In the previous essay I discussed some possible scenarios for the formation of the earth and some of the possible features of the earth and its atmosphere. Now I would like to look at changing conditions of the atmosphere and the earth’s surface that allowed life to be created and survive.
Before life forms could survive, certain chemical changes had to take place. Simple organic molecules that could make copies of itself (replication) had to form. Whether this was DNA or RNA is hotly debated yet today. The original atmosphere lacked the most important gas necessary for present day life forms. Probably sometime between 3.4–3.3 billion years ago, the first form of life arose. This would undoubtedly have been an anaerobic form since there was no free oxygen in the atmosphere. In fact, oxygen probably would have been toxic to obligate anaerobes. (Believe it or not, there are anaerobic conditions on earth now and thus, anaerobic forms of life (a group of bacteria like organisms called Archaea). They may have been the first forms of life. To find anaerobic conditions you have to look no further than your cupboard where you might find a can of vegetables or a swamp where methane gas is being produced by methanogen bacteria. Viruses, which are so simple that scientists still are not sure if they are alive, are composed of either a DNA or RNA core surrounded by a protein cover. They exhibit most of life’s characteristics but may also be crystallized and stored for an indefinite period of time without any need for energy or exhibiting other life functions such as respiration (anaerobic or aerobic), responding to stimuli, adapting to changes in environment, etc. There is much debate on the next point but most scientists believe that the first forms of life were also heterotrophic meaning that they could not make their own food. The following geologic time scale taken from deviantart.com will perhaps put things in proper prospective
Other scientists believe that autotrophic (auto; self and trophic; food) (photosynthetic or chemosynthetic) forms of life evolved first. Chemosynthetic bacteria (chemoautotrophs) use inorganic chemicals such as hydrogen sulfide (H2S), ammonia, and hydrogen gas to obtain energy. But the prize for being the first photosynthetic (oxygen producing) organisms probably goes to the cyanobacteria, formerly called blue-green algae but relegated down from eukaryotes to prokaryotes based on transmission electron microscope (TEM) and scanning electron microscope (SEM) micrograpths. The terms prokaryote and eukaryote require some attention since they are cornerstone concepts in biology. Prokaryotes (bacteria and Archaea) are single celled organisms that lack a true membrane bound nucleus. The DNA which may be circular floats freely in the cytoplasm of the cell (pro; before and karyote; nucleus). Eukaryotes obviously have a nucleus. They may be unicellular, colonial, or multicellular. At any rate, the cyanobacteria probably introduced oxygen into the atmosphere which caused the demise of anaerobic forms of life and paved the way for aerobic forms. This probably occurred around 2.5 billion years ago. Again there is some question here as the next sentence will show.
Perhaps around 3.3 billion years ago oxygen producing organisms called stromatolites, which are still alive today, began the vitally important process that we know as photosynthesis. These autotophs paved the way for all of the food producers alive today. For the next 2 billion years oxygen was pumped into the atmosphere that we know today. Of course, many other changes were taking place on the surface of the earth during this time. Continental drift was constantly at work changing surface features and affecting the distribution of life forms. One collision about 1.5 billion years ago produced a supercontinent, Rodenia. Somewhere around 700 million years ago, the Cambrium Explosion resulted in a tremendous increase in not only the sheer number of organisms but also the diversity of life. Before then life was pretty well limited to the sea because of intense radiation from the sun. Once the ozone layer formed, terrestrial forms began to appear. From about 400 million years ago to about 300 million years ago, land was gradually conquered as both flora and fauna (plants & animals) flourished. During the ensuing Carboniferous Period, dense tropical swamplands over virtually all of the earth produced coal and oil in places like the Middle East. Coal was produced from plants and oil and gas from animals. Here is an interesting side story. Did you ever wonder how that region of the world which is desert today could be so rich in oil fields? Remember, the earth has gone through many climate changes including temperature and rainfall fluctuations. These are the two most important climate factors that determine the type of flora that inhabit an area which in turn, determines the type of fauna. Also at this time arthropods including huge insects dominated the scene and later amphibians followed when reptiles appeared and flourished. It appears from the story told in the rocks and ice that at about this time numerous eruptions in present day Siberia for millions of years produced poisonous gases that covered the globe killing 95% of life. A new supercontinent, Pangea, appeared. Soon dinosaurs dominated the earth.
About 200 million years ago Pangea split thus gradually producing the continents we know today. And, of course, the date that everyone is familiar with, about 65 million years ago, some cataclysmic event resulted in mass extinction of the great dinosaurs. This time probably about 75% of all life disappeared. Louis and Walter Alverez several years ago verified the date using radioactive isotopes (14C dating). A thin layer of Eridium found in various places on earth all date to that date. Below that line dinosaur remains were commonplace; above that line virtually no remains are found. Whether it was a gigantic meteor or an asteroid or some other event is still questionable. But one thing is for sure; something really big happened. About 50 million years ago, the African and European plates collided. This collision produced the Alps including the Matterhorn, of which the northern half is from the European plate and the southern half is from the African plate. About 2 million years ago the first of many ice ages began to occur. Also, overflowing volcanoes in present day Panama probably created the land bridge (Central America) that connected North and South America.
The earth has had a long and varied history and it is still changing. In so many ways mankind has changed it and, unfortunately, not usually for the good. It is time for us to be better stewards of the earth and leave it a better place for our children and grandchildren.