I think that a good definitional quantifier for life would be the ability to gather energy and to put some of that energy towards reproduction.

I think a common trait among life forms is the need to constantly refuel themselves, and being compelled to do so on their own (thus eliminating things like cars). Another possible trait is reproduction, but there are inorganic things that reproduce (computer viruses) and organic things that don’t (anything sterile). Another thought I had is that a living thing maintains a constant temperature disequilibrium with its environment due to its internals systems, but I don’t know enough about cold-blooded creatures and single-celled organisms to say for sure that’s a common trait.

I think that a good metric for life is the ability to independently reproduce and grow. This would limit out viruses, which are incapable of making copies of their own genetic material and rely on other hosts, but it would include all eukaryotes and prokaryotes. I also agree with the above statements about being able to gather energy in order to fuel its processes.

I think life could be defined as anything which converts matter through chemical processes and which could “die” – never be able to perform those processes again – if, for a sufficient amount of time, it was unable to complete its conversion of matter. ie. If I am restricted from oxygen or water or food long enough, I would die due to not being able to produce energy and sustain myself. I think this holds true for all life forms. In other words, life could be defined as anything which, if not able to function for a given, temporary amount of time, would not be able to function ever.

Update: After reading other posts, I realized that I just defined fire as an animal. This is making me ask if there is a comprehensive definition of life.. Objectively speaking, we can define life on earth as anything with cells, but that’s really just saying that all life followed the same evolutionary chain. I’m not sure there is any real meaning to the word “life,” and that it can be defined. Only that we experienced a unique process of evolution and other planets haven’t experienced that same process.

If we look at life from a physics point of view, we can see that all life shares two things in common: First, life resists entropy or decay, and, does so without aid unless nearing death, and, secondly, life can process or respond to information about its surroundings, such as how we notice when it is day or night (circadian rhythm) or how types of bacteria/ microbes/animals can react to droughts by entering into suspended animation (cryptobiosis). Those are, in my opinion, what defines life. What I mean by process or respond to information is that the life form must be able to do this for multiple types of biotic and abiotic factors. These include, changes in water, atmosphere, temperature, lighting, nearby predators, harmful substances, etc. I cannot make a comprehensive list though. Therefore, viruses are not included as these do not respond to most abiotic factors and many viruses do not react to most biotic factors (bacteriophages only infect specific species of bacteria). This is achieved via the various life processes that either repair cells (such as the proteins Rad50, Mre11 and NBS1) or the processes that produce more cells to replace dead cells (such as mitosis and meiosis). This means that a living system remains active and is able to move, convert energy, reproduce, etc. In single cell lifeforms, those life processes are carried out by the organelles within the cell. We can see that after an organism dies, those aforementioned life processes stop, and the body which may have lasted and functioned years or decades decays in a matter of months to equilibrium. Thus, we can see a clear difference between ‘living’ and ‘dead’ things. This definition does exempt inanimate objects or fluids or fire as these are unable to process information and also exempts robots or computers as these require the aid of humans to be maintained and refueled to resist entropy.

I would define life as being any mass of macromolecules which is capable of reproduction and responding through chemical chain reactions to external stimuli. It must also be able to condense outside forms of energy and funnel some towards its own processes and the rest into reproduction, in other words, taking outside substances and exerting portions of its intaken energy towards making an identical base out of those substances (I say “identical base” to allow for genetic recombination). The organism must also be able to stop performing these processes (die), although it will continue to collect energy to fight internal entropy, in other words, it would strive not to die.
These definitions would exclude chemical processes such as fire (which is not a collection of macromolecules but rather an action on such although it can spread (“reproduce”) and takes in energy in the form of its fuel, as well as change form depending on what the adjacent substance is which it spreads to) as well as substances such as non-Newtonian fluids, which respond to external stimuli in the form of different forces, but do not reproduce or take in energy. In response to an expected response asking about robots, I will say that at the present moment, this would not include them, but if they became self-sustaining, I would think they would then resemble humans sufficiently enough to be deemed a life-form.

Doesn’t life also have to undergo Darwinian evolution, I think they created cells in the 1960s that could reproduce, consume energy, etc all the things we have discussed that constitutes life, but they still dido not give it the title of being alive as the artificial cells did not evolve over time. Thus I think many people (myself included) feel Darwinian evolution is a major factor in deciding whether something is alive.

