Here's one definition of the properties of life:

Conventional definition: The consensus is that life is a characteristic of organisms that exhibit all or most of the following phenomena:

Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
Organization: Being composed of one or more cells, which are the basic units of life.
Metabolism: Consumption of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
Growth: Maintenance of a higher rate of synthesis than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.
Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and chemotaxis.
Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.

On another forum, I made a case that Martian rocks were living, based on all those criteria. Reproduction was when a piece of rock fell off and was eroded to form part of another consolidated deposit.  Of course I wasn't serious  

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What about growth?

You make a good point about how 'wishy-washy' the definition is. It doesn't take us far enough away from 'you know it when you see it'. I also think that definition is far too specific on some issues. I would toss out any reference to cells, as it is concievable that alien life forms will not be cell based. They would still be organised, though organisation itself takes us back to 'you know it when you see it'. I suspect they put that in to reject viruses. I would also toss out homeostasis. A single celled organism could be considered alive even if it could only live in a stable environment identical to it's internal conditions. Again, this comes down to 'you'd know it if you saw it'. The smaller the organism, the more trivial this criteria. Response to stimili is unnecessary. If something satisfied the other conditions, including adaptation, it would also satisfy this one.

The only time I have tried to used a definition was in an argument about the Great Barrier Reef. I am sick of journalists saying it is the largest living organism on earth. That is like saying that a herd of cows is a living organism. I would include something about discretisation or irreducability. That is, if something is alive, you can break it down into single organisms, but breaking it down further kills it.

I would also include death, on an individual basis.

Muso, thanks for the link, I will read it later tonight.

The definition of a life form you gave sounds reasonable except may be for the "exhibit all or most", that is a bit to vague for my liking.

freediver wrote on Jan 14^th, 2009 at 3:08pm:


In fact many people argue about that, sometimes to the death. That's why scientist should be mindful about semantics ambiguity.

They are mindful of it. It's just that rather than getting hung up on a definition of life, the explain in detail what they actually did, and leave it up to others to decide whether it is life.

Here's an idea.

Why don't we go to Mars, and build some structures, wonders if you will, like stuff from the old days we have now we can't understand, and then go dump that RNA stuff all over Mars.

Eventually we might get intelligent life on there and they will spend all their time trying to work out the mysteries of the wonders and make secret societies and stuff.

And by the time they evolve in to intelligent life, we will totally be able to go visit other planets and stuff, this is billions of years away, so then we can be the aliens and go visit them and abduct them and stuff, just for fun, not to hurt them.

Because it is my idea I demand the rights to design the wonders to be placed on Mars.

easel wrote on Jan 14^th, 2009 at 4:10pm:


Mars is too dry and oxidising. In the recent Phoenix expedition, they even found magnesium perchlorate - not a very friendly environment.

Apart from being dry, it's very salty too, which high levels (up to 20% by mass) of calium and magnesium chlorides and bromides.

Back in early 1950s Harold Urey and Stanley Miller used "spark and soup" to produce amino acids.
They passed mixtures of boiling water, ammonia, methane and hydrogen through "electric spark systems".

The media of the time described these as proof for the possibility of spontaneous generation of life on Earth.

http://en.wikipedia.org/wiki/Miller-Urey_experiment