Published online 22 April 1999 | Nature | doi:10.1038/news990422-9

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Special feature Part two; What is life?

What is ‘Life?’ How do we know that an object is alive rather than dead? Questions asked by scientists about the origin of life (see Philip Ball’s paper on this page) give no clue about where to draw the line between a pond full of chemicals on one hand, and a living organism on the other.

Biologists tend to think of life by what it does, rather than what it is. Viruses, for example, are on the fringes of life. They can be imagined as automata that simulate many of life’s properties, but which cannot survive without living hosts - just like computers, which need human hosts to charge their batteries. To take this line of thought further: viruses are made of the same proteins and nucleic acids that constitute what we think of as life. This underlines why we cannot define life on the basis of what it is made of. Life is not life simply because it is made of proteins and nucleic acids. Conversely, computers cannot be thought of as non-living because they are made of silicon wafers and plastic, rather than proteins and nucleic acids.

In which case, a process-based definition might be more helpful. Living organisms do things that dead things don’t. They eat, excrete wastes, and make more of their own kind. They tend to react to external stimuli by modifying their behaviour or internal workings. The problem is that such properties are correlated with life, but they do not define it. These properties, moreover, are found in other systems - robots and computer programs exist that replicate themselves, and modify their own behaviour in response to stimuli. Can robots and computer programs be seen as ‘living’? Most would answer in the negative, but does this answer come from parochial prejudice rather than scientific investigation?

This argument exposes a fundamental problem about what we understand by ‘life’. That is, our conception of life is, inevitably, limited by what we know, and this is based entirely on what we see around us on Earth. Because life on Earth just happens to be made of protein and nucleic acids, and based on the chemistry of carbon compounds in aqueous solution, we tend to assume that all life must be made that way - despite the existence of computers and robots, which mimic some of the processes of life, but need not be made from this recipe; and despite the existence of viruses, which are made according to the conventional protein-and-nucleic-acid plan but which do not fulfil all the criteria of respectable living things.

These arguments expose the popular debate on the existence of life on Mars as irrelevant. Scientists have suggested that Mars, sometime in its past, was warm and wet enough to have hosted life, with the implication that life might still exist there. But this rests on the assumption that Martian life is (or was) made like Earth life. Because Earth life is all we know, this assumption cannot be justified: in scientific terms, you can never reach a statistically valid conclusion based on a sample size of unity. Claims that microscopic blobs inside meteorites from Mars might represent the fossils of bacteria are similarly misplaced. Such claims can only be based on comparisons with Earthly bacteria or their fossils, and there is no reason to suppose that Martian bacteria or their fossils need have looked anything like those on Earth.

Clearly, what we need is an approach to the definition of life that is not prey to parochial prejudice. Physics might provide an answer. In terms of thermodynamics, the branch of physics that deals with energy and its transfer, all processes tend to run down if they are not constantly supplied with energy. Left to themselves, systems - whether molecules or herds of elephants - will eventually decay into uniform disorder, and all energy will be dissipated as heat. Molecules will decay into atoms, releasing heat; herds of elephants, unless they strive constantly to stay alive, will die and dissipate as dust. Such is the Second Law of Thermodynamics - the tendency of all systems to increase their entropy, to reach a situation in which no part of the Universe contains more energy than any other. The content of energy can be expressed in other ways, such as the content of order, organization, complexity or information.

Avoiding entropy increase is impossible in general, but it can be staved off locally. That is, one part of the Universe might strive to maintain its own content of order or information at the expense of other parts. In thermodynamic terms, such a sub-Universe is said to be ‘far from equilibrium’. Living organisms make good examples of such sub-Universes. The processes we associate with life occur far from equilibrium - life demands the active maintenance of structures in defiance of entropy, structures which if left to themselves, would decay and disappear. Cell membranes, for example, are not passive sacs, but active surfaces that work constantly to ensure the integrity of the contents at the expense of the environment. As soon as they stop working, the cell ‘dies’.

In general, life could be thought of as a set of phenomena in which collections of atoms form themselves into temporary aggregations of shifting membership, organized in systems far from equilibrium; that contain more information, order and structure than their surroundings; and whose energetic postion is maintained by activities generated within the system and at the expense of order, information and structure outside the system. The implication is that the living entity is usually (but not necessarily always) separated from the rest of the Universe by a discrete boundary, such as a cell membrane.

This sounds, at last, like progress. But is it? Have we not simply moved the problem a stage further, by equating life with all manner of systems that are far from equilibrium? Some chemical reactions and other physical processes produce, far from equilibrium, patterns of great organizational complexity. Even a pile of sand, or snow before an avalanche, could be thought of in this way. One could argue that stars and galaxies constitute such systems, but are they ‘living’? One could even expand the definition of life to encompass the whole Universe, sometimes thought of as a system far from equilibrium. Does the order we see in stars and galaxies reflect a property of the Universe that allows deviations from equilibrium ? Quantum physicists are used to imagining small and short-lived deviations from the laws of conservation of mass and energy, in the form of the creation and destruction of particles. Does the existence, in the same Universe, of living things -larger, analogous deviations - tell us something about the complexion of the Universe in which we live? If so, then life is an integral part of the Universe, as much a part of the Universe as the Hubble Constant and the curvature of space-time. As such, to seek definitions of life as a discrete phenomenon is as difficult, (some would say pointless,) as a quest for the seat of the human soul. There is no simple answer to the question ‘What Is Life?’ that does not include some arbitrary boundary. Without such a boundary, either nothing is alive, or everything is.

This problem underlies how hard it is for people, including physicians and the clergy, to reach unassailable definitions of life in connection with the propriety of activities such as euthanasia or abortion: questions about the beginning and end of life depend, ultimately, on a definition of what constitutes life, as opposed to non-life. There is a Jewish joke about an ecumenical conference attended by clergy from many denominations. Predictably, a session entitled ‘When Does Life Begin?’ attracts heated debate. After the session, a Catholic priest, a Protestant minister and a rabbi go out to a restaurant and argue the matter further. The priest and the minister argue back and forth about whether life begins at conception or at birth. They cannot solve the problem, and the rabbi (who has so far said nothing) is asked directly by his colleagues whether he can break the impasse. “When does life begin?” he asks. “That’s easy. Life begins at forty, when the children leave home and the dogs are dead.”