LIFE, MASS EXTINCTIONS, THE ANTHROPOCENE (1st part: prokaryotes)

 

Post n. 40 English

The researches of Derek York illustrated in "Gli albori della storia della terra" Le Scienze" 1995, have highlighted how the plate tectonics is a process started in the very first phases of the history of our planet. The continents were drifting, as early as 3.5 billion years ago "[...] at a rate of 1.5 centimetres per year, i.e. similar to the rate at which the North American continent has been drifting away from the Mid-Atlantic Ridge over the last 100 million years".

Drift leads the continents sooner or later to collide and thus to a new drift. Where the 

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continents collide, the underlying material may emerge as lava or give rise to mountains (orogenesis), while over 200 million years all the oceanic crust slides into the mantle. All the material that emerges is subjected to continuous erosion that goes slowly to replace the oceanic crust. The earth since its formation is therefore a planet in continuous evolution and the earth's crust has been since its origin continuously recycled. Nevertheless, some ancient sediments have been spared by erosion and orogenetic processes. From the study of these ancient sediments, called ancient continental shields and from the subsequent sediments up to our days we can trace, even if with many gaps, the tragic and at the same time extraordinary history of life. The discovery, in these sediments, of very ancient fossils shows us our evolutionary link with microorganisms that lived at the dawn of life.

Life originated about 3.6 billion years ago in the form of single-celled organisms. The first organisms were surely heterotrophic, they fed on organic substances that were abundant in the primitive environment and they were unable to synthesize nutrients. When this type of nutrient began to be scarce, some microorganisms were able to produce nutrients from carbon dioxide (CO2) by obtaining the necessary energy from the oxidation of hydrogen sulfide (H2S) or oxidizing iron from Fe + +  to Fe + + + (chemoautotrophs), others obtain energy using solar energy (photoautotrophs), sulfobacteria. These microorganisms called anaerobic because they lived in the absence of oxygen, did not use H2O as a source of hydrogen, and none released oxygen into the atmosphere. How long nourishment was available in the primitive environment, and how the microorganisms learned to produce their own nourishment, we do not know. We can imagine that the time must have been long enough to allow, in some organisms, a series of mutations. Driven by the survival instinct for the scarcity of food, these microorganisms assembled in a colony have made the leap from heterotrophy to autotrophy. An evolutionary process accelerated by environmental conditions and the possibility to exploit an enormous number of ecological niches. These microorganisms similar to today's bacteria, called prokaryotes, were for 2 billion years, the undisputed rulers of the planet.

The term prokaryote also includes another group of protoorganisms: cyanobacteria, often called blue algae, microorganisms capable of photosynthesis that use H2O as a source of hydrogen and as a by-product release oxygen. But when life appeared on earth, and even in later epochs, the atmosphere did not contain oxygen or as is often said was anoxic. This hypothesis has received several confirmations. At the beginning of the sixties of last century, laboratory experiments have highlighted that the abiotic synthesis of simple molecules for the origin of life occurs more easily in the absence of oxygen than in its presence. In addition, the presence of oxygen in the atmosphere, with the consequent formation of ozone (O3), would have destroyed these molecules and life would never have occurred. Further confirmation comes from ancient continental shields, in particular sedimentary deposits of Uraninite (UO2). Uraninite sedimentary deposits, formed in river beds, have been found and show various dates, the most recent dating from about 1.8 billion years ago. No sedimentary deposits of uraninite are found after this date. Uraninite in the presence of oxygen oxidizes rapidly to U3O8, this compound is soluble and is carried away by water. The date around 1.8 billion years ago is therefore a watershed and we can say that until that time the atmosphere must have been anoxic. A confirmation of this dating also comes to us from the striated formations of iron. Iron can be found as iron ore Fe++ that is reduced or, if oxygen is present, as ferric Fe+++that is oxidized, red in colour. Ancient sedimentary deposits contain mainly reduced iron, have different dates with a minimum age of 1.8 billion years ago. It has been calculated that at that time the atmosphere contained small quantities of oxygen, 1/1000 of the present one, and it was produced exclusively by the dissociation of water in hydrogen and oxygen by solar radiation. The oxygen released by this process could not accumulate in the atmosphere because it oxidized the surface of the iron sediments. The same process that, due to the presence of a thin coating of oxidized iron (Fe2O3), turned the surface of Mars red. With such a concentration of oxygen the ozone shield was almost non-existent, the atmosphere was therefore transparent to ultraviolet rays lethal to living organisms. Ultraviolet rays rendered land and oceans sterile down to a depth of 10 meters. The first microorganisms necessarily had to live beyond a depth of 10 meters or in shallow lagoons hidden among the sediments or sheltered among the debris of muddy and sandy areas.  Another category of important minerals are red beds, so named because they are red in colour due to the presence of oxidized iron Fe2O3. They are deposits that were formed in the presence of an atmosphere that contained oxygen. There are different dates, some of 200 million years ago others of 400 million but the oldest are dated 1.4 billion years ago. It has been calculated that at this last date the content of oxygen in the atmosphere had reached 1/100 of the current one. In conclusion, for almost two billion years the content of oxygen in the atmosphere, produced by the dissociation of water by ultraviolet rays, remains constant, while between 1.8 and 1.4 billion years ago is poured into the atmosphere a large amount of oxygen. Given the enormous temporal distance that separates us from these events, it is clear that these dates cannot be taken as definitive boundaries. For example, the study of sulphur isotopes in the various epochs brings back to 2.4 billion years ago the beginning of the appearance of oxygen in the atmosphere. However, all scientists agree on one thing: this drastic increase of oxygen in the atmosphere was caused by photosynthesis of cyanobacteria.

