It is still a challenge for scientists and philosophers to define life in unequivocal terms. Defining life is difficult —in part— because life is a process, not a pure substance. Any definition must be sufficiently broad to encompass all life with which we are familiar, and it should be sufficiently general that, with it, scientists would not miss life that may be fundamentally different from earthly life.
Since there is no unequivocal definition of life, the current understanding is descriptive, where life is a characteristic of organisms that exhibit all or most of the following phenomena:
I always keep this fact in mind anytime someone tries to extract too much significance out of the fact that it is likewise difficult to define things like ‘design’ and ‘intelligence.’ An attempt to build an argument around a selective demand for precise definitions is a sign that the attempt is rooted in a biased agenda and not an open-ended investigation.
But let’s get back to the descriptive criteria used to identify life.
Here’s the challenge – as you read through the list, do you notice a pattern? In fact, are these facets of the same gem?
1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
2. Organization: Being structurally composed of one or more cells, which are the basic units of life.
3. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
4. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism’s heredity as well as the composition of metabolized substances, and external factors present.
6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and by chemotaxis.
7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.