Definitions of life
by Carl Sagan
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 behavior 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 is
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. The average
person also tends to think of life in his own terms. For example, the man in the street, if asked
about life on other planets, will often picture life of a distinctly human sort. Many individuals
believe that insects are not animals, because by "animals" they mean "mammals." Man tends
to define in terms of the familiar. But the fundamental truths may not be familiar. Of the
following definitions, the first two are in terms familiar in everyday life; the next three are
based on more abstract concepts and theoretical frameworks.
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, metabolizing, 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, metabolize, 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
form 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
sulfur.
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 wellknown
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 metabolize 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 behavior 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 the 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 favorable for the organism; organisms with such favorable 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 i n
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--
dealt with more fully below--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 (see thermodynamics). Thus the universe
taken as a 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 increases 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 shown 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 known 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
favored 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. As
detailed below, 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.
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