I believe that one of the keys to being considered a living organism is the ability to reproduce individual units. This discussion of what is life ultimately becomes and attempt to define a line between alive and not alive, but this line does not really exist. There are plenty of things that are undoubtedly alive, and plenty that aren’t, but there is a grey area. Alive and dead cannot truly be delineated into two different groups. This grey area is occupied largely by viruses. Viruses are able to enter cells and insert their viral DNA into the genome in that cell. The cell will unwittingly produce more viruses from that viral DNA. In this way, viruses are able to reproduce individual units, but they need assistance from an outside source – a cell. This makes it very difficult to determine whether viruses are living or not.

Another definition of a living organism that I would propose is that, to be living, an organism having genetic material – on earth, this would refer to DNA, but in extraterrestrial lifeforms there could be a different molecule used in coding genetic material. This definition would include viruses as living organisms. One interesting point to bring up is that DNA itself is not living, but it is able to evolve. The genetic code of human immune cells, in particular, is especially apt to change, as these cells have to always have an answer prepared to ever evolving viruses. Therefore, the genetic code in these cells cannot be set in stone, but must be easily malleable in order to be able to change on cue to fight new forms of viruses.

Returning to the earlier question of whether viruses are living, I would argue that they are not, for the following reason. When a virus inserts its DNA into the genome of a cell, for a time, that is all that is left of the virus – just genetic code (a surprising proportion of the human genome is defunct DNA from ancient viruses that inserted themselves into human DNA many millennia ago, and got “stuck” in the human genome). Some viruses, known as latent viruses, are able to exist unexpressed for years within the genome of a cell, only to one day suddenly strike and take over the cell. For this latent period, during which the virus is nothing more than DNA, it is clearly not alive, but then it is able to “come back to life,” which is impossible, so it must have never been alive in the first place.

To summarize, I find the two most important qualities in defining life to be the ability to reproduce individual units and having genetic material.

In reading previous posts, it is interesting that multiple definitions defined fire as living, as the ancient Greeks actually believed fire to be alive, since it has so many qualities commonly associated with life.

Sorry that I am so late, I actually typed a response and forgot to submit it a few days ago

Life has several traits that have been mentioned above, such as the ability to independently reproduce, consume and acquire energy, and evolve. It is not necessarily required to be cellular in nature, even on Earth, we have many organisms (even some that are quite large) that are unicellular. However, another trait that is important is that life seeks out a better place to be, such as in animal migration and plant growth (away from gravity and towards light).
The reason a virus cannot be considered to be “alive” is because it violates the idea of independent reproduction. It cannot create a new one of itself without taking over an external host. In other aspects, it could be considered alive, but it violates these rules and therefore is not truly alive.
Life in space would probably be found varying based on the planet it is found on, because this would change the chemistry that ultimately forms organisms. While we know certain structures will be likely to form (such as RNA) they may form differently than those on Earth.

I personally see classifying what is “life” as “arbitrary” similar to how classifying whether or not a literary work is “science fiction”. Just like there really is not one set of rules that decides if a work is science fiction or not, I don’t believe there is one steadfast method of classifying what life is other than a loose set of rules and a plethora of exceptions. To the best of my knowledge, I would say life is any individual object which tends to have a “desire” to reproduce itself. In addition, things considered “alive” should tend to respond to stimuli, and should at one point in the future be able to survive on its own without the constant aid of its parents. These rules probably don’t explicitly define what “life” is, as such necessary exceptions are needed to fill in any gaps in the definitions I have.

I personally agree with Ioannis, as defining life is tricky and there is a gray area(with mostly viruses within it). As Ioannis stated, Viruses require a outside host to reproduce. I do not think that necessarily means they are non-living, though, as an “outside source” can be defined in many ways. If an outside source can be nonliving, then all life on Earth is not actually living, as it requires food to survive. If the outside source has to be a living organism for the original object to be considered non-living, then what is defined as a living organism? This results in us circling back to the original question.

I think that a good trait shared by life is that life can respond to stimuli. I don’t mean how a virus will respond to coming into contact with a cell by chemically binding with it. I think that life is defined by how persay a person may have multiple different reactions to the same event or object. Life isn’t just a “biological machine”, it’s something that can truly respond to it’s environment in different ways that may even contradict biological impulses.

