The Earth’s atmosphere formed following the initial coalescence of the Earth-Moon system, estimated to be 4.5 billion years ago. It would be unthinkable for most present-day organisms to attempt to live in that environment. The primary component of the atmosphere was carbon dioxide, which most readers will recognize as a greenhouse gas. “Other gases, such as molecular nitrogen, water vapor, and small amounts of carbon monoxide, sulfur gases, and trace quantities of methane, and hydrogen were also present.” (PDF, page 33) Note particularly the absence of free oxygen.
Chemical processes begin to increase oxygen share
Earth’s orbit, some have pointed out, places it in a temperature zone favorable to the varieties of life which have arisen and evolved here. It may not be known to all of those who hold this view that, at the time the Earth formed and long thereafter, the Sun’s radiance was considerably less powerful. The consequence of this dimmer light output might have been a planet whose oceans and atmosphere were frozen, in which case neither its eventual oxygen-rich atmosphere nor life would have developed at all.
In simplest chemical terms, Earth’s early atmosphere contained abundant carbon dioxide. Carbon dioxide may receive energy from solar radiation that causes it to produce carbon monoxide and oxygen. Likewise, water vapor in the atmosphere could be made unstable by irradiation, resulting in the production of molecular hydrogen (two atoms) and monatomic oxygen. Given sufficient reaction time, these tiny changes resulted in higher levels of oxygen, giving life other than prokaryotes a chance to reproduce and spread.
The greenhouse effect, which is now causing consternation to world governments, trapped solar radiation, multiplied its heating properties and ensured that life on this planet had a warm and wet place to develop. Over the course of millions of years, photosynthetic bacteria transformed the composition of Earth’s atmosphere from “hothouse” mode to one supportive of the evolution of life requiring free oxygen to breathe. “We don’t know how long it took for photosynthetic life to evolve and begin the production of oxygen, but we do know that photosynthesis is the dominant metabolic process that sustains the biosphere today.” (PDF, page 36)
Biological processes take advantage
With free oxygen increasing in the atmosphere, that same solar radiation transformed some of the molecular oxygen present in upper levels of the vast gaseous envelope around this planet into ozone, whose atoms contain three oxygen atoms. Ozone forms a protective barrier blocking some of the dangerous ultraviolet radiation entering Earth’s environment from space. Higher life, having developed through natural selection, might never have arisen or survived in the presence of much higher ultraviolet radiation.
Just as biological processes are largely responsible for the amount of free oxygen in Earth’s atmosphere, so it is believed they continue to regulate that amount today. However, scientific understanding is not complete with regard to the mechanism by which this regulation proceeds. This is quite enough reason for caution in human engineering projects on colossal scale that may impact world oxygen production levels. “The current level (20%) is maintained by processes not yet understood.”
How this happened, where it’s going
While it is possible that human science will find hard and fast answers to the questions that evolutionary history raises with regard to the gaseous environment of the Earth, care must be exercised in the research. Human activity is already believed to be responsible for adding back into the atmosphere excess amounts of greenhouse gases, along with other pollutants that may have the potential for ecological disaster.
Regardless, the study of this material will supply fascination for all who wish to learn nature’s “how-to” for the creation, care and feeding of life. It is likely that some of the answers they find will be surprising, as is the presence of life on this beautiful blue planet.
