The early earth, the evolution of the early atmosphere and life

 Authors_:
*Mr. Rupak Bhattacharya-Bsc(cal), Msc(JU), 7/51 Purbapalli, Sodepur, Dist 24Parganas(north) Kol-110,West Bengal, India**Professor Pranab kumarBhattacharya- MD(cal) FIC Path(Ind), Professor and HOD of Pathologyconvenor incharge of DCP and DLT course of WBUHS Faculty DM  courses, Calcutta school of Tropical Medicine,108 CR Avenue, KOlkat-73  West Bengal,India, Ex Professor and HOD  Ophthalmic pathology RIO KOl-73, Ex professor of Pathology WBUHS and EX Professor IPGME&R KOl-20 W.B, India** Miss Upasana Bhattacharya – only daughter pf Professor Pranab Kumar Bhattacharya *Mr.Ritwik Bhattacharya B.com(cal), Miss Rupsa Bhattacharya 7/51 Purbapalli, Sodepur, Dist 24 parganas(north) ,Kolkata-110,WestBengal, India*** Mrs. Dalia Mukherjee BA(hons) Cal, Swamiji Road, South Habra, 24 Parganas(north) West Bengal, India*** Miss Oaindrila Mukherjee-Student, Mr Debasis Mukherjee BSC(cal)  ,Swamiji Road, South Habra, 24 Parganas(north), West Bengal, India           

The atmosphere we enjoy today in earth is radically different one, from the atmosphere that formed with the earth billions of years ago. What was the early atmosphere of the planet the earth, consisted around 3.8Ga on earth? Because the atmosphere played a major role in evolution of life as per Miller Urey jar experiment (S.L Miller- Science Vol117; P528;1953 and H.C. Urey ibid VolX601; P245;1959). The Earth formed with the Sun 4.6 billion years ago. At this point, it was nothing more than a molten ball of rock surrounded by an atmosphere of hydrogen and helium. Because the Earth didn't have a magnetic field to protect it yet, the intense solar wind from the young Sun blew this early atmosphere away. As the Earth cooled enough to form a solid crust (4.4 billion years ago), it was covered with active volcanoes. These volcanoes spewed out gasses, like water vapor, carbon dioxide and ammonia. This early toxic atmosphere was nothing like the atmosphere we have today. Light from the Sun broke down the ammonia molecules released by volcanoes, releasing nitrogen into the atmosphere. Over billions of years, the quantity of nitrogen built up to the levels we see today.

Early earth probably had an atmosphere dominated by carbon dioxide similar to the atmosphere of Venus today. Miller Urey experiment showed that many biologically important macromolecule, important organic compound including sugars and amino acid [Glycine] could be formed by a spark discharge simulating Lightening [During impact bombardment period and steamy weather to rain fall on earth surface &further vaporization of upper layer of ocean] in a jar containing CH4- NH3-H-H 2O, the early atmospheric gas on earth’s surface. Oxygen was nearly absent in the atmosphere of early Earth. So photosynthesis would have created a net gain of oxygen first in the ocean and later in the atmosphere. Eventually with sufficient oxygen in the atmosphere respiration would have balanced photosynthesis except when burial removed the organic material from the oxygenated water or air. Before oxygen could build up in the atmosphere it must have oxidized reduced ions in seawater. There are a group of one-celled organisms that can live in an oxygen free environment. These are the bacteria or prokaryotes. During the period 2.7 to 2.2 billion years ago, these early bacteria – known as cyanobacteria – used energy from the Sun for photosynthesis, and release oxygen as a byproduct. They also sequestered carbon dioxide in organic molecules. They do not have a nucleus and reproduce only by cell division. These creatures are the earliest evidence of life on earth. They were the first organisms to develop photosynthesis. Photosynthesis today is balanced by oxygen using respiration.  According to Miller Urey subsequent reactions between these compounds organized a self replicating RNA molecule- The first life appeared in the earth.