I actually think that considering life on a spectrum instead of having a definite line between what is alive and not alive, as Tessa mentioned earlier, is a good way to approach this question. I believe that the ability to reproduce, acquire and consume energy, maintain a stable body temperature, respond to outside stimuli, and evolve are all important qualities of living things, as mentioned by the posts above. However, in the process of trying to arrange life on a spectrum, which qualities or traits that can be used to define life would be considered the more important than the others? For example, would an organism that performs homeostasis but does not reproduce be considered more alive than one that can reproduce but cannot perform homeostasis? Removing just one of these traits would not necessarily make an object not alive, as sterile organisms are still considered alive, but how many of these traits can an object lack and still be considered a living organism?

In addition, there are some things that we all consider to be living, such as humans and plants, and those we consider to be nonliving, such as rocks and water. If we put all objects on a spectrum of how “alive” they are, would this spectrum be only for objects in the gray area of the definition of life (such as viruses) or could this apply to the objects that are clearly defined as nonliving or living as well? Some may argue that humans are more alive than other animals because humans have more advanced cognitive functions or that animals are more alive than plants because of their ability to move more freely. However, if we do not consider all objects on the spectrum and only the ones in the gray area, it would lead us back to the original question, as we would only be trying to define the difference between objects clearly defined as living and the gray area instead of the difference between living and nonliving objects.

I agree with a lot of what you all are saying. I agree that an organism needs to be able to gain and use energy, evolve, respond to any outside stimuli, reproduce, and so on. But I also think that a big part of a living organism is their consciousness. A being that has the ability to think and make complex decisions when reacting to outside stimuli. I agree with the idea of life being a spectrum when it is applied to consciousness. A being with higher consciousness and intellect, like humans, can be considered more alive than other species. Consciousness definitely has to be considered when looking at extraterrestrial life.

I would agree that when classifying life, one should use a spectrum. I think that some of the top parameters for such a spectrum should be to be comprised of cells (either multicellular or unicellular), the ability to respond to stimuli, the ability to reproduce, and the ability to maintain some sort of homeostasis. This would put common animals at the top of the list. Following this, I would say the ability to seek out means of staying alive would be below that. Since viruses can only replicate once inside a host, this would push them to the bottom of the list as they cannnot do any of the functions previously mentioned. This spectrum does pose a question however of how many of the functions one must have to be considered alive. I would consider being comprised of working cells to be one that something must have to be alive if nothing else. This spectrum would however still leaves a large grey area.

If we classify life using a spectrum, we have to create a method of differentiating where each organism is placed on the spectrum — a way to categorize organisms to different parts of the spectrum (this is mandatory for any spectrum or classification system). Thus I first thought that we should create the spectrum from most aid required to least aid required. What I mean by this is that for example, viruses require a host cell to reproduce and activate most of their other life processes. This means that they require aid for most of their life processes. This would put them at the bottom of the list. Other parasites such as the hair worm would be ranked higher than viruses as these parasites only use other organisms (crickets to transport them to water) as a mode of reproduction, these organisms can still move and interact with the environment without the aid of a cricket. This would go on further till animals, which are at the top of the spectrum. These require no aid at all for their life processes. Items such as rocks or fire are at the bottom of the spectrum as these do not participate in most of the life processes or require extensive help for most of them. Eg a fire requires fuel to reproduce. One key thing is reproduction, a major requirement for life in most definitions, many could argue that all non-asexual reproduction requires aid, but I will define self reproduction as reproduction that can be achieved only requiring a member of the same species, such as sexual reproduction (not of a different species such as in the case of viruses or parasites) or only one member of that species required, as asexual reproduction. One problem with this idea, is that most organisms are clustered at the very top of the spectrum and in many cases, it could mean that bacteria are “more alive” than hair worms or other parasites. This is because most bacteria can achieve all the definitions of life stated above without the requirement of another species. This is obviously wrong. I am thus keen to hear other methods of categorization for the spectrum. One other method I think is useful is categorizing organisms based on the complexity of their genetic material. This solves the bacteria-parasite dilemma as parasites are now placed above bacteria (having more chromosomes and thus more genetic material). This would however mean that prehistoric organisms are less alive than modern organisms based on what I stated. This ultimately holds a host of problems when trying to create a spectrum for life and then trying to place organisms on them. As mentioned above, there are massive grey areas on any spectrum with most organisms clustered at the very top, and most of everything else clustered at the very bottom. I think that going back to the original idea that something is either alive or not should be considered. This method of classifying gives no grey areas, viruses are not alive, humans, plants, bacteria, etc are alive. We can change the definition all we want, (as we have done above), but it will still give us the clearest answer. Moreover, a spectrum require a sample size. By sample size, I mean a set of groups of organisms with similar characteristics. The group in our case being Earth’s ecosystem. All life on Earth share key similarities. All life on Earth would be in one group. Well then we have a sample size of one. The discovery of alien life (if it exists), would definitively help by giving a new set of organisms that may have different common similarities to those on Earth. This new set would increase our sample size and therefore make a spectrum a much more reliable option. Until then, it may be a better idea to use a simpler classification system (such as either dead or alive) so that when new data is added, we can expand that simpler classification system to a more complex and thorough system (like a spectrum).

