Of all the planets in our solar system, there is only one planet that we know that has an atmosphere of about 21% oxygen and in such abundance that if possible it could not only be detected from afar but if it was it would stand out as having an oxygenated atmosphere compared to Venus and Mars which are composed of carbon dioxide, while Jupiter, Saturn, Uranus, and Neptune are mainly hydrogen, helium, and a few organic compounds such as methane and ammonia.
Earth is that unique planet which truly stands out for not only having an atmosphere of oxygen but the bulk of that oxygen is truly the result of that one ubiquitous biological process, photosynthesis where green plants use sunlight along with water and carbon dioxide and produce oxygen as a waste products. Animal life including humans make use of the oxygen for their metabolism and since we and all the animals big or small use oxygen you may think that without , there would be no complex animal life and indeed if it weren’t for oxygen, there would be no evolution of animal life.
Here is a paradox regarding oxygen that may strike you as the more surprising. Oxygen, from a chemical standpoint , is really a poisonous gas and its ability to act as a poison lies in it ability to react to many substances such as organic or carbon based compounds. A most obvious example is combustion where in the presence of oxygen, any material such as wood when exposed to high temperature reacts with oxygen by producing fire, heat, light, along with carbon dioxide.
Why the reactivity of oxygen? To understand why is to understand the atomic structure of oxygen. Oxygen , like all atoms, is composed of three subatomic particles, protons and neutrons which compose the nucleus and an oxygen atom has a nucleus with eight protons and eight neutrons and the number of protons or those particles with positive charges while the neutrons lack electric charges. The other particles, the electrons, orbit around the nucleus and are composed of negative charges but also like every other atoms, the electrons are not arranged randomly and because of quantum mechanics, there is a specific arrangement of the electron’s and that arrangement makes a difference in whether an atom can form bonds with other atoms or not and that ability is called the “valence” or the number of bonds an atoms forms since chemical reaction and chemistry in general depends on the electrons of each atom interacting with the electrons of other atoms which is the essence of a chemical reaction.
The number of protons determines the identity of every element and for oxygen, there are eight protons and eight electrons and as how the electrons are arranged, two electrons are arranged in a orbit called a “shell” and the innermost shell can, for reasons of quantum mechanics, can hold up to two electrons and no more than two can exist in the first shell. It is in the second shell where the chemical reaction can occur and in that second shell, a total of eight electrons can be held but since oxygen only has eight electrons, only six electrons can be found in that second shell but only two electrons are missing so because of the vacany of that second shell, the oxygen atom “wants” to gain two more electrons in order to complete the outermost shell and it is because of the vacancy, the reason why oxygen is reactive and usually oxygen forms what is called a “covalent” bond which is a strong chemical bond the result of sharing electrons with other atoms.
Usually oxygen forms bonds with other oxygen atoms but also it can form bonds with other atoms and in every molecule of life, oxygen is present in various amounts. It is of course the gas that we breathe in the atmosphere as well as the fact that oxygen when it binds to two hydrogen atoms and since each hydrogen atom has one electron in its shell it will accept one other electron such as an oxygen atom and along with an electron from another hydrogen forms that most important molecule that can support life as we know it, which is water.
The fact that oxygen has a valence of two which is another way of saying that it can accept two electrons from other atoms is also the reason for its reactivity and there lies the power of it being a poison and this would have to do how it reacts in that process called combustion.
In combustion or more simply burning, oxygen combines with an element usually carbon and under high temperatures , the reaction can be pretty violent that in a rapid step by step process oxygen in combination with carbon will produce smoke and heat if the carbon is part of wood or wax and in the process there are what called “radicals” which are atoms of oxygen with unpaired electrons that will quickly react with other atoms and it is the rapid violence of the radicals combining with atoms that is the source of the thermal energy resulting in heat and smoke.
From this you can now see that oxygen with its ability to react is the reason why it is an effective poison or rather destroyer or organic compounds but in living organisms that are aerobic or use oxygen for metabolism, the use of oxygen is needed to react with carbohydrates, proteins, and fats but it is done in a slow and controlled way and in a step by step fashioned. How is it possible for living systems to use a gas that should end up in a puff of smoke?
The presence of oxygen has undoubtedly influenced the evolution of animal life and it is oxygen that for one thing may have been the likely trigger in that unusual explosion of animal diversity known as the “Cambrian explosion” where many unusual kinds of species of animals , many of which are now extinct leaving only a handful of species from that unusual and singular evolutionary event and which survived up to the present day and it is also oxygen that some biologists believed was what favored the evolution of multicellularity which led to the evolution of animals, perhaps as a response to oxygen’s reaction with important molecules, the proteins and the nucleic acids, and it is through multicelluraity that not only allowed the evolution of animals, but in doing so could evolve systems of keeping unwanted radicals in the cells in check . Oxygen, in amounts that have may been higher than today’s levels, may have been the reason why insects and other prehistoric beasts were large and it also the part of the reason of that well known unavoidable fact of life, which is ageing and death.