But when did cyanobacteria appear?

The ancient continental shields give us evidence that 3.5 billion years life already existed and was also well diversified. Around the end of the sixties the famous palaeontologist Elso S.Barghoorn, "I fossili più antichi" Le Scienze 1971, and his collaborator J. William Schopf, discovered microfossils in very ancient rocks. Specifically in the Fig Tree formation in South Africa, dated 3.2 billion years ago, they found 28 specimens of microfossils that resemble present-day bacteria. Of these, 2 specimens look no different from today's cyanobacteria. Microfossils have also been discovered in sedimentary deposits of North America called the Gunflint Formation and dated 2 billion years ago. In these microfossils, 8 different genera have been identified of which 4 are similar to present day cyanobacteria. Probably the discovery of only 2 specimens of microfossils capable of photosynthesis, dating back to 3.2 billion years ago, suggested to Barghoorn a certain caution and in fact he concludes: "There are valid reasons to believe that the organisms found in the Fig Tree flints and dating back to three billion years ago were not capable of photosynthesis. Instead, it is probable that the Gunflint forms, a billion years more recent, were photosynthesizing."

In 1978 J. William Schopf in "L'evoluzione delle prime cellule" Le Scienze, publishes an article on the evolution of biochemical systems by natural selection from the first organisms until the elaboration of the biochemical apparatus of photosynthesis, the process that generates oxygen. He concludes, "The first photosynthesizing organisms made their appearance before three billion years ago. They were anaerobic [thriving in the absence of oxygen] precursors of modern photosynthesizing bacteria. [...] The appearance of aerobic photosynthesis in the middle Precambrian introduced a change in the global environment that was to influence all subsequent evolution." Recall that, for paleontology, Precambrian means the period from the formation of the Earth to the Palaeozoic that begins with the Cambrian about 600 million years ago. By middle Precambrian we therefore mean about 2 billion years ago, in line with Barghoorn's conclusion. To conclude, at the beginning of the 80s of the last century it was believed that the first organisms were divided into heteroautotrophs, chemoautotrophs and photoautotrophs (sulfobacteria), appeared 3.5 billion years ago and capable only of a primitive metabolism, and that cyanobacteria, with a more complex metabolism responsible for the increase of oxygen in the atmosphere, appeared around 2 billion years ago. This kind of scenario is consistent with the Darwinian view of a slow transition from the less complex to the more complex.