When classifying weather something is living or not I think that one should consider if that organism has a genetic code, if they reproduce, if their subject to evolution, and if they respond to environmental stimuli. I think the part about evolution is especially important because all living things we know of have responded to their environment in some way. Personally though I think classifying life is a bit hard since at the basis life is just the summation of a bunch of chemical reactions that occur due to the movement of “dead” things (organelles). Really life comes from a bunch of tiny dead things reacting with other tiny dead things, so where do we draw the line between dead chemistry and life.
Also an unrelated question I have is can there exist life that isn’t carbon based?

Carbon can form four covalent bonds with other atoms meaning that it can form many strong, resistant bonds that are the basis for polymers such as DNA or RNA. Thus, all life that uses carbon like this is carbon based. Other elements could easily be a substitute, such as silicon which also has four covalent bonds. Thus silicon based life could be a possibility. Going back to the definitions of life, I think a trait amongst life forms is that there are periods of time when one or more of there life processes cease or slow down only to return after a period of time. These traits include: Cryptobiosis, hibernation, sleep (in which case I would not be considered alive as its 4 am here), or even a slowing of the metabolism or other cellular processes. Could this also be taken into consideration when describing our spectrum or general definition of life?

Yes, it would also mean that silicon based life forms would be undetectable to most of our rovers, (which use mechanisms based on carbon based life), silicon based life would only appear as rocks. The only way to observe a silicon based life form is directly through a camera/microscope.

I believe that what I consider a life form must meet three criteria.

After dying or being broken, it cannot ever return back to its original state.
There is no life form that after dying can become alive again.
When being born or created, it is not entirely identical to its parents or what created it.
This is not meant in the context of being genetically identical, for even genetically identical cells vary in composition. What is meant that fire is created from a spark, which is basically a miniature fire, so its parent is identical to itself in composition.
It consists of more than just one chemical reaction or process and has more than one state in its life not including birth and death.
All life forms have a purpose or some will to extend their time of existence, but one reaction or one state is not enough to be able to achieve this.

Examples judged by the criteria:

The Sun- This will not be alive, because by criterion 3, it cannot just be made of once chemical process, that is nuclear fusion.

A Human- Satisfies all three criteria

A being that is immortal- Only immortal beings that don’t age are alive, since they can still die and not be able to come back..

A flame- Is just one chemical process, combustion, so it is not alive.

A virus- Alive, as it satisfies all three criteria

A bacteria- Alive, it satisfies all three criteria

A rock- Only one state in its life, so not alive.

Well, it sure looks like there are a lot of views on the definition of life (exemplified by this message board). =)

I, personally, think of life as anything that is able to mantain order, and respond to its environment intelligently in ways that further mantain that order and complexity. By order, I mean that it is has internal complexity that is isolated. This simplifies the definition a little, but it does take into account future cases, as discussed, where AI and technology come into the picture, along with extraterrestrial life. Beings from other planets might not resemble anything we’ve ever seen, so I think it’s really cool we’re having this conversation to kind of ‘redefine’ the standard definition of life found in most textbooks.

They’re always going to be examples of life that are on the fringes of our definition, that’s just the reality of trying to define them in the first place. By defining it simply by a thing that is capable of mantaining internal structure and responding intelligently to its environment, not just the physical laws of nature, it broadens the definition in a way that I think might apply to life we haven’t even begun to imagine yet.

The smallest known unit of life is a cell. However, scientists have yet to agree on what makes something alive (though they generally agree some common traits such as the maintenance of homeostasis and reaction to the environment present in all living things): why is an algae cell alive and not a virus that can self replicate? Likewise, crystals grow, and their growth is affected by their surroundings, but they are not considered living.
More importantly, what is it that makes us living? Humans are among the many multi-cellular creatures on this planet: what is it that makes us living in our own right as opposed to the net-sum of our cell components? If there exists a single life in us, when we ourselves are composed of a multitude of tiny life forms, could we not make the extension that a country (or any large cohesive body of people) is likewise alive, as the sum of it’s population?
I would also like to highlight that our definition of “life” is greatly constricted by our very limited exposure to the world. The definition of “life” must follow the study of the natural world and change as our understanding of it does. To pronounce constrictions on the definition life seems dangerous to me. Simply because we do not know of such a life form or have not thought of it as living does not mean it is not. There must be an understanding that whatever definition we settle on is greatly malleable and must be allowed to evolve over time.

