| Live, reproduce, die|
|Life as we know it|
|Divide and multiply|
|Greatest Great Apes|
“”Nobody exists on purpose, nobody belongs anywhere, everybody's gonna die.
|—Morty, Rixty Minutes|
“”Life!. Don't talk to me about life.
|—Marvin, the Paranoid Android|
“”Zaphod: Oh yes you will. There's a whole new life stretching out in front of you
Marvin: Oh! Not another one.
|—Marvin again (of course).|
Life (Superdomain Vitae) is the name given to itself by a four-billion-year-old infestation on a rock located about 150,000,000 kilometres from its host star. Despite having given itself a name, this strange thin crust is very unsure as to the true nature of its identity. It has likened itself to a box of chocolates, as it never knows what it is going to get. That, and the fact that chocolate comes from the seed of the tropical cacao tree, which is alive; therefore, the infestation feeds itself and thrives. The living chocolate, in turn, feeds on the light coming down on its host rock.
A difficult definition
“”Life is what happens while you're busy making other plans. — John Lennon
Life is very difficult to define but "everyone knows it when they see it".[note 1] In the macroscopic world of humans, there is a sharp divide between living and non-living things. Unfortunately, when we get down to microscopic life forms the obviousness of life is not so universal, as the microscopic level houses a tremendous blur between animate and inanimate. Most living things perform the following essential functions: adaptation, metabolism, and some mode of reproduction. However, these functions are debatable with organisms such as viruses.
Bots and viruses
Some may complain about a couple of "mechanical" things, one which exists naturally, the other doesn't. First are viruses. Are they alive by our definition? No. They can't actively respond to the outside world. One might also imagine a robot with solar panels that will move toward light and mineral deposits, avoid water, and copies and/or repairs itself. Is this "alive"? Yes. Is that really a problem?
In a nutshell
Views on life
The intuitive 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 (naked-eye visible) level, knowing that something that moves on its own 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 sans magic. This idea's advantages are that it is intuitive and satisfies many humans' need to feel special, unique, and important (just like everybody else).
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.
Lists are fun!
Looking at the vast diversity of life, some may propose attempting to make a list of characteristics that all living things share. Below is an attempt made by someone on here, with later edits by others:
- The ability to move; plants turn toward light, sponges' collar cells' flagella turn round, fungi (excluding the "little pot fungi", which have flagellate spores) and bacteria without locomotory apparatuses… huh.
- The ability to respire, extracting energy from food. For life on Earth, this process is glycolysis, and the storage of energy is maintained by ATP.
- Sensitivity to the outside world, and an ability to react. Lions maul you if you poke them with a stick, plants will grow their roots out, E. coli bacteria will change direction toward sugar and away from acid.
- Growth is self-explanatory; seeds or spores turn into plants or fungi or whatever, cute babies turn into annoying teenagers, bacteria… huh.
- Reproduction is also self-explanatory; amoebae undergo mitosis, plants release pollen to fertilise other plants, bacteria divide, mules and sterile humans… huh.
- Respiration generates waste, which is excreted or otherwise removed from the organism. Diffusion is a legitimate way to do this.
- To meet the above characteristics, the organism needs nutrition. Even plants are going to need water and minerals.
- All living things have some form of genetic material, like DNA. EVERY. LIVING. THING.
This is definitely an improvement on defining life, but there are examples which still challenge it (as inserted above). For example, our intuitive view classes a mule as alive, but as it is (with very rare exceptions) sterile from birth due to its genetic make-up, it cannot reproduce. Telling a man who got a vasectomy that he's not "alive" because he can't reproduce will only get you a punch in the face. Viruses and prions also stretch many of these features to the breaking point.
One might define alive as applying to genetic code, expressed in an animate individual, rather than depending on the immediate state of the animate individual in question. To elaborate, the example of a man with a vasectomy would no longer apply. Were he to be cloned, he would be perfectly able to reproduce like any other male human. (Take that, Lamarck!)
However, this approach has its flaws. Specifically, while it does eliminate all problems with acquired/non-genetic differences, some people are genetically infertile. Even applying the category to the species level does not solve the problem; mules are intuitively life, yet do not quite fall under horses or donkeys (depending on the definition of species used) due to their status as a hybrid.
One might propose, having grown tired of discussing mules, to solve this problem, to knock out some of these criteria. As shown above, the criteria of movement, growth, and reproduction are to be removed. Our list goes down to nutrition, respiration, sensitivity, excretion, and genetic material.
Simplifying a definition comes with some costs. By loosening our definition, we have also allowed for things that aren't alive to be considered "alive" by our definition. As the picture and title of this section indicate, fire is one of those things. Fire requires some sort of fuel (i.e., nutrition), like wood or petrol. A fire will respire by burning its fuel; it will thus produce waste, like soot, smoke, and ash; convection currents take this waste away, meaning it actively excretes its wastes. Fires will respond to their outside surroundings; it will excrete waste in different directions due to wind, it will (depending on the kind of fire) avoid water, among other things. It can also reproduce. We'll need another criterion.
