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Characteristics of Life

  • Page ID
    1090
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    This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the updated and corrected version.

    As part of our search for life beyond the Earth, we try to understand how life began on the Earth, i.e. the origins of life on Earth, so that we can figure out how life might arise on another planet or moon. There is also the question of what will be the future of life on the Earth and the universe. However, whether you are looking for now-existing life or for how life began or at how the environment will affect life in the far future, you need to know what life is. Unfortunately, we don't have an universally, agreed-upon definition of life. A couple of approaches that complement each other try to answer what is alive vs. not alive in the origins of life research: the "top-down" and "bottom-up" approaches. The top-down approach looks at all sorts of living and fossil life forms to figure out the most primitive forms that are or were alive. Unfortunately, all the life forms we know of are already very sophisticated. There is a big gap between the life forms we know of (either current or fossilized) and the never living material and reactions. The bottom-up approach uses laboratory experiments to mimic the emergent chemical processes of environments of ancient Earth or of current conditions on another planet (e.g., Mars) or moon (e.g., Europa or Titan) in order to create a self-reproducing chemical system. But debate still rages on what self-reproducing system would be considered truly alive.

    We are limited by a sample size of one—the Earth—in figuring out what life is. All of the life forms on Earth share many common processes and features and therefore, seem to have evolved from a common ancestor. These common processes and features include such things as: (a) all life on Earth uses the "left-handed" version of amino acids to build the proteins it needs for carrying out the processes of life (look up "chirality" in a search engine for more on this); (b) all living cells use adenosine triphosphate (ATP) to store and release energy; and (c) the genetic code is used by nearly all cells in reading the information stored in the deoxyribonucleic acid (DNA) to build the proteins. These commonalities among all living things on Earth presents a challenge to us in looking for Life without an Earth bias.

    Although an universally, agreed-upon definition of life does not yet exist, there are some necessary characteristics of life (life as we know it). The following list of characteristics are necessary but not sufficient to define life. Many counter examples of non-living things can be given for the characteristics.

    1. Organization. All living things are organized and structured at the molecular, cellular, tissue, organ, system, and individual level. Organization also exists at levels beyond the individual, such as populations, communities, and ecosystems. Possible counter examples could be rock crystals, machines, and electronics. However, it is a nice characteristic because it is visible in a short amount of time and it can also be used with past life.

    2. Maintenance/Metabolism. To overcome entropy (the tendency of a system to become more disorganized and less complex), living things use energy to maintain homeostasis (i.e., maintain their sameness; a constant, structured internal environment). Metabolism is a collective term to describe the chemical and physical reactions that result in life. Although non-living things such as electrical or gas appliances use energy too, this characteristic is easy to observe in a short amount of time and the reactions could leave residues or changes in the environment that would tell us of past life.

    3. Growth. Living things grow. The size and shape of an individual are determined by its genetic makeup and by the environment. Items 2 and 3 are related. Life grows by creating more and more order. Since entropy is decreased (the amount of structure and complexity is increased), life requires an input of energy. Life gains local structure at the expense of seemingly chaotic surroundings on a large scale. Possible counter examples could be fire and crystals. Looking for growth in living things might be hard to do if the life form grows slowly or has stopped growing and this characteristic cannot be used on past life.

    4. Response to Stimuli. Living things react to information that comes from outside or inside themselves. Counter examples include rivers, clouds forming, or thermostats. While it is possible to see responses on a short time scale, we would need to know what the particular response is ahead of time to look for it and this characteristic would not be helpful in looking for past life.

    5. Reproduction. Individuals reproduce themselves. Life also reproduces itself at the sub-cellular and cellular levels. In some instances, genetic information is altered. These mutations and genetic recombinations give rise to variations in a species. Some possible counter examples include some types of robots or computer codes or definite living things like mules that are the sterile offspring of a mated female horse and male donkey. "In between" cases like viruses and prions cause debates among biologists about whether they are truly alive. Viruses can reproduce only by infesting other life forms. Prions are infectious proteins that make copies of themselves by causing existing, properly formed proteins to change into the prion form rather than by actually replicating themselves (examples include "mad cow disease" BSE, and CJD in humans). The act of reproduction in truly living things is not always convenient to observe at a given time and it certainly will not work for past life.

    6. Variation. Living things are varied because of mutation and genetic recombinations. Variations may affect an individual's appearance or chemical makeup and many genetic variations are passed from one generation to the next (heredity). Looking for heredity, though, requires a whole series of the organisms and perhaps a long time to see what trait is passed on vs. random chance, as well as, the inherited trait might not be readily visible.

    7. Adaptation. Living things adapt to changes in their environment. Items 5, 6, and 7 are related. Life reproduces---complex structures reproduce themselves. Life changes itself in response to natural selection on the macroscopic level and to changes in DNA on the microscopic level. On the positive side, it is possible to use this characteristic when examining past life but on the negative side, it would take a long observation time to look for adaptation and evolution by natural selection.

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