Should Science Rethink the Definition of "Life"? 299
ambermichelle pointed out a story about the search for life on other planets, and the likelihood that it would be much different than what we find on Earth. With the increase of extremophile discovery in recent years perhaps it's time to reassess what the definition of "life" should be. "In November 2011, NASA launched its biggest, most ambitious mission to Mars. The $2.5 billion Mars Science Lab spacecraft will arrive in orbit around the Red Planet this August, releasing a lander that will use rockets to control a slow descent into the atmosphere. Equipped with a 'sky crane,' the lander will gently lower the one-ton Curiosity rover on the surface of Mars. Curiosity, which weighs five times more than any previous Martian rover, will perform an unprecedented battery of tests for three months as it scoops up soil from the floor of the 96-mile-wide Gale Crater. Its mission, NASA says, will be to 'assess whether Mars ever was, or is still today, an environment able to support microbial life.' For all the spectacular engineering that's gone into Curiosity, however, its goal is actually quite modest. When NASA says it wants to find out if Mars was ever suitable for life, they use a very circumscribed version of the word. They are looking for signs of liquid water, which all living things on Earth need. They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive. In other words, they're looking on Mars for the sorts of conditions that support life on Earth. But there's no good reason to assume that all life has to be like the life we're familiar with. In 2007, a board of scientists appointed by the National Academies of Science decided they couldn't rule out the possibility that life might be able to exist without water or carbon. If such weird life on Mars exists, Curiosity will probably miss it."
It's life, Jim, but not as we know it... (Score:2)
not as we know it, Captain!
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"We come in peace! (shoot to kill, shoot to kill)" - James T Kirk
(reference for those not in the know: Star Trekkin' [youtube.com] by The Firm)
Don't you know what aliens look like? (Score:3)
If you know anything about TV science fiction, then you would know that all sapient life forms look like white people with maybe some ridges on their forehead or something (and they speak English). All flora looks just like what you find in California. And animals look like shambling people in horribly fake costumes.
Re:Don't you know what aliens look like? (Score:4, Funny)
Some planets resemble forests in British Columbia, or quarries in Wales. :P
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There may even be planets that resemble the Tunisian desert, or Norwegian glaciers.
http://www.youtube.com/watch?v=WfwNZO2OqMQ (Score:3)
Unless they're brutal, bloodthirsty warriors with some primitive sense of honor. Then they resemble black people with maybe some ridges on their forehead or something.
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Predators only showed up in a couple of fringe Trek episodes, they can hardly be considered sci-fi canon.
Dumb article (Score:5, Informative)
Life is defined as something that feeds and reproduces.
The requirement for water or carbon is not part of the definition, it's simply properties we thought all life forms had.
Re:Dumb article (Score:5, Informative)
Life is defined as something that feeds and reproduces.
The requirement for water or carbon is not part of the definition, it's simply properties we thought all life forms had.
Mod up. Biologists have indeed told me that life is defined by a collection of properties such as metabolism and reproduction. Maybe NASA needs to change its definition, but not "Science".
Re:Dumb article (Score:5, Interesting)
There IS no set definition of life. Viruses, prions, crystals, fire, mules, computers, you can come up with obvious exceptions to any criteria. Reproduction? Fire does that, crystals do that, mules do not. Metabolism? A car battery undergoes some metabolism, bacterial spores and seeds I think don't, though I could be wrong.
The closest thing we have is like Justice Potter Stewart's definition of porn: we know it when we see it.
Re:Dumb article (Score:4, Interesting)
Without disputing that there is no consensus on the definition of life, I am of the opinion that there is a possible definition which includes everything we take to be alive and nothing we reasonably shouldn't take to be alive. It is in terms of thermodynamics, specifically entropy.
Let us define mechanical work as "productive" upon some system when it changes the said system to a state which is less entropic. We can then say that a machine X is productive upon some other system Y when the product of its work is a decrease in the entropy of Y. There may be limited circumstances under which X is productive upon Y; to do work X will need some sort of energy flow through it, but not any energy flow will do. So for example, an electric machine which sorts and stacks coins coined can be said to be productive upon the coins when it is plugged in to an electrical circuit of the proper frequency and voltage, and turned on, and otherwise in its operating conditions, but not just when it is being heated, say.