Changes in the surface temperature of the earth through out it’s history were very important for understanding both the geological development of earth surfaces and origin and development of life in earth. Late Prof. Carl Sagon FRS and Muller G, FRS carried out a theoretical investigation of living term changes in the earth temperature on assumption that major infrared absorbing gases in earth atmosphere had always been vapor and carbon dioxide [Carl Sagon & Muller.G Science Vol177, P52; 1972]. But in view of accepted boundary condition for early earth, they concluded that original terrestrial atmosphere must also have contained additional absorbing gases. The earth’s early atmosphere must have gone significant changes in chemical composition as the postulated additional absorbing agency was removed. Secondly,  because any physically probable additional absorber was likely to belong to a chemical species that figured in concerning the origin of life. Sagan & Muller considered “The Ammonia” to be most probable candidate. The surface temperature could be calculated in two stages. The first involved the computation of effective temperature of the planet earth Te-S (1-A)=fóTe4 where S= Solar constant, A= the spherical Abe do of earth, f=the flux factor ó =the steafan Blotzman constant. For a rapidly rotating planet, with a thick atmosphere the area of emitting radiation is taken as 4Ï R2 where R= planetary Radius. Since the area receiving solar radiation as 4ÏR2, the flux factor becomes 4. In case of slowly rotating planets with thin atmosphere the area of emitting radiation is similarly 2Ï R2 and the flux factor f=2. The second stage of their calculation relates Te to the surface temperature Ts by an equation Ts=Te+ÄT   where ÄT was the green house effect in the earth, which also played a vital role in the appearance of life in this planet. On the lifetime of the Earth for the period 4.5 to 4.0 Ga the solar constant (S) had increased by 40-45%, since the origin of solar system. If this was to fit into the model Ts= Tet ÄT, then surface temperature of earth was bellow the freezing point of water during the early phase of earth’s history i.e. the earth had to pass a “Ice cold Stage also”. But the geological evidence suggests the presence of extended sheet of liquid water on earth’s surface was the pre-requisite condition for appearance of life at least3700MYR ago. Carl Sagan and Muller.G also suggested that the infant biosphere of earth was warmed by an atmospheric gas which exerted a ‘Green house effect “by transmitting sunlight while hindering the escape of heat to space. Water vapor made the most significant contribution to the green house effect in that contemporary atmosphere. Sudden fall in temperature could result it an increase in the size of polar ice caps of earth and seasonal snowfields and a corresponding fall in the atmospheric humidity. Both effects would contribute further drop in the temperature. On the other hand a sudden rise in temperature increased the water vapor content of atmosphere. Sagan and Muller suggested that early atmosphere was very rich in ammonia gas and this ammonia provided the blanket to keep the earth sufficiently warm for life to emerge. Recent works suggested that primordial atmosphere probably contained little ammonia but relatively high partial pressure of CO2. CO2 also acted as blanket gas as green house gas. What ever the green house gas, ammonia or CO2, mean surface temperature of earth exceeded that time over 500c with 25% increase in solar heat flux. Now the question stands for CH4 and NH3. However CH4 and NH3 might not have been present in the atmosphere of early earth. Whether methane and ammonia were present or not in primitive earth’s atmosphere were a debatable situation and might depend on whether oxidation state of upper mantle of atmosphere varied over time or not. For that required volcanic sources where from methane and ammonia become a significant component of volcanic gas. Yet the volcanic gases were there, the mantle could have been oxidized gradually by recycling of water from surface to atmosphere and atmosphere to surface, followed by volcanic out gassing of hydrogen. These process of course could have required hundreds of millions billions years to bring the mantle to be it’s present oxidation state. Not by mere0.7 billion years. So in absence of volcanic sources of methane and ammonia gases, the past history of bombardment atmosphere was probably dominated by carbon di oxide and nitrogen gas with traces of CO, H2, NO, N2, reduced sulpher gas.

