Life
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===NASA definition of life=== | ===NASA definition of life=== | ||
| − | NASA has good reason to need a working definition of life, so they | + | NASA has good reason to need a working definition of life, so they will be able objectively to say if and when they see it. Their working definition is: |
{{cquote|1=''Life is a self-sustained chemical system capable of undergoing Darwinian evolution.''|4=''Bruce Jokosky<ref>Anna Lee Strachan, "Ask An Astrobiologist", ''NASA Astrobiology'', 2002-05-01. "While there are many contending definitions, one that is generally accepted comes from Bruce Jakosky's book, 'The Search for Life on Other Planets.' NASA scientist Jakosky defines being 'alive' in general terms if the object 1) utilizes energy from some source to drive chemical reactions, 2) is capable of reproduction, and 3) can undergo evolution. Of course, this definition is subject to several complications."</ref>}} | {{cquote|1=''Life is a self-sustained chemical system capable of undergoing Darwinian evolution.''|4=''Bruce Jokosky<ref>Anna Lee Strachan, "Ask An Astrobiologist", ''NASA Astrobiology'', 2002-05-01. "While there are many contending definitions, one that is generally accepted comes from Bruce Jakosky's book, 'The Search for Life on Other Planets.' NASA scientist Jakosky defines being 'alive' in general terms if the object 1) utilizes energy from some source to drive chemical reactions, 2) is capable of reproduction, and 3) can undergo evolution. Of course, this definition is subject to several complications."</ref>}} | ||
Revision as of 17:23, 19 June 2010
Life is like a box of chocolates, in the context that chocolate is extracted from the seed of the tropical cacao tree, which is alive. We are all alive. You, me, that guy at the train station, and that mole on your back with the long hair sticking out of it, are all "life".
Contents |
Defining life
Life is very difficult to define[1] but "everyone knows it when they see it".[2] Unfortunately when we get down to microscopic life forms the obviousness of life is not so universal. Most life performs the following essential functions: adaptation, metabolization, and some mode of reproduction. However, these functions are debatable with organisms such as viruses.
Classic or narrative view
This view is the most complex to explain, but it is the one that most people are familiar with. In this definition of life, "life" is what we say it is. Literally, the definition gets no more rigorous than "everyone knows it when they see it", as mentioned above. Because the human race has evolved to deal with looking at life at the macroscopic level, knowing that an organic thing that moves must be alive, we can intuitively tell what is and what isn't alive. In this model of life, life begets life, non-life begets non-life and there is a distinct boundary between life and non-life. In some views, such as creationism, this line cannot be crossed. In other views, this line can only be crossed by essentially a "magic spark" - with the nature of this so-called spark hotly debated. This idea has an advantage as it is easy, intuitive and satisfies the human need (or at least the needs of some humans) to feel special and unique.
The disadvantages of this view are plenty. Firstly, the border between life and non-life presents a challenge; when does life begin? How did life begin? When do gametes become "alive"? Is it possible for us to create artificial life? In essence, this definition raises more questions than it answers and is even more useless because it doesn't, in fact, give us a definition. With this in mind, we can proceed to more objective ideas of life which we can possibly agree on universally.
Biological view
As most children will probably be taught in biology class, life has a series of features (a checklist, if you will) that define it. This list has 7 items, remembered with the mnemonic acronym "Mrs Gren". These features are:
- Movement - the ability to move and react to surroundings, animals have muscles, plants grow towards light.
- Respiration - the processing of "food" into "energy" (most commonly this is the conversion of glucose to ATP, which can power biological cells).
- Sensitivity - to changes in the environment, noting these changes and reacting to them.
- Growth - self explanatory, seeds turn into plants, babies turn into annoying teenagers.
- Reproduction - self explanatory again, life must reproduce.
- Excretion - respiration generates waste, which is removed from the organism.
- Nutrition - the living organism requires nutrients to do all of the above.
This is an improvement on defining life, but there are examples which still challenge it. For example, our narrative view classes a mule as alive, but as it is (with very rare exceptions) sterile from birth due to its genetic make-up (rather than an abnormality or surgery as in the case of castrated animals) it cannot reproduce. Viruses and prions also stretch many of these features to the breaking point. The question of whether we could generate artificial life - a truly sentient computer, for example - is still open and not answered with this definition. Nevertheless, we have a better, more objective definition.
Energy and information
A more abstract definition of life involves energy and information. Firstly, energy. As we can see from the Mrs Gren example above, the use and control of energy is pivotal to life. Life, therefore, must use energy. A rock doesn't consume energy, but bacteria and viruses do. However, a candle flame consumes energy in a controlled manner also, is it alive? This is where the second criteria of information is required. Life consumes energy in order to process and conserve information - essentially life is bucking the thermodynamic trend towards favourable chaos. This is true for all life that we know of, the energy is a self-explanatory use of energy to move, grow and react while the storage and preservation of information is controlled by DNA. However useful this definition is, it runs the risk of being too broad. A computer, taking energy from an electrical socket, using it in a processor and pumping it out via a cooling fan and taking information from its hard drive could be considered life in this context.
