The Hadean Eon
During the Hadean eon, which represents the first 800 million years of the Earth’s life (between about 4600 and 3800 million years), the heat released by the collisions with planetesimals was enough to keep molten the surface of the planet. As the frequency of the collisions decreased, the crust gradually solidified, until the most ancient known rocks were formed.
Hadean eon: lava landscapes (Bernhard Edmaier)
The Archaean Eon
It was during the following eon, the Archaean, that life arose for the first time (at least 3600 million years ago).
In the beginning, the Earth must have displayed a huge density of craters and it would be covered by an atmosphere very rich in carbon dioxide, with some nitrogen and traces of sulphidric acid and hydrogen. It’s possible that there was also some methane and ammonia.
The fact that the Sun was 25% colder in this epoch was compensated by the curtain of greenhouse gases that covered Earth, namely the carbon dioxide and the water vapour. Because the Earth, at that time, still preserved a large proportion of radioactive atoms (generated in the supernova that preceded the formation of the solar system), the volcanic activity was 3 times higher than today.
It’s also probable that, as the Earth cooled, an unusually high rainfall occurrence would have created the oceans or reinforced some previous oceanic volume. These oceans were kept liquid due to the high atmospheric pressure, because the temperatures were well above 100 ºC (373 ºK). They were well loaded with iron and oxygen-reductor composites (which absorb it and prevent its free existence) such as chemical species rich in sulphur and nitrogen.
The hydrogen was produced from the reactions between the ferrous iron existing in the rocks and the water, being these reactions encouraged by the intense vulcanism. The existence of hydrogen opposed to the appearance of free oxygen in the atmosphere (because it tended to react with it to form water molecules) and favoured the accumulation of chemical products that were important for life. If the process of hydrogen release (and subsequent escape to space, given the fact that it is a very light gas) would have continued indefinitely, this could have driven to the exhaustion of the oceans, as it happened on Venus.
Gaia and the First Organisms
It’s quite possible that life could have had an important role on the preservation of the oceans, because it supplied the atmosphere with the oxygen that combined with the hydrogen to form water molecules, which prevented the hydrogen from escaping to space.
At the same time, the production of oxygen meant that the other component of the carbon dioxide consumed by the first photo-synthesizers (the carbon) wouldn’t have been given back to the atmosphere. This way, a large proportion of the carbon that previously existed there was buried in the rocks.
As It All Emerged
It’s possible that the first organic molecules (based on carbon) were synthesized in the interstellar space and brought to Earth by the comets that collided with it. Some scientists even advocate that these objects could have also supplied our planet with the water presently existing on it.
It may also be true that the chemistry and the initial conditions on the Earth’s surface (in the atmosphere, close to submarine volcanos or anywhere else) would have provoked the appearance of complex organic molecules.
In the aquatic environment there would have been formed, from the primitive organic molecules, the aminoacids (from which the proteins are made of), the nitrogen bases and the sugar (being both fundamental foundations for the genesis of nucleic acids, or in other words, the RNA and DNA, responsible for the transmission of genetic characters between generations). A reproduction mechanism subjected to mutations would have been then established. Simultaneously, from the phosphates and fat acids they would have been formed the membranes, which were important for the generation of the first cells, because their existence would be impossible without the solid (consistent) and isolating support provided by them.
The First Organisms
The food of the first cells would have consisted on organic chemical materials that abounded in the oceans, as well as on the corpses of the less successful competitors.
After some time, these supplies of energy and raw materials may have started to become scarce and that provoked the emergence of organisms able to produce their own food through photosynthesis (the capturing of solar energy in order to produce chemical reactions that split the links that bound the oxygen to the hydrogen (in H2O - water) and to the carbon (in CO2 – carbon dioxide)). These bluish-green coloured organisms are known as cyanobacteria and would have emitted some oxygen, although the wealth in reducer chemical elements kept it in quite low proportions. The regular addition of this element to the atmosphere and the subsequent exhaustion of the elements that reduced it would, however, further enable its existence in large quantities in a much later epoch.
Colony of cyanobacteria (David R. Madison)
The methanogenes were decomposers of organic products that were also present inside this primitive biosphere. They got materials and energy through rearrangements of the molecular products manufactured by the producing organisms. Given the absence of oxygen they couldn’t, however, digest the cyanobacteria directly. The methanogenes were responsible for the production of CO2 (carbon dioxide) and CH4 (methane), which are greenhouse gases.
The abundance of cyanobacteria destroyed the carbon dioxide layer that covered the Earth, although this molecule was continually replaced by the intense vulcanism. The existence of the methanogenes, which gave the carbon dioxide back to the atmosphere, was then essential for keeping the thermal equilibrium of the Earth (if CO2 had been entirely consumed, our planet would have become frozen).
The First “Ozone” Layer
The methane (CH4) reactions, induced by the fall of the ultraviolet rays, would have endowed the upper atmosphere with a layer that absorbed the ultraviolet and visible radiation of the Sun, which was therefore equivalent to the actual ozone layer. Gases like ammonia or the sulphidric acid could have survived at the lower atmosphere due to this protective effect.