Personally, I believe that in order to establish a method of quantifying life, we should look toward the patterns or characteristics of life itself:

1. composed of cells
2. uses energy to grow, move and process information (metabolism)
3. organized into different units (cells, tissues, organs, systems, etc.)
4. able to respond to environmental stimuli
5. maintains stable internal conditions (homeostasis)
6. able to reproduce or pass down genetic information
7. contains DNA to encode genes

Of these properties, certainly the cell composition, energy consumption and DNA encoding are able to be quantified. We can count the number of cells in a living organism and the number of base pairs in its genome, or we could observe other measurable characteristics such as basal metabolic rate (Joules/hr/kg body mass). However, doing so also raises the question of whether life on other planets would resemble how it operates on Earth. Are cells truly the basic building block of life; must all life be genetically encoded by DNA or RNA that passes from generation to generation? Though certain properties should remain present such as the ability to manipulate the environment or have a stable, supporting biological structure, we still need more data from extraterrestrial life and matter to formulate a robust scientific theory.

I agree with many of the sentiments proposed. But I don’t necessarily believe that life has to have criteria that we typically learn in biology, such as the fact that life has to consist of cells. This is because much of the generalizations that we draw there are based on what we observe on Earth. For instance, if we were to observe life elsewhere, it may not meet those criteria. Therefore, although this is not quantifiable by any means, the need to survive and live seems to be a good way to categorize life.

However, now the issue becomes, how does one quantify an entity’s “need to survive”? I’d appreciate hearing others thoughts on this.

I believe that something cannot be simply qualified as alive or not alive. There are some that can be clearly defined as one or the other, but there are many things that lie somewhere in between these two ends on the spectrum. The biggest factor that I believe is the most significant to classify something as “living” is its ability to reproduce on its own (without outside influence) with individual units. Of course, the biggest problem that arises out of this definition is where we draw the line between reproduction on its own and with outside help. How can we define this outside help if we cannot even determine if this outside help is alive or not?

What defines “alive”? This is of course up for debate when considering the entire universe. Alive is defined as “: having life : not dead or inanimate” but then Merriam Websters first definition of life is: the quality that distinguishes a vital and functional being from a dead body. This is arguable as well, there are many beings that are “alive” but non functional, and the scale of the question comes into play. How does the “life” of a human compare to that of a bird, or a bird to a microbe? The level of consciousness varies greatly as well, would we consider finding bacteria on another planet as exciting as finding equally conscious and similar beings to ourselves? In this sense we have some prejudice, many would not be, but this life would of course still be considered quantifiable life, but on a anniversary scale, there could be massive beings with no consciousness, or beings the size of microbes with more intelligence than us. Do they have DNA, cells, reproduction, etc.? How would we go about quantifying such a vast possible difference? I suppose we will just have to wait until we find something.