 With regard to origin of life, the key and very important question was whether photochemical reaction in such an atmosphere could have generated Formaldehyde (H2CO) and hydrogen cyanide (HCN)? The formaldehyde was needed to synthesis of backbone sugar molecule of RNA and HCN was for synthesis of amino acid for base sequences of RNA nucleotides. Pinto [J.Ppinto, GR Gladstone, Y.Lyung et al- Science Vol 210;P 183: 1980] showed that an efficient path way for formaldehyde synthesis existed even in carbon –di- oxide dominated atmosphere, when these molecule should have readily available. But formation of HCN was very much and almost difficult because it would require then breaking up both an Nß N and a CßC triple bond [if it started N2& CO2 to form HCN] both bonds can be severed in very high temperature core of lightening discharge. Yet the resulting N and c atoms are more likely to combine with O2 atoms then with each other unless the atmospheric C:O ratio exceed unity. However Zahnle[K.Z Zahnle, - J of geophysics Res.Vol91; P2819; 1986] showed that HCN could be formed by ion spherically produced N atoms reacting with photolyses by product of trace elements (1-10ppm) ofCH4. However such a scenario requires an atmospheric source of CH4. So explaining how HCN could have formed is still a major hurdle for theories of origin of Life that rely on atmosphere as a source of starting materials. When one thinks of varied molecular process at the origin of life, one can imagine that the first replicating molecule that brought life in earth was a RNA molecule. Possibly about 4.6Billion years ago (Ga) lightening and ultraviolet radiation from sun were enough to break up simple hydrogen rich molecule of the primitive atmosphere. The fragments spontaneously then recombined into more and more complex molecules. The products of this early chemistry were dissolved in water of ocean or ponds forming a kind of organic soup, which gradually had an increasing complexity, until one fine day, quiet by an accident? - A molecule arose that was able to make crude copies of itself using buildings blocks of their molecules in that soup- which was the master molecule of life – The DNA. It took approx one million years to develop a DNA molecule from RNA molecule in the earth. It was possible that life was largely confined to sea during the Archaean period. In the ocean the atmospheric partial pressure of Co2 maintained a continuous flux of particulate organic matter for life into the deep ocean. These flux resulted from   in the surface layers which was limited by rate of supply of nutrients notably nitrogen and phosphorus, from reverie inputs and from the slow circulation of nutrients-CO2 rich deep ocean water .In 1950 Stainley Muller and Harold Uray did an experiment individually and isolate with all possible primitive gasses present in early atmosphere of earth in an airtight thick non breakable glass bottle and gave constant electrical sparkling discharge at the glass bottle. After 100 minutes of constant sparkling, resulted a product looking like Tar. It was It was extremely rich in collection of amino acids (constituent parts of protein) and nucleic acid and amino acids. But not the life. This experiment was widely known as “ Muller Jar Experiment”.

 How Oxygen came in atmosphere of Earth-our hypothesis?

 Oxygen was nearly absent in the atmosphere of early Earth. Oxygen didn't start to build up in the atmosphere probably until about 600 million years ago and it wasn't until the end of the protozoic era that it started to approach today's levels 21%. It appears before us that photosynthetic organisms appeared about 2.2 billion years ago in the earth. At that point, neither the sea nor air of earth contained free oxygen. The oxygen content of the atmosphere then became 1.5 billion years ago at 1%, at the 600 million year point mentioned above, the oxygen content is thought to have exceeded 6% in 1.5 billion years. This would have been about 50-80 million years before the Cambrian explosion . By relating atmospheric composition to the chemistry of various ancient rock types, geologists have inferred that Earth went from largely oxygen free to oxygen-rich 2.4 billion to 2.5 billion years ago .For some untold eons prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes  must lived the real old-fashioned way: an aerobically. These ancient organisms—and their "extremophile" descendants today—thrived in the total absence of oxygen, relying on sulfate for their energy needs.  The fossil record shows that cyanobacteria existed about 2.7 billion years ago, leaving scientists to wonder why 200 million to 300 million years of oxygen production by these bacteria resulted in no accumulation of the gas.  So photosynthesis would have created a net gain of oxygen first in the ocean and later in the atmosphere. Eventually with sufficient oxygen in the atmosphere, respiration would have balanced photosynthesis except when burial removed the organic material from the oxygenated water or air. Before oxygen could build up in the atmosphere it must have oxidized reduced ions in seawater. During the period 2.7 to 2.2 billion years ago, these early bacteria – known as cyano bacteria – used energy from the Sun for photosynthesis, and release oxygen as a byproduct. They also sequestered carbon dioxide in organic molecules. They do not have a nucleus and reproduce only by cell division. These creatures are the earliest evidence of life on earth. They were the first organisms to develop photosynthesis. Photosynthesis today is balanced by oxygen using respiration. There are a group of one-celled organisms that can live in an oxygen free environment. These are the bacteria or prokaryotes. For some untold eons prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes lived the real old-fashioned way: anaerobically. These ancient organisms—and their "extremophile" descendants today—thrived in the absence of oxygen, relying on sulfate for their energy needs Later the researchers also had discovered a possible new species of bacteria that  would survive in early earth by producing and 'breathing' its own oxygen. This finding suggests that some microbes could have thrived without oxygen-producing plants on the early Earth — and thus  possibly on other planets of our solar system even — by using their own oxygen to garner energy from methane (CH₄). The oxygen-producing bacterium, provisionally qas named Methylomirabilis oxyfera, could grow in a layer of methane-rich but oxygen-poor mud at the bottom of rivers and lakes of early earth. These microbes live on a diet of methane and nitrogen oxides, such as nitrite and nitrate. These nitrogen-containing compounds are especially abundant in sediment contaminated by agricultural runoff. The discovery of this new pathway also has implications for life on Mars, where methane exists as a trace gas in the atmosphere, and on Titan, Saturn's largest moon, where there are shallow pockets of liquid methane. In such environments, alien microbes could use this pathway to live off the carbon and energy supplied by methane

