As in the case with short term development, as described earlier in this post, the long term evolution of ecosystems is shaped by the interaction of allogenic geological and climatic changes and autogenic processes resulting from the activities of the living components of the ecosystem. In a broad sense the ‘’strategy’’ of long term evolutionary development is the same as that of short term ecological succession, namely, increased control of, or homeostasis with, the physical environment in the sense of achieving maximum protection from its perturbations.

     Although we may never know exactly how life began on earth, the generally accepted theory is that the first living things were tiny anaerobic (living without free oxygen) heterotrophs that lived on organic matter synthesized by abiotic processes. The first successional development then, may been more like the hay infusion culture model (see that post) than the autotrophic culture model. The atmo phere at the time of the origin of life 3 billion years ago containe nitrogen, hydrogen, carbon dioxide, water vapor, but little or oxygen. It also contained carbon monoxide, chlorine, and hydrogen sulfide in quantities that would be poisonous to much of present day life. The composition of the atmosphere in those early days was largely determined by the gaseous stuff that comes out of volcanos. The geologist would speak of this as ‘’atmospheric formation by crusts outgassing.’’ The earth’s early reducing atmosphere (a term to contral with oxygenic atmosphere) may have been similar to that now found on venus or jupiter. Because of the lack of gaseous oxygen there was no ozone laver, as there is now. Molecular oxygen o₂ acted on b short – wave untraviolet radiation produces ozone, or 0₃ which in turshields out the deadly radiation. Thus, at first, life could existonly inshielded by water or other barrers, but straange to say it was the short waved radiation that is thought to have created a chemical evolution leading to complex organic molecules such as amino acide that became the building blocks of life. This synthesis also provided food for the first organisms.

      For millions of years life apparently remained as only a tiny foot hold, limited in habitat and energy source, in a violent physical world. The big change began with the appearance of the first photosynthetion algae which were able to make food from simple inorganic substances and which released gaseous oxygen as a by product. As the oxygen diffused into the atmosphere, the ozone shield developed and life could then spread to all parts of the globe, and there followed an almost explosive evolution of increasingly complex aerobic organisms. The broad pattern of the evolution of organisms of organisms and the oxygenic atmosphere that make the biosphere absolutely uniqe in our solar system is shown in this post. Over long stretches of time production exceeded respiration (P/R > 1) so oxygen increased and CO₂ decreased. Our fossil fuels were also formed during periods when P exceeded R by a wide margin.

      Incidentally, I can think of no batter way to dramatize man’s dependence on his environment and to become a wise custodian of this frail earth than to recount how our atmosphere came into being, emphasizing, of course, that it was built by microorganisms, not by men. I think the story of our air should be told to every school child every citizen. It is a fascinating drama of living history with enough mystery and potential tragedy to intrigue teacher and pupil alike. It is a subject lends itself to student participation in learning since the possibilities for study projects, artwork, plays, and the like are unlimited. Berkner and marshall have writter both a population account (1966) and a more technical treatise (1964) that provide good reference.