Project Cyclops, A Design... - Department of Earth and Planetary ...

3. Is it possible that living systems could be constructed
from molecular subunits different from
those making up our biosphere? Carbon, hydrogen,
oxygen, and their numerous compounds are
eminently suited to serve as the raw materials for
living structures. It is not considered probable that
living structures could be based on other chemistries
or that they would survive the competition
with carbon-, hydrogen-, and oxygen-based life if
they were. It is noteworthy that all living forms on
Earth not only have a common elemental composition,
but are in fact composed of very similar
molecular units-for example, amino acids, nucleotides,
porphyrins and riboses-arranged in different
ways. Living systems on other planets could
well have similar fundamental components.
Although the steps leading from the organic precursors
to DNA are presently unknown, biologists are
confident that one or more likely routes will be found.
There is thus a general consensus among exobiologists
that life has originated and is now present on the
planetary systems of a large proportion of the stars in
the
Galaxy.
BIOLOGICAL
EVOLUTION
At the time of the emergence of the first living cells
on Earth the atmosphere contained little or no oxygen,
and the energy needs of the cell had to be derived from
reactions other than direct oxidation. The energy was
mostly derived from the pool of organic compounds
built up by the processes already described, but may
have included energy derived from a few inorganic
compounds. Such anaerobic processes are still used
today by a variety of primitive organisms. At some point
in the early evolution of cellular systems, a mutation, or
genetic change, gave certain organisms the ability to
transform the energy of visible light into chemical
energy stored in an oxidation-reduction system. The
critical biochemical compound for this reaction is
chlorophyll, in which the absorbed photons raise electrons
to a higher energy le, _.1.An accompanying series of
reactions enabled the plants to extract CO2 molecules
from the air, and reduce them with the new found
energy to form the complex compounds making up the
structure of the organism. The combined process is
known as photosynthesis. The date at which photosynthesis
began is still unclear, but Kvenvolden (ref. 21)
has recently shown that pre-Cambrian rocks, in which
microfossils are found, show an unusually high ratio of
_2C to '3C. Photosynthesis is known to discriminate
between these carbon isotopes in the same way. This
evidence suggests that photosynthesis was present on
Earth 3.4 billion years ago.
One of the major results of the evolution of the
photosynthetic system was an increasing amount of
molecular oxygen, formed by a by-product of the
reaction, in the early atmosphere. At first the oxygen
was probably used up fairly rapidly in oxidizing a wide
variety of chemical substances. Later free oxygen became
available for the oxidation of nutrients in living
systems. Such oxidation possesses a clear advantage over
the anaerobic system in that a great deal more energy
per molecule of the organic nutrient is available to drive
cellular reactions.
Over the next three and a half billion years the
primitive organisms slowly evolved into the vast array of
living systems we see today. The basic mechanism
underlying this long epoch of biological evolution is the
chance modification of the chemical structure of the
DNA molecule known as a mutation. Most mutations are
detrimental. They give rise to lethal chemical structures
and reactions in the cell or prevent vital ones. Very
occasionally the reverse is true and the mutation is said
to be favorable. A favorable mutation confers a greater
chance of survival, since the cell can now compete more
successfully for the available energy sources and can
more successfully withstand the rigors of the environment.
Over many generations the organisms possessing a
favorable mutation will gradually displace those without
the new characteristic. This process, known as natural
selection, was proposed by Darwin (ref. 22) and independently
by Wallace in the middle of the nineteenth
century as a rational explanation for t.he whole history
of the evolution of the widely differing species that
make up the plant and animal kingdoms, although the
genetic mechanism by which it operates was then
unknown.
The theory of natural selection at once explained the
phenomenon, previously attributed to divine will, that
the vast majority of living organisms seem so perfectly
fitted for the particular environments they occupy.
Experimental evidence supporting the Darwinian hypothesis
is now so complete that few have any major
reservations as to its correctness.
Evidence is available from the fossil record, and more
recently from the studies of the comparative biochemistry
of present-day species, to give us a reasonable
picture of the sequence of events as new species
emerged. Many died out en route either as a result of
changes in the physical environment, or by competition
with superior mutants better able to use the available
resources. A number of species reached a certain stage of
2O