A simple analysis of the definition of life leads to the conclusion that living matter is inanimate matter that evolves. Evolution is the sole feature that differentiates living matter from non-living matter.
Consider a definition of life from the old college days:
Life is the property of a highly organized molecular complex, the stability of which is maintained by its constant utilization of energy.
It seems a bit flowery, so to simplify:
Life is organized matter self-maintained by energy utilization.
At first glance, this definition seems inadequate by not including such life properties as metabolism, movement, response to a stimulus, growth, self awareness, communication, and reproduction. But these properties are primarily specializations for energy utilization so it's not necessary to include them in a general definition. Even reproduction can be thought of as merely a specialization that provides for energy utilization in the future. Energy utilization is essential to maintain the stability of life forms because without it, life forms are very unstable and decompose into non-living matter. In fact, life forms are so unstable that, even with energy utilization, they all eventually die. Of course we know that life itself persists for periods of time far exceeding the lifespans of any individual life forms. How is this possible?
Does the Definition Exclude All Non-Living Matter?
There are many examples of non-living matter maintained by their utilization of energy. Some examples are waterfalls, volcanoes, hurricanes, and stars. So, a definition of life requires something more to exclude such non-living matter. Adding something about reproduction might help:
Life is organized matter self-maintained by energy utilization and a process that reproduces Its structure for self-maintained energy utilization in the future.
Special cases such as obligate symbiants and viruses are excluded from the definition because they are not “self-maintained” but depend on other life forms, although that distinction is almost arbitrary. By specifying that life must include a reproductive process, inanimate matter like waterfalls and hurricanes are excluded. However, some scientists might argue rather convincingly that stars reproduce. To eliminate stars as living matter, a process that more faithfully reproduces itself could be added:
Life is organized matter self-maintained by energy utilization and a process that accurately reproduces its structure for self-maintained energy utilization in the future.
This simple definition seems to eliminate stars and other forms of inanimate matter. Specifying a nucleic acid-based reproductive mechanism rather than just a reproductive process would accurately define all of the known forms of life. But the general definition above allows for the possibility of other hypothesized mechanisms such as those based on lipids, polycyclic aromatic hydrocarbons, or polyphosphates.
Perfect Clones Can't Cut It.
Any life form with a reproductive process that creates an exact copy or a perfect clone of itself would satisfy the requirement of a process that accurately reproduces itself. Such a process would always produce the same exact structure and function for energy utilization and self-maintenance in a given environment. However, in a changed environment, functional energy utilization and self-maintenance might benefit from, or even require, new structure(s).
Every Environment Changes With Time.
In order for a life form to persist through time, it must have a mechanism that provides for structural changes to function in a changed environment. Some life forms can produce exact copies or perfect clones, but most importantly, they also provide mechanisms for introducing structural changes by mutation or recombination. No life forms exist that cannot provide such changes. So, the definition of life becomes:
Life is organized matter self-maintained by energy utilization and a process that accurately reproduces its general organizational structure with a mechanism that allows for structural changes to utilize energy for self-maintenance in a changed environment.
Based on this definition of life, the sole characteristic that distinguishes living from non-living matter is a reproductive mechanism that allows for structural changes to utilize energy for self-maintenance in a changed environment.
Adaptation is Essential
When successful structural changes to utilize energy for self-maintenance in a changed environment are produced, adaptation is said to occur. Over time, with continuously changing environments and adaptations, life forms undergo natural selection and classical Darwinian evolution. Thus, for periods of time exceeding the lifespans of individuals, the sole characteristic that distinguishes living and non-living matter is the process of Darwinian evolution. So, what is life?
Life Is matter that evolves.
Most fundamentally, the definition becomes:
Life is evolution.
The two are inseparable. Given the fact that all life forms die, how do they persist through time and changing environments? Every environment harboring life forms must change, simply because of their existence, so evolution is the only way life forms can persist through time. Not only did Charles Darwin discover what makes life possible despite the fact that all life forms eventually die, he unwittingly discovered the sole feature that distinguishes living from non-living matter. Charles Darwin defined life.
Life is Evolution.
Sources:
Goode, M. Dennis, professor who taught biophysics course in 1974, Department of Zoology, University of Maryland, College Park, MD.
Dawkins, Richard, The Selfish Gene. Oxford: Oxford University Press. 1976. ISBN 0-19-286092-5.
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