Life on Earth

Heterotroph Hypothesis

Conditions on the early earth
The most accepted hypothesis of the origin of life is called the heterotroph hypothesis. The scientist most often credited with the hypothesis was A. I. Oparin. Oparin's heterotroph hypothesis assumes that both physical and chemical conditions on the earth were very different from those today. There is evidence that early temperatures were much higher on the early earth than they are at present times. The oceans, when they were first formed, were probably not much below the boiling point of water.

Natural synthesis of organic compounds
Under these conditions, simple compounds in the atmosphere and in oceans could have reacted to form more complex organic compounds. The synthesis of these organic compounds requires energy. Many sources of energy are thought to have been present on the early earth. There was heat given off on the eary itself; radiation from the decay of radioactive elements in the earth's crust; electrical energy from lighting; and ultraviolet visible light and x-rays from the sun. Under these condtions there would have been enought energy available for the breakdown and chemical formation of bonds. The first nucleotides, amino acids, and sugars, could have been formed during this period.

Respiration and photosynthesis
The heterotroph hypothesis shows the idea that protien complexes could develop into nonliving structures that have some of the characteristics of life. Since the atmosphere of the early earth contain no free oxygen, it is thought that the first organisms carried on some form of anerobic respiration to produce energry. This process would release carbon dioxide into the oceans and the atmosphere. Eventually organisms will develop that could use light energy directly for the synthesis of ATP. These photosynthetic autotrophs would change the environment by adding oxygen into the atmosphere. The presence of oxygen led to the development of organisms with the ability to carry on aerobic respiration.