With that out of the way, we can now define a system as "alive" when it is productive upon itself, or more simply define "life" as "self-productive machinery". The given conditions under which a given system is self-productive will of course be limited and vary, but those constitute the conditions under which a given system can live, which are also limited and vary.
Crystals, fire, and car batteries all seek lower-energy, higher-entropy states, and so are not life, though they might fuel life (like fire) or be instrumental in the construction of living things (like crystals).
Viruses, by this definition, can only live inside of more complicated cells, the way that a severely deformed baby might only live on life support, or humans in general can only live in an atmosphere of appropriate composition, temperature, and pressure, with sufficient water and appropriate chemical fuels available. A virus floating around by itself is dead, though under the right circumstances it can come to life (unlike humans, but that's because dead humans decay more readily than dead viruses, being big complex things instead of simple molecules). Spores and seeds likewise: not alive just sitting there inert, but alive when put into the conditions in which they grow. Prions I don't know enough about to say.
Mules are definitely alive, whether or not they are a viable species; reproduction is just one way for life to continue living, not a prerequisite for living at all.
The only really controversial part of this definition is that computers may count as alive, when plugged in and turned on etc, the same way that viruses may count as alive when inside another cell of the appropriate type. But that's only controversial because they are artificially constructed machines made from something other than the stuff we are made of. Artificially constructed organic nanomachines modelled after the ones we're built on are indisputably alive: artificial life, but life nonetheless. Computers differ only in being bigger, and made of different materials. Their information-processing functions certainly reduce the information entropy of their storage media.
This factor has the nice benefit to this definition of ensuring that intelligence, sentience, sapience, etc, are a proper subset of life; you can't have something which takes in information about the world around it and does something productive with it, without that thing being alive by this definition in the process.
Re:Dumb article (Score:5, Insightful)
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Life is defined as something that feeds and reproduces.
So... fire is alive by your definition and a bumble bee drone isn't.
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So... fire is alive by your definition and a bumble bee drone isn't.
Huh? A (bumble)bee drone reproduces. Essentially, that's the only thing it does, and then it dies. But its descendants live on, starting from the fertilized eggs inside the drone's mate(s).
Fire does sorta throw water on the definition, though. It definitely "feeds", and reproduces in the sense of expanding. So do crystal formations, which aren't considered alive.
Methinks this definition needs a bit of revision. Maybe some actual biologists could chime in with something with better wording.
Re:Dumb article (Score:4, Informative)
Fire does sorta throw water on the definition, though. It definitely "feeds", and reproduces in the sense of expanding. So do crystal formations, which aren't considered alive.
I'm not a biologist.
I think you can distinguish fire vs. life by metabolism. With metabolism, chemical processes inside the organism occur to fuel the organism. With fire, those processes do not happen within the flame, they happen inside the fuel. I can't think of an organism which fuels itself by chemically transforming fuel outside of its "body". This may be shaky, because you could argue that the chemical processes would not happen were it not for the presence of the flame. Fire doesn't sound like it fits the definition of metabolism though, unless you view the flame as simply the result of the metabolism and the organism itself which produces the fire through metabolizing the fuel is unknown.
With crystals growing in a liquid, matter which is dissolved in the surrounding liquid gets added to the crystal. There is no real chemical change that takes place, the dissolved particulates just coalesce on something else. When the liquid and its particulates is removed the crystal no longer grows, but the crystal itself does not take the particulates out of the liquid. The particulates simply adhere to the crystal (or any other structure on which the crystal starts).
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Combustion.
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External digestion does exist; fungii are a good example of this (heterotrophy II).
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Cooking, or other food processing, could fall under this broad concept.
This whole debate is silly, just like the definition of planet that people were arguing over a few years ago---people who should be doing science and not trying to define and then redefine words to re-fit reality into their preconceptions. Definitions are human constructs, attempting to create bright lines between what is an x and is not
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With crystals growing in a liquid, matter which is dissolved in the surrounding liquid gets added to the crystal. There is no real chemical change that takes place, the dissolved particulates just coalesce on something else. When the liquid and its particulates is removed the crystal no longer grows, but the crystal itself does not take the particulates out of the liquid. The particulates simply adhere to the crystal (or any other structure on which the crystal starts).
