Defining life is difficult as definitions are either too broad or too specific. The most common definition requires meeting seven criteria: growth, stimulus response, metabolism, homeostasis, reproduction, mutation, and autonomous movement. Archaea, animals, plants, fungi, and bacteria meet these criteria, but viruses and prions do not. Specific criteria may cause classification problems if life is discovered on other planets. As science evolves, criteria for defining life will likely change to encompass new forms of life.
Defining life, while an easy proposition at first glance, turns out to be incredibly difficult. Every proposed definition of life suffers from one of two problems: the definition is broad enough to allow things generally accepted as non-living to be defined as alive; or is it so specific that if more exotic lifeforms come to light, they may 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, while others require additional components. The basic criteria for life are:
Growth: A living organism must grow in some way, 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 to which one responds can vary, as can the specific responses, but there must be some interface between the organism and the outside world. Stimuli can cause simple metabolic changes or cause complex behavioral changes.
Metabolism: Living organisms must be able to convert the 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 capable of modifying themselves on a certain level to stay within pre-set parameters. This is related to stimulus response, but builds further on that idea.
Reproduction: All living organisms are able to replicate. This can be done through interaction with other organisms (sexually) or independently (sexually).
Mutation: In addition to being able to reproduce, a living organism must be able to change and develop spontaneously between generations.
Autonomous Movement: A living being is able to move under its own power. This movement can be very slight and does not require locomotion, but somehow the movement has to happen.
The above criteria for defining life easily encompass most of the things we commonly think of as living. Members of Domain Archaea, which include animals, plants, and fungi, among others, are all readily capable of exhibiting the seven characteristics of life. Bacteria also meet the seven characteristics, as do archaea, or creatures found in thermal vents on Earth.
Viruses are an interesting case, as they can often show no metabolism at all. Viruses in this stage are considered by many to be non-living, in much the same way that a human corpse would be considered non-living. Unlike a human corpse, however, a virus could regain its life state in the future.
Some entities that are often assumed to be alive are shown not to be by virtue of our definition. Prions, the most famous of which is mad cow disease, appear at first glance to display many of the characteristics of a life form. On closer inspection, however, we find that they do not truly metabolise or reproduce, and therefore cannot be said to be alive.
To further define life, many scientists include highly specific criteria, such as the requirement for the presence of lipids and proteins. While this makes sense when dealing solely with life on Earth, other scientists fear 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 takes on new challenges, be they extraterrestrial microbes or classes of artificial intelligence, our criteria will likely evolve to encompass new forms of life and our understanding of them.
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