What is the Definition of Life?
The definition of life, while at first glance an easy proposition, proves to be incredibly difficult. Every proposed definition of life suffers from one of two problems: The definition is sufficiently broad that it allows things generally accepted as non-living to be defined as alive; or is so specific that should more exotic forms of life come to light, they might not fit the classic view.
The most common definition of life requires that a thing meet seven criteria to be considered alive. Some definitions do not require all seven be present, while others require additional components be present. The basic criteria for life are:
- Growth: A living organism must grow in some manner, most often by converting external materials into progeny or additional mass.
- Stimulus Response: Living organisms must respond to stimuli in their environment. The amount of stimuli responded to may vary, as may the specific responses, but there must be some interface between the organism and the external world. Stimuli may result in simple metabolic shifts, or provoke complex behavioral changes.
- Metabolism: Living organisms must be capable of converting energy in their environment into a new form. This definition is often given in much more scientifically-precise terms, to ensure the exclusion of pure-energy reactions such as stars.
- Homeostasis: Living organisms are able to modify themselves on some level to remain within set parameters. This is related to stimulus response, but builds further on that idea.
- Reproduction: All living organisms are capable of replicating. This may be done by interaction with other organisms (sexually), or autonomously (asexually).
- Mutation: In addition to being able to reproduce, a living organism must be able to spontaneously change and develop between generations.
- Autonomous Motion: A living thing is capable of moving under its own power. This motion may be very slight, and does not require locomotion, but in some way movement must occur.
The preceding criteria for the definition of life easily encompasses most things we commonly think of as alive. The members of Domain Archaea, which include animals, plants, and fungi, among others, are all readily able to exhibit the seven characteristics of life. Bacteria also meet the seven characteristics, as do archaea, or the creatures found in thermal vents on Earth.
Viruses are an interesting case, as they may often exhibit no metabolism at all. Viruses in this stage are considered by many to be non-living, in much the same way a human corpse would be considered non-living. Unlike a human corpse, however, a virus may regain its living status at some point in the future.
Some entities that are often thought of as alive are shown not to be by virtue of our definition. Prions--the most famous being Mad Cow Disease--appear at first glance to exhibit many of the characteristics of a life form. On closer inspection, however, we find that they do not metabolize or truly reproduce, and therefore cannot be said to be alive.
To further define life, many scientists include extremely specific criteria, such as the requirement that lipids and proteins be present. While this makes sense when dealing exclusively with life on Earth, other scientists worry it will present classification problems should 'life' be discovered on other planets or deep-space objects.
For now, the qualitative definition of life must suffice; but as science makes new discoveries and meets new challenges--be they extra-terrestrial microbes or classes of artificial intelligence--our criteria will likely evolve to encompass new forms of life, and our understanding thereof.
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![Paramecia are unicellular life forms.]()
By: micro_photoParamecia are unicellular life forms.
Discussion Comments
I believe that all life is resurrection. Without death there really is no life: dirt is dead matter, and plants grow out of it. Fungi survive on dead matter, we eat meat all the time. Life leads to death and death leads to life. I think this makes perfect sense metaphysically and scientifically. It is also a psychological basis for understanding the world.
@BigBloom
Life is simply a predefined condition of science. There is nothing more to it than that. I don't think that life is a matter of some preconditioned factors which are anywhere beyond our current comprehension. As such, the idea of life should be a strictly definable concept which requires no redundant connection with "living."
@jagers
Wouldn't those bacteria which were frozen be considered alive because of their potential for life? Perhaps they were dead at the time, but could be made alive. But then we could say that all things are alive, because there is potential for life everywhere. If the life of an object is defined by the structure of an organism which it is built from, we could effectively state that all things which are not yet living are alive, since all things will be built from what currently exists. Perhaps life is just a state of living. The semantic relation between "life" and "living" makes them inseparable.
Unfortunately, the seven points mentioned in the above definition of life seem OK, but are misleading. A sterilized cat cannot reproduce, but is alive and has life.
If you are under anesthesia, you don’t respond to stimuli, but you are still alive and have life. Bacteria that over many generations don’t show mutations are living organisms. It is easy to find more critical points.
For the largest part, the troubles are caused by the confounding of a definition of life with a definition of living.
All the aspects of growth, stimulus response, metabolism, homeostasis, reproduction, mutation and autonomous motion apply to the dynamic state (the “living”) of an entity. For this reason, they may contribute to a definition of “living”. But, already in 1860, the famous Société de Biologie in Paris wrote a lengthy report on this subject (Broca 1860-1861). All seven aspects were debunked as requirements for a definition of life. The reason was that frozen bacteria and dried protozoa showed none of the above criteria, and nevertheless remained “life”, simply because they could be thawed/moistened and become living again.
It was concluded, therefore, that life was defined by a specific structure/organization of the matter that the organism was built from.
If the argumentation so far has convinced you of the problems that are caused by a definition of life (not of “living”!) that is based on the aforementioned seven criteria, and if you are interested in a new, more fundamental way of defining life, you are invited to read the following article: Towards a hierarchical definition of life, the organism and death (Philosophy of Science, 2010).
A popular version of this paper is presented in Astrobiology (see article: Bringing the definition of life to closure).
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