Hmm ... That needs a bit of rewriting. It reads a whole lot like a description of most of the higher plants growing out in our garden. They require much of their nutrition from the water that pervades their soil (and they die if that water is removed). They even get their carbon from the CO2 that has dissolved in the water in their tissues; they don't take it directly from the atmosphere. They absorb the nutrients from the water, transport them via diffusion and capillary action (two things that are n
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A mule is alive because it was created though reproduction. Sterility does not mean that something is not alive.
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Something which is comprised of living things is by class inheritance alive. Hence mules via their cells. And corporations via their "citizens united."
Perhaps it's not a discrete concept (Score:2)
That would exclude viruses, which are quite subject to natural selection. And Eunuchs don't reproduce, but they are still "alive" by most accounts.
We've had a long debate about this at c2.com. We considered robots, prions, parasites, etc. I've concluded there is no simple definition.
My eventual favorite was a weighted definition involving combinations of the following:
* Shaped by natural selection
* Ability to adapt to changes
* Reproduces
* Maintains self
* Consumes energy
* Complex
It had to have at least one o
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1) What sort of life-detection-capabilities should we build into a mars lander?
2) What is the permanent fundamental definition of "life"?
The first is pretty easy to answer - we've got limited resources, so it makes sense to look for the kind of life we know how to look for. True, we might miss other things that (had we not missed them) we'd consider "life" - but what choice do we have? We don't know what "different life" might look like, so we can't b
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Life is defined as something that feeds and reproduces.
Meh, not so simple. I won't even talk about things like mules, or other infertile animals; but are for example erythrocytes (red blood cells) alive? They feed, but don't reproduce, you know; how about viruses? They do reproduce, but surely don't feed. Or prions? They also reproduce, but don't even carry nucleic acids. What about Sydney Fox's protobionts [wikipedia.org]? They both (kind of) reproduce and (kind of) feed, but can form spontaneously from inorganic matter.
self-reproduction with variations (Score:5, Interesting)
FTA: Simply, Life is "self-reproduction with variations" - like mutating computer viruses?
Sure... (Score:5, Insightful)
Re:Sure... (Score:5, Insightful)
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Fact: We know carbon based life forms exist
Fact: we know they need water.
So it makes sense to start looking where you know for a fact life COULD exist withing know parameters.
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And thus, my lifelong pursuit to create a carbon-based life-form that does not need water is born.
You can't look for everything (Score:2)
so you pick things that at least you can work out how to look for and that you know can exist.
Of course we should. (Score:2)
And these constructs totally depend on the greater system they are part of. All the conditions, present compounds and elements in a given environment, would cause any such self developing and propelling systems to evolve shaped according to that environment. it does not necessaril
Thermodynamic definition of life (Score:5, Interesting)
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> but life could generally be defined as the ability to actively resist entropy
Indeed, that's what I thought of too - I think it was in Heinlein's Stranger in a Strange Land that he used a phrase something like "phenomena of localized negative entropy".
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Here it is:
Three of them were big enough, as planets go, to be noticeable; the rest were mere pebbles, concealed in the fiery skirts of the primary or lost in the black outer reaches of space. All of them, as is always the case, were infected with that oddity of distorted entropy called life~, in the cases of the third and fourth planets their surface temperatures cycled around the freezing point of hydrogen monoxide-in consequence they had developed life forms similar enough to permit a degree of social co
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Meh... life isn't negative entropy except with very very rose coloured glasses on, and only if you look at it from one side.
Human life is on some level just a chemical fire that needs a near continual influx of fresh fuel to burn.
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You mean like fire?
We should rethink the false dichotomy first. (Score:3, Insightful)
I am always depressed about the primitivity of human thought, when I hear people discuss "Is this alive, or is it dead?". As if that was some binary either/or question or switch.
We have to face, that for every step between completely dead and whatever we define as completely alive, there exists something that fits that. And why wouldn't there?
Then we can rethink our egocentrism, and accept that we are neither special nor unique, and that that is OK. :)
It really is.
Life has to follow the constraints that we defined it to have, and therefore logically is between some bounds. E.g. the elements it uses, if it needs water, what temperatures it requires, what processes it uses and consists of....
But really it's just a definition thing. And nothing else. Since "life" is just a word. Nature itself does not know the concept of a "concept".
So I see this from a relaxed point of view. All this bickering about definitions and "ME, ME, ME, ME, ME, ME!" doesn't matter.
