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.