The emergence of life elegantly illustrates the intimacy of the connection between atmospheric evolution and biological evolution. The pre-biotic atmosphere, with its low concentration of free oxygen, not only provides favorable conditions for the development of biologically important molecules; the atmosphere also contains the chemicals from which life's precursor molecules themselves can be synthesized.
The famous Miller-Urey experiment in the 1950s demonstrated that a gas mixture containing methane, ammonia, hydrogen, and water subjected to simulated lightening discharges is capable of producing amino acids (essential biochemicals) within a very short time. Subsequent experiments have shown that gas mixtures more closely resembling the actual composition of the pre-biological atmosphere produce essential biomol-ecules under the influence of electrical discharges. Hence, laboratory experiments demonstrate that the atmospheric conditions on the early Earth were sufficient to give birth to the molecules of life.
It is unknown how life arose from this primordial mixture of chemicals, though plausible mechanisms can be postulated. There is certainly plenty of time in which rare events leading to unusually stable states could repeatedly occur; about a billion years separate Earth's formation and the dawn of life, sometime around four to five billion years ago.
The number of places on the planet's surface where life could form, however, must have been somewhat limited. The organic molecules needed for living systems are susceptible to destruction in the presence of free oxygen, and sunlight is a catalyst for this oxidation. Even though the concentration of oxygen in the early atmosphere was low, its presence would have confined the initial emergence of life to the water. Water acts as a filter, preventing the damaging ultraviolet wavelengths of light penetrating much below the surface. However, the oceans would have been unsuitable environments for early life because the turbulent vertical mixing of ocean waters would bring biomolecules and early organisms to the surface where sunlight could initiate oxidation. The first living things were, therefore, confined to the sub-surface waters of stagnant pools.
These early life forms were different from the majority of living things alive today; there was so little oxygen available that they evolved to survive without it. Relatives still survive on Earth today as the anaerobic bacteria that live in stagnant water. These early bacteria were photosynthetic, using visible wavelengths of light to make food and biomolecules from CO2 and water. An important by-product of this activity of the first living things was oxygen.
Over the next billion years or so, these simple cells carried on a chain of life that steadily increased the oxygen concentration of the atmosphere, from its low point of less than one-billionth of today's value, to somewhere around one percent of the present concentration.
This increasing oxygen concentration was an important development, for with it the came an increase in the complexity of biochemistry available to living systems. Most importantly, the development of structural proteins that can be formed only in the presence of oxygen allowed the evolution of the cell nucleus.
With this change, biological complexity could increase rapidly by means of sexual reproduction; genetic material could be shared among members of a species. The concomitant increase in variability gave rise to simple animals and plants, as well as to more varied bacteria. Aerobic respiration became dominant, as it is today, and life was able to explore many more configurations and exist in many more niches.
All the while, photosynthesis was generating more and more free oxygen.
An important atmospheric change was occurring alongside all this biological activity; the action of sunlight on the increasing amount of oxygen in the air was generating ever-higher concentrations of ozone. As the ozone concentration increased, the intensity of damaging ultraviolet light at the surface of the waters was rapidly decreasing. Then, as now, the ozone layer served to protect life from harmful radiation from the Sun. The depth of water required to screen the Sun began to be reduced, and life could finally enter the open ocean.
This led to an explosion in the numbers of living creatures on the Earth, as life expanded into the newly available space. More life meant more photosynthesis and rapid increases in the atmospheric oxygen concentration. With increasing availability of free oxygen, even more complex biomolecules became possible, and multicellular life arose. Fossil evidence of jellyfish-like creatures from 670 million years ago gives valuable evidence of the oxygen concentration. These creatures had no lungs, and must have relied upon the ability of oxygen to diffuse across their skin. It is estimated that to support their existence, the atmospheric oxygen concentration must have been around seven percent of its present value at that time.
About 550 million years ago, fossils with strong and impervious exoskeletons suggest that the oxygen concentration had risen to 10 percent of present values (around two percent of total atmospheric composition), and the ozone concentration was approaching levels that would enable life to exist on the land, unmolested by ultraviolet radiation. Land-based life emerges in the fossil record at 420 million years ago, and by 380 million years ago the complexity of land-based life has multiplied remarkably leading to the appearance of the Carboniferous period's Great Forests (from which fossil fuels derive). The rapidly increasing oxygen and ozone levels allow ever more complex land-based life, and the forests soon find themselves home to amphibious animals, mammals, and eventually the flowering plants. With this blossoming of life, the atmosphere finally reaches present oxygen and ozone levels by 300 million years ago.
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