What matters, is that we are on the brink of discovering things on other planets... Things that can be so vastly different from ourselves, that our knowledge may leapfrog forward... And that yet may be so very similar to us in so many aspects, that is will tell us things about ourselves we could never have imagined.
Exciting times, baby. Exciting times indeed.
Re:We should rethink the false dichotomy first. (Score:4, Insightful)
The dichotomy isn't false. Death is simply defined that way. If it's not alive, it's dead. If it's not dead, it's alive.
You may as well say that it's a false dichotomy to only have 0 and 1 as valid significant figures in binary.
Conjecture (Score:2)
It's Mars though. (Score:2, Interesting)
IF we're going to find any life on Mars, it's probably going to be the carbon stuff we've been hearing so much about. Silicon life and other sorts of voodoo biology might exist in stranger environments but Mars is basically a big dry dust-ball sitting next to a big wet swamp-ball. Odds are that whatever splashed our planet in the first place also got Mars, and Mars just so happened to be tinier, lighter and colder than us enough that its water cycle kind of evaporated. Or at least that's the theory they'
That's not how science works (Score:3)
We don't assume something just because we can't rule it out completely, we assume something because there are signs indicating it's true. We have pretty good proof that shows that carbon-based life can exist, but there is neither physical nor theoretical proof of other exotic lifeforms. Not being able to rule it out is not enough reason to send another expensive probe when that money could finance far more promising research.
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If it isn't very much like us.. (Score:4, Interesting)
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Just because it's not like us, doesn't mean it's technology wont be the same. We might end up using oscillating radio waves to communicate.
News flash (Score:4, Interesting)
You have to be able to define life before you can redefine it. Turns out to be pretty tricky.
"Howler" alert (Score:5, Interesting)
They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive.
This is likely to trigger red flags in the minds of a lot of people with biological training. Just what is "organic carbon"? That's a media phrase that isn't too well defined in scientific circles. There's a great variety in the "organic" carbon chemistry of our world. But we should expect that any life on other worlds, even if it uses carbon, will produce compounds and radicals that are different and/or more varied than what we see here.
Another problem is that astronomers long ago pointed out a probable path for Earth bacteria colonizing the rest of our solar system, and possibly beyond. Earth has a thin "dust tail" produced by the same solar light pressure that produces comet tails. This is a problem for some kinds of astronomical observations in the plane of the solar system, since our dust tail reflects back back to us. Anyway, back in the 1970s, satellite and upper-atmosphere probes verified the presence of both fine dust particles and bacterial spores at all altitudes. The planet's dust tail thus contains such dust and spores. So the Earth has been contaminating the outer solar system with bacterial spores, presumably for some billions of years. We don't know whether any of those bacteria can survive on the outer planets. But the default assumption should be that some of them have, and have adapted to some degree over those billions of years to their new environments. Maybe they have; maybe they haven't. But if we find Earth-like bacteria out there, they probably came from here.
Some astronomers have also calculated out that part of our dust tail (and comets' tails) escapes the solar system. So we've been contaminating the galaxy with bacterial spores for billions of years. A billion years is around 4 or 5 orbits of the galaxy, up to 20 or so orbits since life arose here. The chaotic nature of galactic dynamics mean that our dust has spread through the entire galaxy, as has the dust from other planets with atmospheres.
This argument is more often used by the "panspermia" supporters, who point out that life from anywhere else in the galaxy could have colonized Earth in its early years, since the galaxy is around 13 billion years old, while our solar system is only about 1/3 that age. But some astronomers use it to explain how earthly life could have colonized the rest of the galaxy before humans evolved here. And, of course, both could be true.
Of course, the main problem with all this is that we have no data on how well bacterial spores can survive the millennia in interstellar space. Probably not well, but it doesn't take a whole ecosystem to establish a colony. For bacteria, it only requires one spore (and hundreds of millions of years ;-).
Probably the best prediction is that eventually, some probe will find a few bacteria on Mars and/or other planets, and they'll be somewhat similar to bacteria on our planet. This will raise more questions than it answers, as is common in most scientific fields.
We have two choices. (Score:3)
Two choices:
A) We can look for the sort of life we understand the best, with sensors that are very good at doing that, in places which are likely to harbor such life.
B) We can look throughout the universe for something completely unknown. We have no criteria to define it, no instruments to detect it, no idea where to look for it, and no way to interpret it.
Which of these two choices is the more feasible for a small unmanned probe?