Evolutionary view
Another view of life based on the principles of evolution, often adopted by evolutionary biologists (surprise, surprise...), can be postulated in light of the above. In addition to the consumption of energy and preservation of information, life must be able to use this information or add to it. Namely, it must undergo evolution, preferably by natural selection. This definition of life satisfies most criteria for both practical biologists and proponents of odd thought experiments. With this definition, it is possible to create artificial life by making "artificial DNA" which can mutate. It is possible for a computer to evolve, and also for a simulation of life to evolve. While this may also be a very broad definition, the advantages of it are numerous. We can now satisfy the problem of a boundary of life and non-life by blurring it a little. While people may have difficulty in accepting a string of molecules as "being alive" it is certainly possible to have a gradual evolution from non-life to life via this definition - starting with simple chains that reproduce via auto-catalysis, then to longer ones that mutate, before complexity builds up to the point where the definitions above become usable.
NASA definition of life
NASA has good reason to need a working definition of life, so they will be able objectively to say if and when they see it. Their working definition is:
“”Life is a self-sustained chemical system capable of undergoing Darwinian evolution.
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| —Bruce Jokosky[3] |
This definition is imperfect, of course, but going through the caveats in the above sections should make you reasonably aware of the imperfections - should you ever actually need to answer the question and only have a copy of this RationalWiki page to hand.
Requirements for life (as we know it)
Life has to be inherited through something hard enough and/or protected enough to survive while simultaneously being soft enough to allow mutation. On Earth, following abiogenesis, this is the molecule DNA.
Origins of life
Something very similar to DNA, or of course RNA, is almost certainly required for life, as we know it now, to originate. Precursors to DNA, such as individual nucleotides or short polymers of nucleotides would be an obvious step in the right direction. Various other compounds have been proposed but all are carbon compounds, for reasons of carbon's abundance and otherwise unique polymeric chemistry. Often electrical charges, lighting strikes, are associated with giving molecules the energy to react; inorganic and gaseous N2 in the atmosphere is "fixed" into an organically usable form of nitrogen via lightning strikes and is one of the largest sources of organic nitrogen, even today when efficient nitrogen fixing bacteria have evolved.
In addition to abundant appropriate chemicals, no scenario has been realistically proposed which does not require liquid water in sufficient amounts to completely envelop a basic reproducing unit such as is represented by a cell on Earth. Viruses and some primitive archaebacteria can exist in totally dry environment but only truly operate in the presence of water. The first life on earth probably existed by trapping a small volume of water in a sphere of emulsion created from hydrophobic/hydrophilic molecules - this is basically what a cell is now.
The only other requirement for life is an energy source. Two classes of energy source are recognized:
- Primary - This is mostly sunlight, however life has evolved elsewhere to gain its primary energy from thermal vents or chemical reactions.
- Secondary - This is energy gained by interaction with other life forms that use primary or other secondary sources of energy.
Fire
A wildfire demonstrates the characteristics needed for life based on some definitions and is often cited as an example of where the definitions present ambiguity or non-intuitive consequences. Fire "breathes" oxygen, consumes food (wood or other fuel), grows (expands across the solid fuel), reproduces by division or by spreading spores (embers) through the air, and eventually dies while excreting waste ash. Sensitivity and reaction to surroundings is more difficult, but fire does react to water by retreating or dying. By the definitions involving energy, fires can consume energy in a controlled manner, however, their primarily destructive nature makes them fail at being alive using definitions based on information or evolution.
Interestingly, even though fires may spring to life spontaneously under the right conditions, and there is no obvious physical link between various individual fires, they all appear to be very closely related - or even identical - in terms of their chemistry.
How to get a Life
- See the world (but not on a cruise ship)
- Enjoy good food
- Get drunk (occasionally)
- Get stoned (at least once, but not too often)
- Have great sex with people you really like
- Enjoy a wide range of music, literature and art
- Make a stranger happy and ask nothing in return
- Treat other people as you would expect them to treat you
- Appreciate what you have, rather than strive for what others have
- Remember that life is not a dress-rehearsal for something later
- Laugh (as often as you can)
- Don't concern yourself with the perceived failings of others
- Leave the world in as good shape as possible for those who come after you
- You're gonna die. Accept it!
- Click on the little red box with the white "x" on the top right of this screen.[4]
See also
Footnotes
- ↑ An attempt to define life
- ↑ Like Pron!
- ↑ Anna Lee Strachan, "Ask An Astrobiologist", NASA Astrobiology, 2002-05-01. "While there are many contending definitions, one that is generally accepted comes from Bruce Jakosky's book, 'The Search for Life on Other Planets.' NASA scientist Jakosky defines being 'alive' in general terms if the object 1) utilizes energy from some source to drive chemical reactions, 2) is capable of reproduction, and 3) can undergo evolution. Of course, this definition is subject to several complications."
- ↑ Don't have a red box and white "x"? Then you definitely need to get a life!