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

essential component of someof themolecules responsibleforthetransferofenergyinlivingsystems.
It isclearthatsimilarprocesses weretakingplacein
theatmosphere oftheEarthfourbillionyearsago.Many
of the organicmoleculesoproducedsubsequently
dissolved in the primeval sea. Further organosynthesis
took place in the upper few millimeters of the primitive
ocean as a result of absorption of ultraviolet light.
Beneath the surface, organic molecules were modified at
a much slower rate since they were largely protected
from the energy sources responsible for their origin.
So far the story of the origin of life is reasonable,
easy to understand, and well supported by present-day
experimental evidence. We visualize the primitive ocean
containing in dilute solution a wide variety of organic
compounds suitable as precursors for living systems. The
environment is fairly stable over millions of years. As the
compounds degrade they are replaced by more of their
kind falling into the sea from the atmosphere, and they
are modified at the surface by ultraviolet radiation. The
process is called chemical evolution since living systems
were not yet present.
In South Africa, Barghoorn found in a rock formation
called Figtree Chert, microstructures that could be
the fossils of single-celled organisms (ref. 19). The age of
the formation, obtained from radioisotope dating, is 3.1
billion years. The sequence of events between the time
when only the mixture of organic precursors existed in
the early oceans and the time when the first living cell
appeared, 3.1 billion or more years ago, is still unclear. It
is the only portion of the entire chain of events
constituting biological evolution that is not yet understood.
It is a crucial step, for it marks the transition
from the nonliving to the living system. Somehow the
organic molecules of the primitive ocean were assembled
into that complex unit of life, the ceil.
The cell is a highly organized structure containing a
large amount of information in a very small volume. It
has all the characteristics we associate with living
systems. It is self-reproducing. It contains within its
nucleus a molecular code for the ultimate control of all
cellular activities and structure. It degrades high energy
food or light into lower forms of energy and extracts
useful work to maintain its activities. It has sophisticated
systems for maintenance and repair, and is able to
remove waste products and to protect itself from a
variety of environmental threats.
In recent years it has become more difficult to arrive
at a rigorous definition of a living system. The chemical
and physical basis of life, now generally accepted, makes
it hard to find any characteristics peculiar to living
systems as a whole. The most apposite definition,
suggested by Schr6dinger in 1943 (ref. 20), is that living
systems maintain and propagate localized states of
decreased entropy over very long periods of time. At
first sight living systems appear to be contradictions of
the second law of thermodynamics. The total system in
which life occurs does not disobey the second law:
decreased entropy represented by the high level of
organization of the biological part of the system is
achieved and maintained at the expense of a greater
increase in entropy in the rest of the system.
The critical problem remains. What was the sequence
of events in the evolution of the cell from the mixture of
organic precursors in the ocean? One possibility is that
autocatalysis played a significant role. In a sequence of
reactions in which compound A -_ compound B -* compound
C, if compound C turns out to be a catalyst for
the reaction A_ B, it will accelerate the yield of
compound B and of itself. Many examples exist.
Particulate radiation can cause polymerization of certain
classes of compounds. Today certain polymers, such as
the DNA chain, can be made to produce replicas of
themselves in solution given the raw materials, an
appropriate enzyme, and an energy source. It is likely
that the evolution of the cell required a long sequence of
individually improbable events. Nevertheless, given the
millions of years available and the vast supply of organic
raw materials, these infrequent crucial events must have
occurred many times.
The numerical values for the probabilities must await
the construction of models of the chemical interactions,
in a variety of environments, at a level of complexity
unobtainable with present knowledge and computer
capacity. As will be seen later, the numbers are very
important because they will help to answer three
fundamental questions, each crucial to the problem of
interstellar communication:
1. On those planets of solar systems throughout the
Galaxy that had evolutionary histories similar to
that of Earth, what is the probability that life
began? Clearly if the number is small, say 10-1 o,
then Earth has the only life forms in the Galaxy.
However, it is generally believed that the number
is high, perhaps high enough that life could have
begun on many separate occasions on the Earth,
and correspondingly on all planets similar to the
Earth.
2. To what extent do the probabilities of the
development of life change as the geophysical,
atmospheric, and chemical characteristics of other
planets deviate from those of Earth? If the
numbers remain high, then life is the rule in the
universe. There is less general agreement about the
answer
to this question.
19