The most complex evolution of life, and the most unlikely, took place long before the hominid appeared. It was a billion years after life appeared before the cell. It was the cell which become the basis for all modern life forms.
The information in this section is not hard science. Known evidence and measurement support this account. No known evidence or measurement dispute it. The conclusions drawn are not dependent on this introductory material.
The evolution of life from its beginning through the development of the metazoa (primitive multicellular organisms) took billions of years. The earth's atmosphere did not contain oxygen when the earth formed 4.6 billion years ago. This reducing environment provided favorable conditions for the natural synthesis of the first organic compounds. The first phospholipid bilayer membranes formed along with primitive RNA and DNA genetic molecules. The membranes adsorbed proteins and the hereditary DNA/RNA material. From these organic molecules, the first primitive prokaryote (simple single cell organism lacking a nucleus) arose. Natural selection began.
The carbon dioxide in our atmosphere contains both 12C and 13C isotopes. When the carbon atoms of CO2 are captured by organisms in photosynthesis, the organisms show a definite preference for the light 12C isotope of carbon. They will incorporate the 12C isotope into the proteins, sugars, and other molecules that they synthesize preferentially to the heavier 13C isotope. Rocks that are 3.4 billion years old have been discovered which are enriched with the 12C isotope. The concentration of the 12C isotope shows the presence of photosynthesis. These early photosynthetic organisms used H2S as a source of hydrogen atoms instead of water and did not produce oxygen as a by-product.
Ancient sediments are found which are rich in iron (Fe2+) as water soluble compounds. Beginning at about 2.5 billion years ago, the sediments began showing iron oxides containing Fe3+ ions that were not water soluble. This shows that the atmosphere had changed from a reducing environment to an oxidizing one where oxygen was present. The oxygen in the atmosphere appears to have been generated by living organisms carrying out oxygen-producing photosynthesis. These newer organisms could obtain hydrogen by breaking down water instead of H2S to produce the hydrogen needed to synthesize carbon compounds. As a by-product of breaking down water, oxygen gas is formed.
In the next billion years following the evolution of aerobic photosynthesizing bacteria, the first cells arose which had lost the ability to carry out photosynthesis and thus the ability to manufacture their own food from inorganic material. These cells relied on organic material (other life) as their source of food. The food chain was begun.
Once complex organisms began evolving, the DNA became quite complex. Pictured is a human genome, some 3 billion base pairs long.
For a comprehensive study of early evolution try University of Arizona.