Here, the Second Law of Thermodynamics makes itself known. Living things fight against this law; they will, when alive, attempt to resist becoming equalised with the outside world. A fire is a vassal of the Second Law, doing nothing but breaking things down. Living things will, despite breaking things down, also build things up. Our definition of life is now that which:
- Can take in food, converting it to energy and waste,
- Actively responds[note 2] to changes in the outside world, and
- Builds complex structures (including molecules, cells, tissues, and organs, depending on the organism's complexity) from simpler ones, thereby attempting to resist the Second Law.
This also provides us with a handy definition of death: whenever the organism as a whole can no longer meet all these criteria. A jellyfish dies when it can no longer bring food to its mouth or resist decay (hence "dead" beached jellyfish can still sting, although stinging is an active response); a person is dead when said person will permanently no longer be able to respond to the outside world. As a person's actions are controlled by his brain (even reflexes are controlled by his reptilian brain), this fits well with the concept of brain-death. The regenerative abilities of sponges, hydrae, and planarians show that they haven't died after being chopped up because they can recover from it.[note 3] We've got a nice definition of life here.
The 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 self-organisation and -maintenance, life 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. Unfortunately, this has a problem itself; evolution requires reproduction to occur, so any "dead ends" that do not leave any descendants would not be "alive".
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.
|—Bruce Jokosky[note 4]|
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.
Various bullshit views
As in literally anything, there are some people who wish to ditch all the hard work of countless generations that they take for granted and instead prefer to deal with their primitive fantasies.
- Vitalism is an out-moded concept that "living" things have some supernatural component to them, i.e. a "life force" or soul, without which the organism just wouldn't be "alive". This is a very primitive dualistic viewpoint, and at least in the Western world has been put aside. Mostly in the East (although some people latched onto it in the West), concepts such as qi have been around for millennia and will persist as a meme most likely until Homo sapiens goes extinct.
- Answers in Genesis, the one-stop shop for all your True Christian® "creation science" needs, has come up with their own, severely restricted definition of "life", tied up with a biblical concept of "nephesh" or "soul". AiG defines "living things" as having nervous, muscular, respiratory, and circulatory systems. This restricts "life" to being within the Bilateria, and alienating many of the simpler animals like the flatworms. They go on to then claim that only the vertebrates "definitely" have all these characteristics. Why do they do this? Before the original sin, there is supposed to be "no death". Since heterotrophs (e.g., most animals) can't just hang around eating nothing and surviving, they will need to eat something. Therefore, plants aren't "alive". Invertebrates? We have no clue.
The origin of life
Ongoing life has to be inherited through something hard enough and/or protected enough to survive while simultaneously soft enough to allow mutation. On Earth, following abiogenesis, this is the molecule DNA. This may very well change, though, with artificial life made from scratch.
Life on Earth, in order to build its complex structures (see above), requires some means of storing information, that is hard and/or protected enough to survive, but is soft enough to allow for mutation. For life on Earth, this is currently universally DNA (well, some viruses use RNA, but see above; also, we may soon make our own life that doesn't use DNA). As DNA is a polymer from the same few repeating molecules, it does not become that hard for a primordial soup to produce these simple chemicals (e.g., the Miller-Urey experiment); it wouldn't be incredibly difficult for these nucleotides to fuse and form the first primitive genes. Although other chemicals (so-called "XNA"s, xeno[note 5]-nucleic acids) can replace DNA (and very well could have done so 4 billion years ago), all rely on carbon for their base, as carbon is abundant and reacts with other chemicals very interestingly, to the point where an entire branch of chemistry is devoted just to carbon and its antics. Often electrical charges, e.g. 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 realistic scenario has been 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 Archaea can exist in totally dry environments but only truly operate and perform vital functions 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 recognised:
- Primary/autotrophic sources — Typically sunlight, although things like the glow of lava, or radioactive decay, or chemicals coming from hydrothermal vents (which would have occurred on the early Earth) can also be included.
- Secondary/heterotrophic sources — A very wide group, including herbivory (eating plants), carnivory (eating flesh), scavenging, decomposing, and parasitism. Anything you aren't doing for yourself to get energy falls under here.
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 (if you're asexual, have great cake 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[note 6]
- 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!
- Press Ctrl+Q.[note 7]
- Evolution — The development of life's diversity.
- Second genesis — Another case of abiogenesis beside that which led to us.
- Like pron!
- Versus a passive response like a rock being forced down a hill; the rock is not doing anything itself to alter its movement.
- Yes, these animals can survive being dropped in a working blender. Hell, they can use that as a means of asexual reproduction.
- 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."
- Meaning "foreign" or "strange".
- South Asians can read this as "(Teenage years of) life are dress-rehearsal for marriage. Then everything goes dark!
- If you're using a Mac OS, get rid of it all together.
- An attempt to define life
- Right in the fossil record
- What are the 'little pot fungi?
- "What, waste all our lives raising children, to feed ruddy lions? Not me!"
- Interestingly, some female mules have born offspring with non-mules.
- A nice handy guide to one plausible abiogenesis scenario