To understand how life on earth could not only produce oxygen but evolve to cope with its deleterious effects, we must go back and imagine and based on geological and astronomical evidence, about the history of earth.
The earth is about 4.6 billion years old, and earth formed from the condensation of gas and dust along with the formation of the large sphere of gas that would later become the sun while earth and the rest of the seven planets were congealing out of the swirling gas and dust. The earth along with the terrestrial planets formed mainly from the collisions of planetesimals or large rocky objects and with enough mass was accreted, additional planetismals collided on to its surface and from such massive collisions along with radioactive elements, early earth would likely have been a hot, battered molten planet. Rapid outgassing along with collisions with comets would have provided a substantial atmosphere but earth’s first atmosphere would not have contain any oxygen except for water vapor and since 4 billions years ago when the sun formed, it may have emitted much more ultraviolet light than it does now and with so much ultraviolet light, the energy would have split any oxygen which could have been present but in small amounts while the dissociated hydrogen, due to it being the lightest gas, would have simply floated away.
The surface of the young earth would have been appropriately be described as hell yet as the bombardment slowly but inevitably ceased altogether and as the surface become warm enough to allow any water vapor to condense as water resulting in the first oceans, and with water on the surface, the stage was set in what would eventually the origin of life for the conditions were present and although no evidence of whatever process where a set of inorganic chemical reactions would end up forming the molecules of life since whatever process or processes was present is now long gone, we can at least attempt by making reasonable guesses in regards to abiogenesis or the origin of life using a combination of scientific fields as diverse as physics, chemistry, biology, and geology and through these approaches together with geological evidence that can reach as far back as the oldest rocks ever present which are about 3.8 billions years old together with the oldest fossils which are around 3.8 to 3.5 billion years and these fossils are the remains of the simplest life forms which are ancient bacteria.
The origin of life starting from the assumption that the first atmosphere was reducing or hydrogen based and it was from that assumption behind the first experiment in regards to the origin of life which was carried out at the University of Chicago in 1952 by Stanley Miller along with his professor, Harold Urey and in this experiment a laboratory vessel which used water vapor along with hydrogen, ammonia, and methane were allowed to circulate and pass through an electric spark and in a week this reaction produced amino acids, the building blocks of proteins, along with a few other amino acids which could be formed in artificial conditions and various experiments using energy sources other than electricity such as heat, ultraviolet light, and shock waves which results in the formation of organic molecules including nucleotides, or the building blocks of DNA, amino acids, and fatty acids which inevitably result.
The problem is that it is far likely that there were carbon dioxide and nitrogen and less methane or no methane and organic compounds cannot form if there is an abundance of carbon dioxide but somehow organic molecules had to have formed and it would have to have formed in less than 200 million years but whatever the conditions, and for reasons that are still unknown, molecules and eventually life did form and it was simple life as what the oldest fossils will demonstrate.
What were the earliest life forms on the ancient earth and what did they look like? It is very likely that they were not much different than today’s bacteria with it simple or prokaryotic structure. It was possible that they were anaerobic and likely so since there was little or no oxygen present but eventually as the conditions went from hot to mild, this result in evolutionary pressure on bacteria to adapt to new form and the first bacteria may have been what are today’s thermophilic or “heat loving bacteria” and depended on organic compounds to survive but eventually the supply of organic molecules would have run out exerting pressure for some of those simple cells to adopt a new strategy for survival and that is to evolve into autotrophic life forms or those life forms that can synthesize their own food from simple inorganic compounds together with a source of energy and it was possibly the shortage of organic compounds that favored that one form of autotrophy that would have a profound influence on the biosphere and that is photosynthesis.
With the evolution of photosynthesis, oxygen was being produced at such an enormous amount and gradually at 2 billion years, the atmosphere was changed and this increase in oxygen indeed had a profound effect that was at first deleterious. As you probably know by now, oxygen is a reactive gas that will destroy any organic molecule and as oxygen was being built up, not every bacterium survived and even today there are still bacteria that are anaerobic or can live without oxygen but with oxygen some cells evolved that could handle oxygen as part of the process of metabolism and in oxygen based metabolism, there is a molecule that acts as a short term supply of energy for every living process and that is ATP and it is ATP which is produced in both oxygen and non oxygen based metabolism and for those cells that later become aerobic there was an advantage in using the oxygen for energy needs in that with oxygen about 36 molecules of ATP are produced while in fermentation which is a biochemical process that is used by cells in the absence of oxygen, about 3-4 molecules of ATP.
