing forms thousands of times.
But where were the first DNA molecules created?
According to some researchers, life began in the pockets of water left by the oceans when they receded due to strong tides. The waves would break on the shoreline creating a large number of bubbles, some of which did not burst and instead attached themselves to the rocks, becoming a kind of micro chemical laboratory.
The bubbles were able to capture even external substances, then, in the small protected space, the different elements combined with each other in every possible way, and following a very high number of attempts the first DNA strands were formed, or so some researchers believe. Shortly after their appearance, the first microorganisms began to evolve and diversify, but even today some primitive life forms can be found on the seabed.
Biologists are studying with particular interest the seafloor off the Mexican coast, where an underwater volcanic eruption disrupted the ocean in 1991, recreating conditions similar to those of 4 billion years ago. At a depth of 2,600 meters, a special camera captures the craters from which a black smoke rich in hydrogen sulfide is rising. Scientists bring back to the surface a platform they had left for a week at the bottom. Analysis confirms what seemed impossible: the platform is full of microorganisms that used hydrogen sulfide to feed themselves: they are bacteria probably similar to the first living things that appeared on earth.
Bacteria evolved quickly, some tried to protect themselves with hard shells others preserved a mobile membrane, but all are our most distant progenitors.
The earth today is surrounded by an oxygen-rich atmosphere, but in the beginning the situation was quite different. Oxigen was not found in the air, and in any case it was a poison to the first livings.
There are now about ten million animal and plant species, and almost all but a few types of bacteria and other microscopic organisms use oxygen. Animals in particular use it to extract energy from food through a series of complicated chemical reactions.
But how did this gas, which was toxic four billion years ago, become indispensable to every form of life from microorganisms to humans?
Oxygen only spread across the earth when continents began to form. Our planet was surrounded by a dense layer of clouds that were created as a result of the very strong evaporation of the seas. The temperature was high because large amounts of carbon dioxide enveloped the earth, creating a powerful greenhouse effect. Then the winds began to disperse the dust coming off the continental masses, and this dust rich and calcium and sodium combined with carbon dioxide reducing the greenhouse effect, so the earth began to cool and the clouds gradually dissipated and sunlight was able to reach the water of the oceans directly; and here a revolution took place: some kinds of bacteria were able to use the energy of light to produce sugar from water and carbon dioxide: it was the fundamental mechanism of life, called photosynthesis, which led among other things to the release of oxygen, and as the amount of oxygen grew, the sky became lighter, so the earth that had seemed orange for millions of years became the blue planet we live on today.
The presence of oxygen forced the most primitive bacteria to flee or modify themselves, some sought refuge in areas still rich in hydrogen sulfide, while others protected themselves with a sturdier membrane and accepted the challenge by learning to use oxygen that provided much larger doses of energy and soon attacked smaller rivals. To defend themselves, the intended victims tried to fuse their respective genetic codes, and this was another revolution because the DNA strands intermingled in ever-changing combinations-so life was able to experiment with endless variations resulting in the ten million species living on earth now.
In the waters of the oceans thousands of strange creatures now gone invented new forms of survival, but only those organisms that were best adapted to the environment were able to reproduce. Unpredictable mutations in the genetic makeup distributed advantages randomly, providing some living species with an extra chance for survive. Many of these characters arrived to us after a journey of millions of years.
Mountains rich in living organisms extended under the sea. Some have remained under the water to the present day while some, in more recent times, have emerged, such as those more than 2,000 meters high found today in the western regions of Canada.
One day, five hundred thirty million years ago, these mountains suddenly crumbled under the sea, causing tons of dust and rock to fall to the bottom. The avalanche of debris imprisoned thousands of primordial creatures whose fossils tell us some important pages of evolution.
The gradual settling of the continental masses and the increasing presence of oxygen gave new impulses to evolution. As rains brought large amounts of nutrient-rich soil and sand into the sea, the environment became increasingly favorable for life. In the shallow waters lived, for example, Trilobites, which defended themselves with a real shield. In contrast, the Wiwaxia possessed long, sharp spikes on their backs. These animals evidently had to protect themselves from some really fearsome enemy, and in fact many fossils bear the traces of aggression: this one shows the broken tips of a Wiwaxia; some Trilobites show the sign of bites, the killer was probably much larger than its prey and had sharp teeth. In some fossils, traces of a peculiar double w-shaped bite have been found. Probably the perpetrator of the attack was one of the most ferocious animals living at that time: the Anomalocaris.
