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
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
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
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.