Why "rethink"? (Score:5, Interesting)
James Lovelock came up with a perfectly good definition that doesn't stipulate any specific chemistry - he merely stated that life is that which will actively sustain a dynamic equilibrium when the non-living parts of the system passively change*. (He also argued that the distinction between living and non-living was stupid anyway, since there are too many inter-dependencies to make such a distinction in a productive way. Since his work forms the backbone of almost all modern life science, it seems pointless NASA resorting to definitions of "life" that have been considered obsolete for a decade or more.)
Indeed, Lovelock's theories on life are exceptionally useful to astronomers, because you CAN monitor the chemistry of the atmosphere of an exoplanet and you CAN monitor things like the solar radiation it gets. You can therefore utilize Lovelock's work to determine if the planet has life on it or not, remotely, without any regard whatsoever to the chemistry of that life or the mechanisms it utilizes.
*The basis of Lovelock's definition is that all life MUST geo-engineer. It has to, with no exceptions. That goes for viruses, bacteria, algae, etc. Not only must it geo-engineer, but in order for a system to be in dynamic equilibrium, the geo-engineering HAS to contain a negative feedback loop. The mere presence of life will alter the planet, but if it were to alter it without creating a dynamic equilibrium it would necessarily create a positive feedback loop that would destroy itself. In his view, you cannot treat the geology, the meteorology and the biochemistry as distinct fields - they interact and compartmentalizing will never let you understand the processes going on.
Analyzing soil samples will help on Mars but really it shouldn't be necessary. Dormant's another matter. If life exists in an active form, there will be variables that are held to a value and do not passively fluctuate with the seasons. If life *ever* existed on the planet, then the chemistry of the rocks will show that variables were held to a specific value and did not fluctuate with the seasons. The geology will record the feedback processes that all life (in this model) must have. The soil samples would let you identify what that life was/is, and to understand HOW it operated, but to merely detect if it was there to begin with you need look no further than the chemistry of the sedimentary rock we already know exists on Mars.
That is, if his theory is correct.
Evidently, despite the views of the life sciences, NASA is not following this path. Ergo, NASA thinks that despite the fact that it doesn't know what to look for, it shouldn't look where Lovelock said. I would hope they have a really good reason -- it's exceptionally bad science to ignore the prevailing theory, particularly if you have none of your own. They have to be rejecting his theory because if they accepted it then they wouldn't need to care about carbon, water, etc. They'd merely need to care about whether the chemistry could or could not be explained by passive processes alone. What the process was would simply not matter.
If at first you don't succeed... (Score:2)
expand the definition and hope you get lucky.
Obviously not a scientist. (Score:3)
There is perfectly good reason (Score:2)
how many lifeforms have you seen not on earth?
changing the question to fit your answer is not science, its religion
focus on the habitable zone (Score:3)
Alien sentience is more interesting than life (Score:3)
How we define "life" when searching the cosmos is entertaining, but to me the bigger philosophical question to consider is alien sentience (not just intelligence) -- self willed, thinking, rational or irrational beings who think, feel, and act for themselves similar to us, but likely following completely different structures of society and morality.
Life as a Taxon (Score:2)
After posting the article linked in the summary, Zimmer followed up by posting the comments of evolutionary biologist David Hillis [discovermagazine.com] on his own website. For those that don't want to read the entire post, the basic idea is that we ought not try to define life as a collection of characteristics (i.e. reproduction, inheritance of traits, existence of metabolism, &c.). Any such definition is likely to exclude things that we think of as alive, or include things that we think of as not-alive. Instead, it ma
What's the point? (Score:2)
There's only self-replication. (Score:2)
For convenience, when it happens in the chemical domain and contains hydrogen, oxygen, carbon and nitrogen, we call it "life." Quaint and parochial, but convenient.
Salt crystals in hypersaturated solution, bacteria, books, religion, money. Self replicators all. Some more limited than others.
Yay for stupid titles (Score:3)
This is not a definition. It doesn't even claim that all conceivable or possible things that have these properties are life, and I would not discount the possibility of finding something that did not have one of these properties that still seems worthy of calling life. Maybe science should change its definition of asjkdhljkfg while we are at it. The 'definition' doesn't even say that life is carbon based or water based as the summary seems to suggest, rather it goes out of its way to stress that this is just true for what we have seen so far.
