Environment has been an important factor in shaping the biosphere to its present state and composition. However, in the process it has to undergo drastic changes itself. The environment in which life appeared was altogether different from the present-day environment.
About 600 million years ago, the biosphere must have existed under oxygen-deficient atmosphere which was warm, humid and rich in carbon dioxide while lithosphere and hydrosphere probably lacked sufficient nitrogen. Biocidal ultra-violet radiations penetrated right unto earth’s surface as there was no ozone layer. This confined early life to water only.
It was the evolution of photosynthesis which was responsible for the gradual modification of environmental conditions. In the process of photosynthesis, carbon dioxide is taken up from the environment and oxygen and organic matter are produced. With evolution of this process gradual depletion of atmospheric carbon dioxide started.
We have a little more than 5000 x 109 tons of carbon locked in fossil fuels which represents the photosynthesis of past ages. At present there is about 8 3 0 x 109 tons of carbon locked in the biosphere. Thus photosynthesis up to date has trapped an enormous amount of carbon in the biosphere and the fossil fuels. Today, the atmosphere contains about 720 x 109 tons of carbon as carbon dioxide which amounts to 350 parts per million parts of atmospheric air. About 1880 A.D., atmospheric concentration of carbon dioxide was 200 ppm only. Carbon dioxide is a greenhouse gas, lowering of its content in the atmosphere must have reduced the mean global temperatures considerably (Rothy, 1980).
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Photolytic dissociation of water under ultra-violet rays might have produced some oxygen, however, it was mainly photosynthesis which has added most of this vital gas to the atmosphere. As the level of oxygen in the atmosphere determines the extent of ozone concentration, in oxygen- deficient atmosphere there was no ozone layer to protect the terrestrial life till about 440 million years ago. It was only during the Silurian era, that enough oxygen could accumulate in the atmosphere to sustain an effective concentration of ozone which could filter out ultra-violet radiations. This enabled life to colonize the land surface. The critical level of oxygen in the presence of which an effective ozone layer can be established is about 1/10th of the present-day oxygen content of the atmosphere (Brancazio and Cameron, 1986).
For synthesis of protoplast in addition to carbon, other elements, like nitrogen, phosphorus, sulphur, micro-nutrients etc. are also required. Earth’s surface had enough of mineral nutrients but lacked sufficient nitrogen for luxuriant plant growth. The process of nitrogen fixation, therefore, should have closely followed the evolution of photosynthesis.
Bacteria and blue green algae which were already there about 600 million years ago gradually enriched earth’s surface with compounds of nitrogen. It was the organic matter enrichment of earth’s surface which gradually turned rock fragments – sand, silt and clay etc. – into soil.
The hydrosphere obtained nitrogen from lithosphere as well as from the atmosphere. It caused natural eutrophication of aquatic bodies on earth’s surface. As time elapsed oceans received more and more salts, silt and sediments which gradually raised their salinity.
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As a consequence of these interactions of biosphere with atmosphere, hydrosphere and lithosphere, the conditions of environment were modified to suit the requirement of living organisms. The present-day environment is, therefore, a product of about 600 million years or more of ceaseless activity of biosphere.