Astrobiology 853
2.2 Generalized Theories for Life
There have been many attempts at a definition of life, and
perhaps such a definition would aid in our investigation for
life on other planets and help unravel the origins of life on
Earth. However, it is probable that there will never be a sim-
ple definition of life and it may not be necessary in a search
for life on other worlds. Despite the fundamental unity of
biochemistry and the universality of the genetic code, no
single definition has proven adequate in describing the sin-
gle example of life on Earth. Many of the attributes that
we would associate with life—for example, self-replication,
self-ordering, response to environmental stimuli, can be
found in nonliving systems—fire, crystals, and bimetallic
thermostats, respectively. Furthermore, various and pecu-
liar life forms such as viruses and giant cell-less slime molds
defy even a biological definition of life in terms of the cell
or the separation of internal and external environments. In
attempting a resolution of this problem, the most useful
definition of life is a system that develops Darwinian evolu-
tion: reproduction, mutation, and selection (Table 4). This
is an answer to the question what does life do?
We are able to answer the questions, what does life
need? and what does life do?, even if we do not have a
closed form compact definition of life. Thus, the require-
ments for life listed in Table 1 and the functions of life
listed in Table 4 are very general; it is probably unwise to
apply more restrictive criteria. For example, for evolution
to occur some sort of information storage mechanism is
required. However, it is not certain that this information
mechanism needs to be a DNA/RNA-based system or even
that it be expressed in structures dedicated solely for repli-
cation. While on the present Earth, all life uses dedicated
DNA and RNA systems for genetic coding, there is evi-
dence that at one time genetic and structural coding were
combined into one molecule, RNA. In this so-called RNA
world there would have been no distinction between geno-
type (genetic) and phenotype (structural) molecular repli-
cating systems—both of these processes would have been
performed by an RNA-replicating molecule. In present bi-
ology, the phenotype is composed of proteins for the most
part. This example illustrates the difficulty in determining
which aspects of biochemistry are fundamental and which
are the result of the peculiarities of life’s history on Earth.
In basing our consideration of life on the distribution we
observe here on Earth as a general phenomenon, we suffer
simultaneously from the problem that there is only one kind
of life on this planet while the variety of that life is too
TABLE 4 Properties of LifeMutation
Selection
Reproductioncomplex to allow for precise definitions or characterizations.
We can neither extrapolate nor be specific in our theories
for life.
Some scientists have suggested that living systems else-
where in the universe may exhibit vast differences from ter-
restrial biology and have proposed a variety of alternative
life forms. One postulated alternative life form is based on
the substitution of ammonia for water. Certainly ammonia
is an excellent solvent—in some respects better than water.
The range of temperatures over which ammonia is liquid
is prevalent in the universe (melting point:− 78 ◦, normal
boiling point− 33 ◦, liquid at room temperature when mixed
with water) and the elements that compose it are abundant
in the cosmos. Other scientists have suggested the possi-
bility that silicon may be used as a substitute for carbon in
alien life forms. However, silicon does not form polymeric
chains either as readily or as long as carbon does and its
bonds with oxygen (SiO 2 ) are much stronger than carbon
bonds (CO 2 ) rendering its oxide essentially inert.
Although speculations of alien life capable of using sil-
icon in place of carbon or ammonia in place of water are
intriguing, no specific experiments directed toward alter-
nate biochemistries have been designed. Thus we have no
strategies for where or how to search for such alternate life
or its fossils. More significantly, these speculations have not
contributed to our understanding of life. One can only con-
clude that our unique understanding of terrestrial life is
based on Earth systems, and wide-ranging speculations re-
garding alternate chemistries are currently too limited to be
fruitful. Perhaps some day we will develop general theories
for life or, more likely, have many sources of life to compare
thereby allowing for complete theories. Basing our theories
on Earth-like life should be considered a necessary first ap-
proach and not a fundamental limitation.3. The History of Life on Earth
Several sources of information about the origin of life on
Earth include the physical record, the genetic record, the
metabolic record, and laboratory simulations. The physi-
cal record includes the collection of sedimentary and fossil
evidence of life. This record is augmented by theoretical
models of the Earth and the Solar System, all of which
provide clues to conditions billions of years ago when the
origin of life is thought to have occurred. There is also
the record stored in the genomes of living systems that
comprise the collective gene pool of our planet. Genetic
information tells us the path of evolution as shaped by
environmental pressures, biological constraints, and ran-
dom events that connect the earliest genomic organism,
thelast universal common ancestor, and the present
tree of life (see Fig. 3). There is also the record of
metabolic pathways in the biochemistry of organisms that
have evolved in response to changes in the environment