Hey, I think I might have entered a little bit too late on this discussion. It seems to me that everybody has contributed to a very extensive and complete definition of life. However, I´d like to try making some observations, at the risk of someone else having said the very same.
A little detail I don´t think anyone mentioned, is metabolism. Yes, sure most said that living creatures have an internal mechanism to transform external energy, into some other type that can be exploded. Based on the simple definition of energy transformations, we could consider fire a living thing. But the difference may reside not on the fact that fire doesn’t obey to our common-sense idea of life, and instead in the fact that fire isn’t properly “something”. You see, fire is only the visual representation of burnt gases around a fuel, mainly organic material such as gasoline or wood; we are able to see fire because our eyes have adapted to a certain range of electromagnetic waves (called visible light) and the air surrounding organic heated material brightens up. Also, and quite more important, fire executes only one chemical reaction to sustain itself, and metabolism is way more complex than an exothermic reaction. Transforming potential heat energy into mechanical and chemical energy requires more processes. Metabolism includes this idea that a living form performs a continuous exchange of energy with its environment, in a cycle where it absorbs energy in order to seek for more sources of energy (a cycle that is also meant to stabilize the temperature between the creature and the surroundings, homeostasis). On that matter, I read that someone was arguing about temperature being the same as the average of some places. I disagree with this, as a pluricellular organism can´t live for extended periods of time in a place where the internal and external temperature are the same. Your body is designed to be warmer than the surroundings, it is meant to be exothermic. And the laws of thermodynamics don´t allow the heat transference between two objects at the same temperature. And if your body can’t get rid of the extra heat, homeostasis will be lost, and the primary functions won’t continue for long.
Reproducing is also a way of understanding what life is. All living organisms have a certain way of “copying” themselves, due to a genetic detail that tells them “to preserve themselves, whether through fighting against external dangerous stimuli, or transmitting their forms to a similar or even identical new version of themselves”. It is curious, at the least, the fact that a mixture of carbon-based sequences can control many many things about any being. As many of you must know, the DNA is a large large chain of sugars attached to a nitrogen-base and a phosphate group, which only purpose is to transcribe the combinations in such a way, that a “code detector” in another part of the cell (RNA) can sort of translate it into amino-acids, that will attach to one another to create proteins, that will, eventually, attach to one another to build tissue, and so on, an so on.
In the end, it is quite a question. But dreams have no cost, and imagination is a white board waiting for ideas to be in it.

Hi,
I believe I am entering the conversation a bit late. And I do agree with a lot of the in depth analysis made regarding “life.” These include the existence of stimuli for example. Being able to respond to changes one’s surroundings is one of the fundamental characteristics. It initiates chemical reactions/processes in a “living organism” that can initiate change from its current stage.
Different “living organisms” have different capacity to respond to such changes. Over the period of time this results in an organism’s adaptability to change and survive them. That is where I believe one of the fundamental ideas regarding life is generated i.e. the survival of the fittest. This gradually developed into Darwin’s Theory of Natural Selection, where living organisms with the best suited “traits” are the ones that survive. From my perspective it is of the fundamental properties of life which has lead to evolution and has ensured the continuity of the exitance of life. This can at least hold true from the one firm example we have, which is on Earth.

Hey guys,
Also a bit late here but I wanted to sort of add a new perspective on the question. I think we should start off with consciousness. Consciousness is a tricky topic because we have no definitive proof of its existence, therefore, I think it shouldn’t be considered due to how contested it is. Responding to stimuli, metabolism, maintaining homeostasis, reproduction etc. are all very relevant requirements that we all learn in biology but I think it is too restrictive. For instance, viruses are capable of performing reproduction but they cannot respond to stimuli or undergo any sort of metabolism(as far as I know). They check some of the boxes on the seven characteristics of life but not all. Does this make them alive, or non-living or a mix? I think this uncertainty shows how our current understanding is too “limited” in a way or too tied down on the seven characteristics of life we learned in school. We’ve drawn a line in the sand trying to differentiate between living and nonliving, but the only thing we’ve done is boxed ourselves in. I think we should try to look at all types of life and just pick out the most necessary traits and broadly define it as how NASA did. NASA’s alternate definition of life is “A self-sustaining chemical system capable of Darwinian evolution”, which is a start to truly defining “what is life” especially if we find organisms that completely go again those seven characteristics of life.

There are many characteristics of life that are described in biology. These help us to categorize living and nonliving as we understand them. Some of these characteristics include having a metabolism, being made of cells, being able to maintain homeostasis, responding to stimuli, being able to reproduce, being able to develop, and being able to evolve. These are characteristics of life here on earth that classify an understanding of what life is. Because of these being entirely based on our observations of livings things here on earth, they perhaps can be misleading about what life outside of this world is like. Some of these characteristics may hold outside of this world, while others do not. They are important on Earth, because there is not really a single characteristic that entirely separates everything into living and nonliving. For example, one characteristic is that living things are made of cells, but if this was the only characteristic, then leather and materials made out of animals would be classified as living things. Some characteristics may also be seen as more important than others, such as the ability to evolve.

I think life should be looked at by the basis of biology, like Matthew stated. Life, from a biological standpoint, is defined by something that can grow and can reproduce. I recently went to a few planetary lectures about life outside of Earth at the annual AAAS meeting. Most of these scientists who are looking at dust particles from other plants look for specific organic compounds, like ozone. However, it is important to note that what may be necessary for life on Earth may be completely different for different planets or galaxies.This can make it even more difficult to define what life is. I also believe to gain a better understanding of “What is Life?”, we need to look back at the conditions of the creation of life itself on Earth.