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."
self-reproduction with variations (Score:5, Interesting)
FTA: Simply, Life is "self-reproduction with variations" - like mutating computer viruses?
Re:Dumb article (Score:2, Interesting)
Life is defined as something that feeds and reproduces.
So... fire is alive by your definition and a bumble bee drone isn't.
Thermodynamic definition of life (Score:5, Interesting)
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're testing more or less.
Nobody is going to get funding to put expensive probing equipment on an expensive robot to prove a theory that life exists in a form that it doesn't exist in on Earth, and in a form that nobody seems able to create for testing purposes.
Re:If it evolves by replicating, it's life. (Score:5, Interesting)
If it isn't very much like us.. (Score:4, Interesting)
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.
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.
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.
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.
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.
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: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)