Terminology (Score:3)
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Re:If it evolves by replicating, it's life. (Score:5, Funny)
Life is anything that dies when you stomp on it.
Or, my personal favorite: Life is a monosyllabic morpheme consisting of a fronting diphthong followed by a labio-dental voiceless fricative.
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Re:If it evolves by replicating, it's life. (Score:5, Funny)
... a labio-dental voiceless fricative.
I don't know why, but I feel a little dirty after reading that.... Is that, like, Latin porn or something?
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Fire.
Software.
What about clearly living things that can not replicate? such as a mule.
for the record:
Corwin was a whiny bitch.
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There are "Molly mules" that can get pregnant and bear colts, though the males ARE all infertile.
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Re:If it evolves by replicating, it's life. (Score:5, Informative)
In all seriousness, though, the definition of 'life' taught to young scientists doesn't proscribe any particular construction materials; hence this article (or at least this summary) is deceptive. The requirements are:
1. Homeostasis. It must make a detectable effort to maintain the conditions of its internals, and to adjust to changes in its environment.
2. Reproduction. It must be capable of creating copies of itself (or approximate copies of itself.)
3. Evolution. Its offspring must be able to adapt to changes in the environment through to natural selection.
That being said, there are circumstances in which some of these are suspended, like ancient trees and soldier ants that can't reproduce but are most definitely alive. The maintenance of an internal environment (homeostasis) is considered the most important, and the primary reason scientists have hesitated to consider transposons and viruses to be alive, even though they can reproduce and evolve.
Outside of these guiding principles, though, biologists really have no problem with the Enterprise running into plasma filament creatures, or Doctor Chaotica's henchmen duking it out with photonic life forms (although physicists might.) We're very good at pointing out flaws with some of these ideas (like "silicon is extremely bad at supporting life when compared with carbon") but that doesn't mean chemical evolution will never find a way to do it anyway.
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I have rarely seen a baby that looks just like an adult. And after the birth, both parent and child are smaller than the combined entity prior to the birth. The real difference vs the broken rock shows up in what happens later.
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Re:If it evolves by replicating, it's life. (Score:4, Insightful)
1. Homeostasis. It must make a detectable effort to maintain the conditions of its internals, and to adjust to changes in its environment.
2. Reproduction. It must be capable of creating copies of itself (or approximate copies of itself.)
3. Evolution. Its offspring must be able to adapt to changes in the environment through to natural selection.
I'm not a biologist but I enjoy learning so I have a some questions about these definitions.
What's the current thought on virus? Are they 'living'?
Concerning #2. Shouldn't life have to create approximate copies? If they create [exact] copies, wouldn't that negate #3?
Concerning #3, If life doesn't make exact copies, doesn't evolution have to happen by way of natural selection. In other words are #2 and #3 redundant?
Re:If it evolves by replicating, it's life. (Score:5, Interesting)
We classify viruses in the same way we classify living organisms, but there's still a lot of debate about whether or not they're alive. I could come up with a half-baked underslept computer analogy, but just going to Wikipedia [wikipedia.org] would probably be more useful.
Regarding #2: a truly reliable and perfect form of biological reproduction is asymptotically impossible due to thermodynamics (this is mentioned in the article.) Assuming 'nearly perfect' = 'perfect', I meant 'approximate' to refer to complex mechanics like sexual reproduction, where the traits of multiple parents are mixed, and random evolution is enhanced.
Regarding #3: It means the organisms produced by mutation must be sufficiently different for natural selection to act upon them. A photocopy machine operating repeatedly in the absence of humans will produce imperfect copies, but no one cares.
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a truly reliable and perfect form of biological reproduction is asymptotically impossible due to thermodynamics (this is mentioned in the article.) ,
I am not a physicist either :) but while it is in the article, the author merely mentions that the laws of thermodynamics make it impossible. But presuming he is referring to the second law and entropy, it only applies to isolated systems so I wouldn't think it's impossible.
/. with informed people can lead me to a better understanding of things.
I'm not trying to be argumentative I just find that discussions on
Re:If it evolves by replicating, it's life. (Score:5, Informative)
life, from a CS view (Score:2)
I prefer a simpler definition: universal machines, in the sense of the Church-Turing thesis. Of course, we say that computers are not alive. But we define life in a way that excludes computers, at least, current computers.
