During the last four decades scientists have been performing various experiments to discover how“ the heavens and earth rose out of chaos”, and how life appeared on earth. When our planet was very young , it rumbled with active volcanoes and the skies were crowded with sinister clouds which sent down showers. Then within half a million years the conditions necessary for the development of life were met. Scientists working 4 billion years after the event, have reached a conclusion that the elemental building blocks of life-the amino acids, the purines and pyrimidines the sugars and the fats, arose in the Earth’s primordial broth through the action of lightning and sunlight on simple atmospheric molecules. For decades they have assumed that the recipe for the spup of life contained ammonia, methane, hydrogen and water. But recently the cosmic cookbook spelling out the ingredients of the early atmosphere has begun to be rewritten by dedicated scientists.
At least one atmospheric scientist of the United States is of the opinion that the overwhelming majority of chemical evolution experiments since the first one was done six decades ago, may have been conducted with the wrong atmospheric mixture. According to him none of the experiments included oxygen which is believed to have been present in small quantities in the early atmosphere. And until recently no one was aware of the high levels of ultraviolet radiation the young sun probably emitted–levels considered lethal to living systems as we know them.
This scientist began his new work towards the end of the century, while he was developing a series of sophisticated computer-based photochemical models, designed to assess the impact of man’s activities on the composition and chemistry of the atmosphere. (Photochemistry is the subject that involves the study of the chemical reactions initiated or assisted by exposure of elementary particles to light.) Though these models were made to project into the future, he became fascinated by the idea of running them backward in time with the hope of learning about the origin and evolution of earth’s atmosphere.
Research into the chemical evolution of life dates back to the 1920s when a Russian biochemist and a famous English geneticist independently proposed that organic molecules could not have formed in an oxidising atmosphere (i.e. one containing oxygen molecules). According to them organic molecules could only form in an atmosphere rich in methane, ammonia, hydrogen and water. Their theory of an oxygen-free or highly reducing atmosphere was placed on a firm footing in 1952 when a graduate student at the University of Chicago, Stanley Miller and Harold Urey, the late Nobel prize-winning chemist conducted their now famous experiment. Miller sent a continuous electrical charge through a mixture of methane, ammonia and hydrogen that circulated through a flask of water vapour. A week later they noticed that the reddish brown soup which had formed at the bottom of this collection chamber actually contained several amino acids—the precursor of living systems that had been predicted by the Russian biochemist.
In 2007, after Miller’s death the sealed vials preserved from the original experiments was examined by scientists, who were able to show that there were actually well over 20 different amino acids, produced in Miller’s original experiments. That is a much higher figure than what Miller originally reported, and higher than the 20 that naturally occur in the genetic code.
Three decades later when the American atmospheric scientist mentioned earlier and his colleague entered the classic mixture of gases into the computer models they discovered that an early atmosphere with appreciable amounts of the methane and ammonia would have been chemically unstable and hence short lived. They were of the view that owing to the decomposing influence of ultraviolet radiation and hydroxyl radical, a molecule that results from the break-up of water vapour the life-times of ammonia and methane would have been less than 100 years.
Then what divine breath infused the atmosphere? The answer is provided by the new science of planetology. Scientists now believe that the atmospheres of Earth, Venus and Mars formed through the volcanic emission of gases were originally trapped during the formation of planets about 4.5 billion years ago. The temperature difference explains how the three planets with similar compositions could yet develop quite different present-day atmospheres). The volcanic emissions were composed of about 80 per cent water vapour, 10 per cent carbon dioxide, one per cent nitrogen the remainder being sulphur compounds chlorine and so on. So planetary evolution prescribed that the earth’s early atmosphere would be composed primarily of water vapour, carbon dioxide and nitrogen. Recent laboratory experiments suggest that such a mixture can yield life’s vital organic compounds. Thus all the gaseous precursors needed for the production of the complex organic molecules which Urey and Miller found in their methane, ammonia and hydrogen atmosphere, can also be produced in a carbon dioxide, nitrogen and water vapour atmosphere. The only difference is that the yields one would get with the latter are less. But then it just probably means that it took a little longer and time is one thing you do have when you are thinking about periods running to millions of years.
Until a few years ago scientists believed that the early atmosphere contained little or no oxygen. The small amounts present were produced through the chemical decomposition of the atmospheric carbon dioxide and water vapour by ultraviolet radiation. But it now appears that the early atmosphere may have contained a million times more oxygen then anyone had believed before.
The theoretical insight for this idea was provided by an astrophysicist in the United Sates who began began to wonder how the computer based models of the early atmosphere would be affected by the possibility that the sun had in its youth emitted more ultraviolet light than it does today. Recent measurements of half a dozen young sun-like stars indicate the possibility that the young sun may have emitted as much as 10000 times more Ultraviolet rays than it does today. When the increased Ultraviolet value was inserted in the computer based models it was found that the oxygen levels in the atmosphere rose by a factor of about one million.
The early geological record appears to support this conclusion. Geologists have long suspected that the oxygen level in the early atmosphere had to be much greater than previously calculated. Their analysis of the oldest rocks estimated to be more than 3.5 billion years old found oxidised iron and uranium in amounts that called for the atmospheric, oxygen levels to be a hundred to billion times greater than generally accepted. But scientists who study oxygen’s lethal effects on early life forms believe that even the calculated million fold increase in oxygen indicated above does not raise the early oxygen level enough to be detrimental to life’s emerging organic compounds.
Some scientists would like to believe that a mildly reducing atmosphere would have been essential for producing life, an idea that first occurred to Darwin. However there is controversy regarding the amounts of certain compounds such as hydrogen that were present. The scientists are of the view that hydrogen ought to have been present since in the absence of molecular hydrogen no organic compounds could have been made but they do concede that it was present in very insignificant concentrations compared to the water vapour, nitrogen and carbon dioxide.
Some chemists find it difficult to assume that the earth produced sufficient amounts of these volatile substances by volcanic outgassing amounts. They feel that comets may have been responsible for depositing these volatiles.
All the same, it has to be conceded that some atmospheric scientists have already made a significant contribution to our knowledge of the origin and evolution of the earth’s early atmosphere and that it is through continuation of such work that some day a major breakthrough would be achieved in this field.