In the latter half of the 1980s, J. William Schopf and two of his collaborators collected flints in an area of the Pilbara craton in Australia, named Apex and dated to 3.5billion 

years ago. Microscopic fossils were discovered in these flints. In particular, 1990 specimens of about 200 individuals were identified, grouped into 11 different types. It is Schopf's opinion that 6 of the 11 species are cyanobacteria that in size, shape, and cellular organization resemble modern cyanobacteria. These microfossils are found together with carbonaceous organic debris whose Carbon isotopic composition is typical of cyanobacterial photosynthesis. This discovery convinced Schopf to radically change the previous paradigm, introducing a vision that was in some respects revolutionary. He published his research in 1993 and in 2003 he took a broader view of his conclusions and published "La culla della vita" 2003, where he states, "If this parentage is correct, the presence of cyanobacteria in this nearly 3.5 billion year old community testifies to the fact that evolution came a long way in a hurry at first. All cyanobacteria are capable of performing the kind of photosynthesis that releases oxygen and, like higher animals and plants, can inhale oxygen (via the process known as aerobic respiration). Photosynthesis and aerobic respiration are, however, both advanced life processes, evolved from other more primitive in which oxygen did not play any role. If in such remote times cyanobacteria existed, they had to be already present even the processes evolved previously; they had to have already been part of the living world both photosynthetic organisms that do not release oxygen (bacterial photosynthesizers) in addition to those that release it (cyanobacteria), and microbes capable of living in the absence of oxygen (anaerobes) in addition to those that breathe (aerobes). These are precisely the processes that feed the current living world. If among the Apex fossils we also find cyanobacteria, we must necessarily conclude that the foundations of the world ecosystem were already established in these early stages of Earth's history.  He further adds, "The key metabolic processes of current life, heterotrophy and photoautotrophy, anaerobic or aerobic were invented by microbes that existed billions of years ago. Whether the cycles of CHON (Carbon, Hydrogen, Oxygen and Nitrogen) and energy concern the animals and plants of our times or only microorganisms, as in the distant past, the systems used for their circulation are the same, as well as the same rules apply. The current ecosystem is not recent at all, but is just an enlarged version of the ecosystem originally created by a menagerie of ancient microbes."

This new vision anticipates, therefore, the appearance of cyanobacteria of 1.5 billion years, no longer to 2 but to 3.5 billion years ago. It follows, as we will see shortly, that evolution has undergone a very long period of stasis of over 2 billion years. Because this new paradigm does not so much match the Darwinian view, Schopf revived the term bradytelike developed by paleontologist Georg Simpson for exceptionally slow-evolving life forms. To give an answer to the ultra-slow change of cyanobacteria Schopf coined the term Hypobradithelia and concluded, "Simply put, it is argued that cyanobacteria have maintained the status quo, with little or no change since they burst onto the scene billions of years ago”.

Schopf's conclusions about the presence of cyanobacteria already 3.5 billion years old received a lot of criticism especially since 1996. In that year in fact NASA scientists announced the discovery of remains of microorganisms in a Martian meteorite. In the same year, in a conference organized by NASA to present the discovery and in the presence of many scientists, Schopf was the only one to say that there was no scientific evidence that in the meteorite there were remains of microorganisms.  His opponents insisted on the appearance of cyanobacteria not earlier than 2 billion years ago. Schopf brought more evidence to support him, meanwhile molecular traces were discovered in the Pilbara craton that point to a well-settled presence of cyanobacteria at least 2.7-2.5 billion years ago. (Robert M. Hazen, "Breve storia della terra" 2017).

The debate continues.

Without wishing to enter into a dispute that concerns experts, an outside observer can, however, draw logical conclusions about the facts. In relation to the chain of food cycles, the world ecosystem is a closed system, and the primitive ecosystem must have been closed as well. Could our ecosystem today do without plants? In the hellish conditions our planet was in 3.5 billion years ago, evolution had to explore all possible avenues for survival, and without the affirmation of cyanobacteria the primitive ecosystem would have been stumped and life could have become extinct. For survival imperative was everything and now.

Another consideration to highlight is the following. Scientists agree that one of the first forms of autotrophy, 3.5 billion years ago, was the anoxygenic photosynthesis of sulfobacteria.

6CO2 + 6H2S + light = C6H12O6 + 6S2

The famous paleobiologist Preston E. Cloud in 1983 in "La biosfera" Le Scienze put forward the hypothesis that it could have been a mutant sulfobacterium the first to split water molecules instead of hydrogen sulfide (H2S) molecules, according to the reaction

6CO2 + 6H2O + light = C6H12O6 + 6O2

while preserving its ability to use hydrogen sulphide as an alternative energy source.