Evidence to support the above our hypothesis:

It appears that photosynthetic organisms appeared about 2.2 billion years ago. At that point, neither sea or air contained free oxygen At about that point, bands of iron oxide begin to appear in sedimentary rock. The layers seem to indicate a period of about 200 million years of rising oxygen content in the seas. At about 2 billion years, saturation seems to have been reached and oxygen would begin to accumulate in the atmosphere Iron (Fe) is a very abundant element in the earth's crust so much is released by the chemical disintegration of minerals contained in rocks. Fe⁺⁺ is slightly soluble in seawater while Fe⁺⁺⁺ is insoluble . During the time when the earth had a reducing atmosphere Fe⁺⁺ should have accumulated as dissolved ions in seawater. However at some point the oxygen build-up in the ocean from prokaryote photosynthesis should have oxidized the Fe⁺⁺ to Fe⁺⁺⁺ resulting in the precipitation of insoluble iron compounds. Are such ancient iron rich compounds preserved? Yes there are, in fact the bulk of the iron ore mined to produce steel comes from iron deposits that are about two billion years old . Such deposits are found on all continents and all look much the same . They are reddish and have clearly visible bands hence they are called Banded Iron Formations. The Messabi range of Minnesota is an example of such a deposit. It was for much of US history the primary source of iron ore for the steel mills of Pittsburg, Pennsylvania and Gary, Indiana. If we know the mass of these banded iron formations and the rate at which we mine them we can calculate their residence time and determine how long they will last, or when we will run out of this kind of iron ore .

A second line of evidence, to suggest that the early earth had a reducing atmosphere like Venus and Mars, is the presence of detrital (formed from the products of erosion of pre-existing rocks) pyrite in sedimentary deposits older than two billion years old. Iron pyrite forms in reducing environment and is quickly chemically decomposed in the presence of oxygen. Today such minerals are only preserved in rocks that formed in reducing environments such as swamps etc. However, in rocks older than two billion years old this mineral (iron pyrite) is found in rocks that were probably formed in streambeds.

The possible changing composition of the Earth's atmosphere during its early history is shown in Figure 10. All nucleated cells (Eucaryote cells) require oxygen for metabolism. We and all other plants and animals are built of eukaryotic cells so we all require oxygen. Hence early primitive life (procaryote cells) modified our planet by converting CO₂ and H₂O to organic matter and releasing oxygen to the environment. As a consequence these organisms moved carbon from the atmosphere to the rocks and broke down water molecules releasing oxygen to the ocean and eventually to the atmosphere. Life therefore is a powerful force controlling the composition of the Earth's atmosphere which in turn exerts a powerful control on our planet's climate.

 References

1. Prof Pranab Kumar Bhattacharya., Upasna Bhattacharya; Mr. Rupak Bhattacharya Mr. Ritwik Bhattacharya, Dalia Mukherjee ’ LIFE IN OUR PLANET “ THE EARTH”  e BOOK once was Published in www.unipathos.com, P-2-6 2004- The website  domain owned by Professor Pranab kumar Bhattacharya and his brothers and sister

 http://www.nature.com/news/2010/100324/full/news.2010.146.html#comment-id-9991 See in Nature News Published comments under title The early earth, the evolution of the early atmosphere " by Professor Pranab Kumar Bhattacharya

Published online 24 March 2010 | Nature | doi:10.1038/news.2010.14

http://www.nature.com/news/2010/101201/full/news.2010.643.html  see in NatureNews published comments asTitle= Exo planets or the Super earths- how much probability of colonization of life is there?Published online 1 December 2010 | Nature | doi:10.1038/news.2010.643

http://www.nature.com/news/2010/100428/full/news.2010.207.html#comment-id-18639

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