26. Origin of Life on Earth
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       The Common Biochemical Heritage of Life

We have no fossilized citric acid cycle or glycolytic enzymes for study, and never shall have, so the starting point for understanding chemical evolution must be the reactions that occur in present-day organisms.

Eucaryotes all obtain energy by oxygen-using respiration; and those that are photosynthetic obtain reducing power from water and release oxygen. This metabolic uniformity is missing in the older procaryotes.

Some bacteria do respire with O2, but others can use nitrate as an oxidant if O2 is not available. Since the same enzymes are involved, and O2 always is their first preference, nitrate respiration probably is a relatively recent special adaptation that is of little interest in tracing the evolution of life.

Desuifovibrio sewage bacteria respire and extract energy from their foods by using sulfate as an oxidizing agent, emitting the H2S that contributes to sewage stench. Different enzymes are used in the electron-transport chain of sulfate respiration, and this appears to be a genuinely independent solution to the problem of getting more energy from foods by combining them with an oxidizing agent.

Sulfate is not as good an oxidant as O2, but it is acceptable. Sulfate-respiring bacteria are strict anaerobes, which are poisoned by the mere presence of O2. They are restricted to life in rotting sewage and other microenvironments that are reducing in character. They may be living fossil remains of an era when the planet had little or no free atmospheric oxygen.




Other bacteria such as the Clostridia, which produce botulism in foods and gangrene in wounds, do not respire at all. They obtain all of their energy by anaerobic fermentation (glycolysis), giving off as waste products lactate, acetate, ethanol, butyrate, propionate, or other small organic molecules.

They all are compulsory or obligate anaerobes, for whom free oxygen gas is lethal. (This is why botulism only develops in sealed but imperfectly sterilized cans of food, and why aerating a wound helps to prevent gangrene.)

The ability to respire and oxidize foods is a special talent not possessed by all life, but glycolysis is universal. Glycolysis accompanied by the storage of energy as ATP appears to be the, irreducible minimum for life.

Those organisms that go no farther than glycolysis cannot tolerate the presence of gaseous O2. In contrast, with few exceptions, those bacteria that can live in the presence of O2, also have learned to use it for respiration. It is too good a source of extra energy to neglect.

These facts suggest that life began as fermenting one-celled organisms, at a time when no free oxygen was present in the atmosphere.


  Page 6 of 36 Glossary