During the last phase of the Archaean, it’s identifiable an increase on the quantity of nitrogen. It’s probable that previously most of this element was found under the form of an ammonium ion (NH4)+, which is abundant in the oceans. It is possible that the ferrous iron of the oceans stole a large quantity of ammonium ions, used for producing composites of iron and ammonia, which contained atoms of nitrogen. The decrease of the atmospheric carbon dioxide and the use given by life to the nitrogen may have favoured the importance of this element as a constituent of the atmosphere. The increase of the gaseous nitrogen made the atmospheric pressure grow and, therefore, contributed for the reinforcement of the greenhouse effect.
During this period the Earth should be covered by an opaque and brownish-red atmosphere, being enlightened by an orange Sun. A brown sea (reflecting the colour of the sky) would bathe the sand beaches depleted of shells. There were rocks with strange shapes, formed by calcium carbonate secreted by the cyanobacteria colonies. On emerse land there were stagnated water puddles, blotched by the green and dark bacteria. The sounds that could be heard in such an environment were only the wind, the waves and the bubbles of methane sparkling in the mud. Farther away from the sea, a thin layer of life would permanently corrode the rocks and would release nutrients and minerals into the rain-water flows.
Archaean landscape (Early Life on Earth, S Bengtson; Earth's Earliest Biosphere, J W Schopf)
The first rain droplets may have appeared when bacteria of the pseudomonas’ type arose over the Earth’s surface. Typically, in the absence of solid particles functioning as anchors, the water can be over-cooled down to temperatures of –40 ºC without, nevertheless, congealing. Those bacteria produce, however, a macromolecule that, along with the dust raised by the wind, worked as a solid anchor that made easy the solidification of the water as soon as the water fell under 0 ºC (273 ºK).
The bacteria may have used it to freeze competitors or predators, to break the hard skin of their meals or to fracture rocks.
A solidification favoured through this process would release heat, which would make the cloud become hotter and would, therefore, favour its rising to the higher and colder layers. This would provoke the subsequent freezing of an additional quantity of water vapour that existed in it. At last, when the droplets would reach a volume and weight big enough, they would fall down under the form of rain.
The Oxygen Surrenders the Archaean
The end of the Archaean eon occurred about 2500 million years ago, when the production of oxygen overcame the accumulation of the reducer elements, namely those produced by the volcanos. There started to appear oxic organisms, which were directly feeded by the products of the cyanobacteria, including the oxygen.
The Proterozoic Eon
During the following period, the Proterozoic, the procaryotic organisms (bacteria, or in other words, simple structured cells without nucleus) began to be replaced by eucaryotic organisms (cells with a complex structure and with a nucleus).
The first eucaryot may have appeared when a bacteria tried to fence in another bacteria and, instead of digesting it, made an association with it.
The eucaryots are formed by symbiotic organelles, or in other words, communities of smaller entities that come together in order to get better chances of being successful at manipulating the planet’s resources. For instance, the chloroplasts that exist in plants (which are eucaryots) descend from the cyanobacteria.
On the other hand, the eucaryots can’t reproduce themselves through division (because there would be the risk that some vital organelles were not present in each descendant) and, therefore, it was invented the sexual reproduction.
To the left: an eucaryot, to the right: a procaryot (Christie Lyon)
The Great Glaciation
During this period, the quantity of methane diminished due to the appearance of the oxygen as a dominant atmospheric gas. The methane was oxidised (or in other words, reduced) by products resulting from the fall of the solar light on the oxygen, like the hydroxyl radicals. The decline of the atmospheric methane (greenhouse gas) would have driven the world to a glaciation 2300 million years ago (the glaciation of Gowganda).
On the other hand, the free oxygen present in the atmosphere would have reacted with elements like carbon and sulphur, releasing acid substances into the air. These substances would provoke an increase on the erosion of the crust rocks, driving to the release of nutrients and, therefore, to a higher wealth of living organisms (like the cyanobacteria), which produced more oxygen. All this process drove to the creation of a virtuous circle.
The First Oxic Consumers
The growth of the quantity of cyanobacteria’s corpses drove to an increase on the number of the organisms that consumed them – the methanogenes. The organisms that were able to breathe oxygen (increasingly abundant in the atmosphere) also gained a growing relevance, competing with the methanogenes at the role of organic products’ consumers. Finally they ended up replacing the methanogenes as dominant creatures. Contrarily to the methanogenes, these organisms were directly feeded by living cyanobacteria. The falling quantity of methanogenes would have contributed to the decrease in the proportion of atmospheric methane.
It’s thought that about 2000 million years ago the environment would have stabilized, existing in it an already remarkable quantity of oxygen, a lower quantity of carbon dioxide (consumed by the cyanobacteria) and a much lower quantity of methane. The photo-synthesizers (as the cyanobacteria) existed in large quantities, as well as the oxygen consumers, while there persisted a small population of methanogenes. Given the decrease in the proportion of CH4 and CO2, the temperature would be lower than at the end of the Archaean but it would be already stabilized.
The appearance of the planet during that epoch was already much more similar to its actual aspect than it was the Earth of the Archaean. The sky was pale blue, with a cloud coverage that was eventually more dense than it is today, the sea was greyish blue and on the beaches, besides the sand and pebble dunes, bacteriological carpets would also extend over them. Like in the Archaean, rocky structures built from stromatoliths’ colonies (cyanobacteria) could be found in the sea.
The desolated landscape of the driest desert of the world - the Atacama, in Chile – shall not differ a lot from the aspect of the proterozoic Earth