General Definition of Life
X.Y.
Zhang
5 Aug.
2003
Life is a
concept as old as the human beings themselves and could be found from any
system of human languages. It belongs to those concepts that everyone knows
what it means but nevertheless none can tell exactly what it is. Such a puzzled
feeling is specially strong among scientists, which may be found so vividly in
the words quoted below from Encyclopaedia Britannia (pp. 985~986):
“…
DEFINITIONS
OF LIFE
A great deal
is known about life. Anatomists and taxonomists have studied the forms and
relations of more than a million separate species of plants and animals. Physiologists have investigated the gross
functioning of organisms. Biochemists have probed the biological interactions
of the organic molecules that make up life on our planet. Molecular biologists
have uncovered the very molecules responsible for reproduction and for the
passage of hereditary information from generation to generation, a subject that
geneticists had previously studied without going to the molecular level.
Ecologists have inquired into the relations between organisms and their
environments, ethologists the behaviour of animals and plants, embryologists
the development of complex organisms from a single cell, evolutionary
biologists the emergence of organisms from pre-existing forms over geological
time. Yet despite the enormous fund of information that each of these
biological specialties has provided, it is a remarkable fact that no general agreement
exists on what it is that being studied. There is no generally accepted
definition of life. In fact, there is a certain clearly discernible tendency
for each biological specialty to define life in its own terms. ….
Physiological. For many years a physiological definition of
life was popular. Life was defined as any system capable of performing a
number of such functions as eating, metabolising, excreting, breathing, moving,
growing, reproducing, and being responsive to external stimuli. But many
such properties are either present in machines that nobody is willing to call
alive, or absent from organisms that everybody is willing to call alive. An
automobile, for example, can be said to eat, metabolise, excrete, breathe,
move, and be responsive to external stimuli. And a visitor from another planet,
judging from the enormous numbers of automobiles on the Earth and the way in
which cities and landscapes have been designed for the special benefit of
motorcars, might well believe that automobiles are not only alive but are the
dominant life on the planet. Man, however, professes to know better. On the
other hand, some bacteria do not breathe at all but instead live out their days
by altering the oxidation state of sulphur.
Metabolic. The metabolic definition is still popular with many biologists.
It describes a living system as an object with a definite boundary,
continually exchanging some of its materials with its surroundings, but without
altering its general properties, at least over some period of time. But
again there are exceptions. There are seeds and spores that remain, so far as
is known, perfectly dormant and totally without metabolic activity at low
temperatures for hundreds, perhaps thousands, of years but that can revive
perfectly well upon being subjected to more clement conditions. A flame, such
as that of a candle in a closed room, will have a perfectly defined shape with
fixed boundary and will be maintained by the combination of its organic waxes
with molecular oxygen, producing carbon dioxide and water. A similar chemical
reaction, incidentally, is fundamental to most animal life on Earth. Flames
also have a well-known capacity for growth.
Biochemical. A biochemical or molecular biological
definition sees living organisms as systems that contain reproducible
hereditary information coded in nucleic acid molecules and that metabolise by
controlling the rate of chemical reactions using proteinaceous catalysts known
as enzymes. In many respects, this is more satisfying than the physiological
or metabolic definitions of life. There are, however, even here, the hints of
counterexamples. There seems to be some evidence that a virus-like agent called
scrapie contains no nucleic acids at all, although it has been hypothesized
that the nucleic acids of the host animal may nevertheless be involved in the
reproduction of scrapie. Furthermore, a definition strictly in chemical terms
seems peculiarly vulnerable. It implies that, were a person able to construct a
system that had all the functional properties of life, it would still not be
alive if it lacked the molecules that earthly biologists are fond of and made
of.
Genetic. All organisms on Earth, from the simplest cell to man himself,
are machines of extraordinary powers, effortlessly performing complex
transformations of organic molecules, exhibiting elaborate behaviour patterns,
and indefinitely constructing from raw materials in the environment more or
less identical copies of themselves. How could machines of such staggering
complexity and such stunning beauty ever arise? The answer, for which today
there is excellent scientific evidence, was first discerned by the evolutionist
Charles Darwin in the years before the publication in 1859 of his epoch-making
work, the Origin of Species. A modern rephrasing of his theory of
natural selection goes something like this: Hereditary information is carried
by large molecules known as genes, composed of nucleic acids. Different genes
are responsible for the expression of different characteristics of he organism.
During the reproduction of the organism the genes also reproduce, or replicate,
passing the instructions for various characteristics on to the next generation.
Occasionally, there are imperfections, called mutations, in gene replication. A
mutation alters the instructions for a particular characteristic or
characteristics. It also breeds true, in the sense that its capability for
determining a given characteristic of the organism remains unimpaired for
generations until the mutated gene is itself mutated. Some mutations, when
expressed, will produce characteristics favourable for the organism; organisms
with such favourable genes will reproduce preferentially over those without
such genes. Most mutations, however, turn out to be deleterious and often lead
to some impairment or to death of the organism. To illustrate, it is unlikely
that one can improve the functioning of a finely crafted watch by dropping it
from a tall building. The watch may run better, but this is highly improbable.