It is surprisingly easy to support universal computation. One might think it takes all kinds of complicated logic and machinery, but this is not so. Some two input logic gates, such as NAND, are enough. Conway's Game of Life is a simple cellular automaton that can do universal comp
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There are always exceptions to everything. Science isn't about what happens all the time, it's about what happens 95% of the time and hoping that we're right. For all we know, our definition of "gravity" is wrong and it has nothing to do with actual mass but something else that we can't detect that generally corresponds with mass.
Essentially, what we're arguing is semantics. When we say "life" we can't even be sure what we're talking about without using a sentence to specify. We could be talking about any n
Re:If it evolves by replicating, it's life. (Score:5, Interesting)
And for your information, the exceptions I listed aren't exactly classic exceptions. The question of whether viruses constitute life is under debate [wikipedia.org], and sterile organisms are essentially modifications of other members of their species, which are very much capable of reproduction.
Finally, the definition is supposed to be used to differentiate large groups of phenomena from life, and has widely been recognized as inexhaustive and incomplete for a long period of time. You expect too much of experimental science if you believe that a scientific definition must be so rigourous.
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Fire actually does none of those things. Those things can happen to a fire. Big difference.
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Re:If it evolves by replicating, it's life. (Score:5, Interesting)
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To a biologist, "evolution" refers to the Darwinian variety. Basically, that means reprodcvtion with heriditable variation. On that basis, I consider viruses to be life, but not rocks
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If their heritable mutation affects their probability of reproduction, so that it is capable of evolving in a Darwinian fashion. There is a theory that the earliest form of life might have been crystalline.
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If your computer reproduces, and has heritable mutations that affect reproductive success, so that it is subject to Darwinian evolution, then I would consider it to be life, whatever the its form or behavior.
The answer to the question, "what makes us humans different from animals?" is of course "nothing," because we are animals. If you mean, "what makes us different from other species of animals?," the answer is different genes.
If you ask, "what about our behavior is different from other animals?" there is
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People like you make me fear for my unborn grandchildren. I see folks with no understanding of how computers work thinking that they're "thinking machines", like idiots in the media have been calling them since ENIAC, which was less powerful than a Hallmark greeting card.
A computer is nothing more than an electric abacus. It works on exactly the same principle as an abacus. IT DOES NOT THINK, IT IS NOT ALIVE! I made a pseudoautonomous program thirty years ago on a machine with only 16k of memory and no disk
Excess sustained negentropy (Score:4, Interesting)
measured in bit seconds of locally retained information
divided by bit seconds of locally retained information expected (statistically) given the thermodynamic regime.
More (locally retained information retained longer) is better (more lifelike, or higher life, or what have you.)
That's my proposal for the definition of life.
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more precisely "given the thermodynamic regime and other aspects of the local physical regime (momenta, ranges of other forces)"
Re:Excess sustained negentropy (Score:5, Insightful)
According to your definition of life, all my hard disks are alive!
What you have missed: 1. perception of proximity, sensing 2. behavioral intent
Re:Excess sustained negentropy (Score:5, Interesting)
One word : Parasites
They are highly evolved to a specific environment, but are simplified often to an absurd degree as they do away with unnecessary organs and structures ...
They do not retain information they deliberately lose it, they are not "higher" except in the sense that they are as efficient as possible (which is all evolution produces, a best fit for an environment)
Re:If it evolves by replicating, it's life. (Score:5, Insightful)
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That is a really bad definition of evolution.
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How's this one:
Judging by their reliability, I suspect Fiat is just that.
I give it a C-, personally.
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There's nothing deliberate about the error mechanisms that cause mutation and therefore make evolution possible. "Deliberate" is the kind of word that's more appropriate to discussions of "intelligent design" than evolution.
The error mechanisms exist and are arguably necessary, but in the world of chemistry, randomness is everywhere so you don't need to go looking for deliberate errors.
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Actually, theoretically, the definition of 'alive' is disputed, because some scientists want viruses to be 'alive' and some don't. (Among many other disputes as to what the formal definition should be).
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But parasites, even internal ones that rely on the environment of their host bodies to reproduce, take in food and process it into new parasites internally. For a virus, everything that could be considered a life process is outsourced to the host. A virus, at least by a classical textbook understanding (although I distrust those because they're always oversimplified), doesn't have any life processes of its own. It doesn't aquire food or process it to grow/heal/regenerate, or to make more viruses. It doesn't
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