With the ability to use so much energy wherever there are cells that could use oxygen, the stage was set for the evolution of animal life but also with the evolution of oxygen based metabolism came a price.
Unlike rapid combustion where organic materials release a tremendous amount of heat in oxygen, and destroying themselves instantly, the process of respiration as it is called in biology, carbon based molecules needed for life such as glucose, a component of carbohydrates, can produce energy but in the cell, the energy is released in a step by step manner, each bit of biochemical energy is used in the process of synthesizing ATP and other related molecules that store energy for short term use and the process evolve to be the complex set of reactions known technically as “oxidative phosphorylation” where the presence of oxygen which is really known as a “final electron acceptor” where the electrons are passed from one molecule to the next before reaching oxygen resulting in water vapor along with carbon dioxide as the two waste products of respiration.
Recall that chemical reactions involve electrons being shared between the outermost shells of atoms and also that in the process of combustion, radicals or unpaired electrons of reacting atoms are what determines the combustion process. It is no different in that respect for respiration the only difference is that in respiration it is a slow , and steady process. In every process of respiration, there are radicals that do not get neutralized and some will end up in the cytoplasm or in eukaryotic cells, the mitochondria or the cell structure where respiration takes place and where ATP is made.
With radicals in the cell, is where they do their damage and it is the accumulation of radicals that is one of the reasons for the ageing process in higher animals but early in the evolution of life there had to mechanisms to keep radicals from accumulating in the cell to such dangerous levels and natural selection has favored the evolution of certain class of enzymes, the dismutase enzymes, along with the related enzyme catalase and together these enzymes keep oxygen based radicals in check but cannot eliminate them completely but favored by natural selection and rightly so under an oxygenated atmosphere.
The rise of oxygen, in addition to speeding up the evolution of what would be animal life, has also made its mark on the planet which is well documented in the rocks. In rocks where there are iron, these are usually iron oxide or more simply rust and in layers which are called the banded iron formation is where we have reliable evidence for the transition from an atmosphere devoid of oxygen to an oxygen atmosphere and that was made possible because as Sagan (1986) called oxygen ” the first pollutant” and indeed through photosynthesis, oxygen , a pretty dangerous reactant was altering the landscape causing rust as well as death to those life forms unable to adapt yet life forms wound up using that natural pollutant to their advantage.
Not only the atmosphere was being gassed with oxygen but also another unintended advantage is that with so much oxygen, that gas was being exposed to ultraviolet and with ultraviolet light, ozone, a form of oxygen, which consists of three oxygens, was being formed until little ultraviolet light could strike the surface thus setting the stage for the future evolution of life and with oxygen and ozone, the stage was set for multicellular organisms, with their cells equipped with systems of extracting usable energy along with a set of enzymes to deal with radicals, came the rich evolution of animal life and for all the species of animals that came and went, only to replaced with animals along with other life forms together with their descendants, along with the evolution of sex for creating genetic diversity to cope with new environmental changes, came the Cambrian explosion, later with the rise and fall of the dinosaurs, along with later evolution of humans but for all life’s ability to evolve, natural selection as you may have read in previous blogs, is something of shortsighted “blind watchmaker” (Dawkins, 1986) where it can only make do with what is present in an organism but is unable to look ahead and one such is example is ageing which is common in sexually reproducing animals and with the cell’s ability to counteract the accumulation of free radicals which although useful , doesn’t really avoid the inevitable which is the death of the sexually reproducing organism all because of oxygen’s ability to share electrons, which is all that it takes for oxygen to be reactive and through natural selection for life to successfully deal with that reactive poison that is oxygen.
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Dawkins, R. (1986) The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design. New York, NY: Norton
Lane, N (2002) Oxygen: The Molecule that Made the World Oxford, England: Oxford University Press
Oxygen (n.d). Retrieved December 23, 2015, from https://en.wikipedia.org/wiki/Oxygen
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Sagan, D, Margulis, Lynn (1986) Microcosmos: Four Billion Years of Microbial Evolution Los Angeles, California: University of California Press
Ervin Strauhmanis. Forest Fire https://www.flickr.com/photos/ervins_strauhmanis/9554405492/in/photolist-fyhPTb-psbjwf-pubFmA-pu9NzC-pcGrYR-ptWcqc-pcJky2-ptTFYk-ps8imf-pcFabe-pu948Y-pcFueA-4Z7kDS-fBCB1E CC BY 2.0