Organisms are so much more finely crafted than the finest watch that any random
change is even more likely to be deleterious. The accidental beneficial and
inheritable change, however, does on occasion occur; it results in an organism
better adapted to its environment. In this way organisms slowly evolve toward
better adaptation, and, in most cases, toward greater complexity. This
evolution occurs, however, only at enormous cost; man exists today, complex and
reasonably well adapted, only because of billions of deaths of organisms
slightly less adapted and somewhat less complex. In short, Darwin’s theory of
natural selection states that complex organisms developed, or evolved, through
time because of replication, mutation, and replication of mutations. A genetic
definition of life therefore would be: a system capable of evolution by
natural selection.
This definition places great emphasis on the
importance of replication. Indeed, in any organism enormous biological effort
is directed toward replication, although it confers no obvious benefit on the
replicating organism. Some organisms, many hybrids for example, do not
replicate at all. But their individual cells do. It is also true that life
defined in this way does not rule out synthetic duplication. It should be possible
to construct a machine that is capable of producing identical copies of itself
from preformed building blocks littering the landscape but that arranges its
descendants in a slightly different manner if there is a random change in its
instructions. Such a machine would, of course, replicate its instructions as
well. But the fact that such a machine would satisfy the genetic definition of
life is not an argument against such a definition; in fact, if the building
blocks were simple enough, such a machine would have the capability of evolving
into very complex systems that would probably have all the other properties
attributed to living systems. The genetic definition has the additional
advantage of being expressed purely in functional terms: it does not depend on
any particular choice of constituent molecules. The improbability of
contemporary organisms is so great that these organisms could not possibly have
arisen by purely random processes and without historical continuity.
Fundamental to the genetic definition of life then is the belief that a certain
level of complexity cannot be achieved without natural selection.
Thermodynamic. Thermodynamics distinguishes between open
and closed systems. A closed system is isolated from the rest of the environment
and exchanges neither light, heat, nor matter with its surroundings. An open
system is one in which such exchanges do occur. The second law of
thermodynamics states that, in a closed system, no processes can occur that
increase the net order (or decrease the net entropy) of the system. Thus the
universe taken as whole is steadily moving toward a state of complete
randomness, lacking any order, pattern, or beauty. This fate has been known
since the 19th century as the heat death of the universe. Yet living
organisms are manifestly ordered and at first sight seem to represent a
contradiction to the second law of thermodynamics. Living systems might then be
defined as localized regions where there is a continuous increase in order.
Living systems, however, are not really in contradiction to the second law.
They increase their order at the expense of a larger decrease in order of the
universe outside. Living systems are not closed but rather open. Most life on
Earth, for example, is dependent on the flow of sunlight, which is utilized by
plants to construct complex molecules from simpler ones. But the order that
results here on Earth is more than compensated by the decrease in order on the
sun, through the thermonuclear processes responsible for the sun’s radiation.
Some scientists argue on grounds of quite
general open-system thermodynamics that the order of a system increase as
energy flows through it, and moreover that this occurs through the development
of cycles. A simple biological cycle on the Earth is the carbon cycle. Carbon
from atmospheric carbon dioxide is incorporated by plants and converted into
carbohydrates through the process of photosynthesis. These carbohydrates are
ultimately oxidized by both plants and animals to extract useful energy locked
in their chemical bonds. In the oxidation of carbohydrates, carbon dioxide is
returned to the atmosphere, completing the cycle. It has been show that similar
cycles develop spontaneously and in the absence of life by the flow of energy
through a chemical system. In this view, biological cycles are merely an
exploitation by living systems of those thermodynamic cycles that pre-exist in
the absence of life. It is not know whether open-system thermodynamic processes
in the absence of replication are capable of leading to the sorts of complexity
that characterize biological systems. It is clear, however, that the complexity
of life on Earth has arisen through replication, although thermodynamically
favoured pathways have certainly been used.
The existence of diverse definitions of life
surely means that life is something complicated. A fundamental understanding of
biological systems has existed since the second half of the 19th
century. But the number and diversity of definitions suggest something else as
well. …, all the organisms on the Earth are extremely closely related, despite
superficial differences. The fundamental ground pattern, both in form and in
matter, of all life on Earth is essentially identical. As will emerge below,
this identity probably implies that all organisms on Earth are evolved from a
single instance of the origin of life. It is difficult to generalize from a
single example, and in this respect the biologist is fundamentally handicapped
as compared, say, to the chemist or physicist or geologist or meteorologist,
who now can study aspects of his discipline beyond the Earth. If there is truly
only one sort of life on Earth, then perspective is lacking in the most
fundamental way.
…”
Now we can
see that there are three problems to scientists who pursue a general definition
of life. First, they have not yet found anything that might alone and only be
called as life from any living system they studied, neither at the level of
living body of plants, animals and humans, nor at the cell level, nor at the
level from large biological molecules to quarks and leptons that organisms are
made up. The second, they do not know the difference between life and living
organisms. Phenomena mentioned in many definitions above are those of different
levels of biological life but not pure phenomena of life. And the third, most
of their theories logically need a God to start the activities of life at its
very beginning, such as that in the metabolic definition.
In my
opinion, the most fundament characteristics of life are consuming and creating.
And life may be then defined as the dynamic equilibrium state in which
certain energy is continuously consumed to create a related matter and at the
same time the related matter is continuously consumed to create the energy.
There are two kinds of life in our universe, one of which consumes more energy
and creates more matter and another consumes more matter and creates more
energy. For more detailed discussion on them please refer to my article at: http://home.t-online.de/home/x.y.z/en.html
5.08.2003 Science submission number 34881 and
34941