Origin of Life: The Evolution of Autotrophy

Although primitive cells may have developed very elaborate ways of using the variety of free organic compounds found in their environment (and even some of them have developed other feeding methods including parasitism and hunting, some of which are completely heterotrophic), however, as long as all diets remain heterotrophic. life would end eventually. This is not only because nutrients are consumed much faster than you might think, but the organisms themselves alter the environment by reducing the rate of abiotic synthesis of organic compounds. For example, in the absence of molecular oxygen, the fermentative metabolism of heterotrophs will continuously release carbon dioxide into the atmosphere. Complex organic compounds are much less likely to be synthesized abiotic from CO2 than from materials such as methane, formose or hydrogen cyanide.

Since the free organic compound source is reduced, the living environment has not disappeared. In contrast, some primitive heterotrophs developed autotrophic pathways that synthesize organic compounds from inorganic molecules. Such a first route is almost certain to be chemosynthesis using most of the energy in the covalent bonds of molecular hydrogen. This energy has been used in the synthesis of many organic compounds and even many new compounds that can no longer be obtained from “soup”. Chemosynthetic autotrophs still exist today. They are particularly found in volcanic openings in the swamp and ocean floor. The next autotrophic man to evolve – cyclic (evolutionary) photophosphorylation – was far more important to the story of life on earth. In cyclic photophosphorylation, rays of visible wavelengths are used by the cell as an energy source in the synthesis of ATP. Current anaerobic photosynthetic bacteria may be direct derivatives of the first non-chemosynthetic autotrophs. In the next step, much more complex pathways of non-cyclical photophosphorylation and CO2 flexion have evolved, in which cyanobacteria first appeared 2-2.5 billion years ago, first using energy from the sun to synthesize carbohydrates from CO2 and water (or some other electron sources). Thereafter, the survival of life on earth depends specifically on the activity of photosynthetic autotrophs.

The evolution of photosynthesis based on water as an electron donor likely has put the ultimate in abiotic synthesis of important complex organic compounds. An important product of this type of photosynthesis is molecular oxygen, which is highly electro-negative. O2 released by photosynthesis must have participated in the water cycle and reacted with many minerals, including iron dissolved in the oceans. This has led to the precipitation of iron as Fe304 and the resulting accumulation of large sediments known as “bound iron”. As a result of the dissolved iron separating from the water, free oxygen must have accumulated in the water and then passed from there to the atmosphere.

The ozone (03) layer, now located in the upper parts of the atmosphere, was once formed by some of this oxygen. This layer effectively sheds most of the ultraviolet radiation from the sun and allows only a small amount of high energy radiation to reach the earth’s surface. In other words, living organisms that once appeared change their environment in a way that destroys the conditions that made the origin of life possible. These organisms caused what’s sometimes called the oxygen revolution. With molecular oxygen becoming a major component of the atmosphere, both heterotrophic and autotrophic organisms were able to use biochemical pathways related to oxygenated respiration, which could extract much more energy from nutrient molecules than with fermentation alone.

The continuous increase of atmospheric oxygen and the strengthening of the ozone layer have been the main factors determining the separation of organisms from UV-absorbing oceans and their transition to land.