Now, if you keep in mind that hydrogen sulphide was found only in particular areas of the earth and was not as abundant as water, it is incomprehensible how sulphobacteria could have taken 1.5 billion years to use water instead of H2S and have an inexhaustible source of raw material.

Schopf's conclusions are probably closer to the reality of the time.

We observe that throughout the history of life, four fundamental events produced groundbreaking breakthroughs that upset the course of events. Without these turning points, life on our planet would have remained at the stage of microorganisms. Well, the first of these breakthroughs, 3.5 billion years ago, was the aerobic photosynthesis of cyanobacteria.

But if cyanobacteria appeared 3.5 billion years ago, and if there was no oxygen in the atmosphere until 1.8 billion years ago, what happened to the oxygen produced by cyanobacteria during this long period?

According to Schopf between 3.5 and 2 billion years ago, a huge amount of ferrous iron (Fe++) was poured into the oceans giving rise to the banded iron formations. But initially, in the places where cyanobacteria lived, the oxygen produced was used to oxidize iron in solution (from Fe++ to Fe++). As oxygen spread throughout the oceans and scavenged the ferrous iron (Fe++), about 1.8 billion years ago, it began to spill into the atmosphere. As oxygen slowly increased in the atmosphere, the ozone shield began to form and ultraviolet rays were thus blocked in the atmosphere.

When around 1.4 billion years ago the concentration of oxygen in the atmosphere 

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reached 1/100 of today's, the concentration of ozone allowed ultraviolet rays to reach just the surface of the oceans. Microorganisms could then rise to the surface and have the entire ocean at their disposal. While these photosynthetic organisms developed defences to live in the presence of oxygen, aerobic, for anaerobic organisms oxygen is a powerful poison. All researchers agree that the presence of oxygen has caused an environmental crisis leading to the mass extinction of several evolutionary lines of anaerobic.

In fact, we have no evidence of such events. It is possible that in the marine environment, some species that lived in the open ocean, where there were few niches in which to hide, became extinct. On the coast and on land, in muddy areas, anaerobes could migrate to anaerobic areas. We must also keep in mind that the increase in oxygen concentration occurred over hundreds of millions of years, so anaerobic organisms had plenty of time to work out defences. In conclusion, we do not know whether the change in atmosphere actually produced mass extinctions of evolutionary lines. Ancient continental shields do not give us any information about possible extinctions in this epoch.  However, in these 2 billion years, storms with sudden high and low tides, volcanism, meteoric bombardment even if much more rarefied than in the early stages of planet formation, were definitely present. Periods of overheating and acid rain were therefore, in geological times, frequent. So, it is more likely that all these processes, associated with the drift and later the collision of the continents, and the appearance of oxygen led the environment to undergo continuous changes both in the short and long term. Ecological niches, refuge for living organisms, disappeared and with them their hosts. We can then imagine that, in this long period, prokaryotes certainly underwent continuous decimations that brought them close to extinction and that several times they had to start all over again the conquest of the earth.

All living organisms need energy to keep us alive. The biochemical process that allows prokaryotes to obtain energy is called fermentation. This process makes available to microorganisms an amount of energy equal to about 50 Kcal per mole.

As we mentioned the heterotrophs fed on substances they found in the environment. These prokaryotes were like today of various sizes and often the larger ones fed on smaller microorganisms, like the big fish eats the little one, predators and prey. 

One day, about 1.4 billion years ago, a large prokaryote swallows a small prokaryote. But before digesting it, fortunately for us, the large prokaryote realizes that this tiny being had exceptional abilities: it was able to extract energy from organic substances using oxygen, a process that we call respiration. Through respiration, from the same organic substances used for fermentation, instead of 50 it is possible to obtain 686 Kcal per mole. The large prokaryote was careful not to digest it; rather the two began to collaborate, a process that in biology has been called endosymbiosis. After a stasis of over 2 billion years, the evolution of this symbiosis, through mutations, gave rise to a cell 10 thousand times larger than the cell of prokaryotes, which is called Eukaryote. The appearance of the eukaryotic cell had as consequences a radical change of life on earth, because all higher (in the sense of more complex) organisms, and therefore also us, are made of eukaryotic cells.

 And here begins another story.  

                                                                               Giovanni Occhipinti