Life?

[SkepticLance]

The other problem with adding complex metabolic processes as a criterion for any definition is that it automatically excludes the first life on Earth. I suspect that the first 'living' thing was no more than a self replicating molecule surviving within the primeval soup, and using the molecules around it to build more of itself. Any concept of metabolism will immediately exclude this as 'life'. Yet this first self replicating molecule would be the ancestor of all modern living things.

 

I stand by my three criteria.

1. Based on organic chemistry.

2. Reproduces.

3. Evolves.

 

These criteria include the first life on Earth, even the very simple, includes viruses, and includes all other life on Earth, and excludes fires and computer programs, and anything else that is not living. If you cannot stand the idea of simple life and viruses being included, you modify criterion 1 to 'based on a complex system of organic chemistry.'

[iNow]

Problem with using this definition is that it uses the word life, so perhaps it's not as good candidate for defining life.

 

There are countless definitions of metabolism that don't use the word life, but they do all still apply to biological processes. I didn't feel like wasting a bunch of time finding the perfect reference to rebut a stupid point by Lance, so just offered you the wiki.

[CharonY]

Heh, it has been mentioned countless times now, that formal definitions of life do not exist. But regarding viruses, if one thinks of them as living organisms, so can be our individual genes.

See, they also reproduce and proliferate, albeit with the help of other genes and their respective gene products, not unlike viruses. So is the complete unit "life" or already sub-parts? Again, this is a question that is not possible to answer easily, as there is not a simple property of life vs. non-life. Rather this distinction was historically done instinctively (bird alive, rock not alive) and then a definition was made from the bottom up.

 

Mind you, there are a number of consensus definitions, but they are basically based on a priori distinction.

[alan2here]

I now understand why a virus might not be considered life and am unsure myself.

 

 

Lets take another look at that list.

 

 

1. Metabolism --> *expand* --> The set of chemical reactions that occur in living organisms in order to maintain life. --> *remove circular definition* --> A set of chemical reactions that occur. --> *shorten* --> Chemical reactions.

 

So we rule out a computer run simulation of a large group of ameba like organisms that follows physical rules simerlar to those of our reality in an enviroment simerlar to one they could be in on earth? Even if they go to that state though evolution within the simulation? (we are close to achieving this sort of simulation)

 

 

2. Response to stimuli

 

Why? A plant that dosn't could plauably survive well.

 

 

3. Growth

 

My first reaction was "lol, who wrote this list? Socrates? Plato?" however this should probbably be included in a definition that discards a virus as being a lifeform.

 

 

4. Reproduction

 

Possibly we shouldn't rule out a lifeform designed by something that is alive and also didn't need to evolve however in a formal definition this should probably be included.

Edited January 5, 2009 by alan2here

[iNow]

I'll just pivot back to my original point that the life/non-life distinction doesn't really map so cleanly onto reality, that our desire to force this absract human construct to do so is part of what leads to confusion like this.

[Baby Astronaut]

Just to add something to the computer debate.

 

I think a computer virus does not fit any of the mentioned definitions.

 

Remember, we're talking about occurrences in the universe. A computer virus is a virtual occurrence, the only thing replicating is virtual machine code, not an actual substance, and it can't borrow energy nor metabolism. The thing borrowing energy is the computer hardware, not the software, which exists only if viewed by a human as interpreted light from the screen, whereas the computer hardware actually interacts with the universe.

[CharonY]

2. Response to stimuli

 

Why? A plant that dosn't could plauably survive well.

 

Plants and in fact all cells react to a host of stimuli. Both external and internal. Only because they do not run around does not mean that they do not sense and react to stimuli.

[SkepticLance]

Another reason we should include viruses as 'life' is that they probably evolved from more complex forms that met all the criteria iNow posted - metabolism etc. In fact, all life on Earth descended from early forms 3 to 4 billion years ago, and so we should expect all life to have the sophistication that comes from billions of years of evolution.

 

Viruses appear to be more complex than early researchers thought. For example : a recent discovery is a 'motor' used by one virus at least to help it penetrate a host cell.

http://www.sciencedaily.com/releases/2008/12/081229200748.htm

 

A virus such as HIV contains a number of different proteins with different functions. They are definitely not as simple and 'primitive' as some people think. And as I said, the dependence on a host is not sufficient reason to exclude them as 'life' since there are many more complex life forms that are just as dependent on a host, including using host enzymes in their own metabolism.

[Mr Skeptic]

I now understand why a virus might not be considered life and am unsure myself.

 

 

Lets take another look at that list.

 

 

1. Metabolism --> *expand* --> The set of chemical reactions that occur in living organisms in order to maintain life. --> *remove circular definition* --> A set of chemical reactions that occur. --> *shorten* --> Chemical reactions.

 

Metabolism is more than just chemical reactions.

 

 

2. Response to stimuli

 

Why? A plant that dosn't could plauably survive well.

 

No, it would die a horrible death. For example, plants measure the length of nights to figure when it is time to flower or prepare for winter, some plants respond to insect saliva by releasing chemicals that summon that insect's predators, respond to shade by making thinner but wider leaves, etc.

 

3. Growth

 

My first reaction was "lol, who wrote this list? Socrates? Plato?" however this should probbably be included in a definition that discards a virus as being a lifeform.

 

4. Reproduction

 

Possibly we shouldn't rule out a lifeform designed by something that is alive and also didn't need to evolve however in a formal definition this should probably be included.

 

I'd say that growth and reproduction are not strictly necessary, but of course without them it would mean that the individual/species is dying.

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Merged post follows:

Consecutive posts merged

Just to add something to the computer debate.

 

I think a computer virus does not fit any of the mentioned definitions.

 

Remember, we're talking about occurrences in the universe. A computer virus is a virtual occurrence, the only thing replicating is virtual machine code, not an actual substance, and it can't borrow energy nor metabolism. The thing borrowing energy is the computer hardware, not the software, which exists only if viewed by a human as interpreted light from the screen, whereas the computer hardware actually interacts with the universe.

 

Just remember that data is a physical thing as well. While most people think of data in an abstract form, it has a physical existence as well -- be it as DNA, arrangement of magnetic domains, or volatile arrangements of electrons. Like a biological virus, a computer virus is entirely dependent on its host for energy and replication. If we get DNA computers or computers composed of neurons, a computer virus could even be chemically based. Most people agree that viruses are not alive though.

Edited January 5, 2009 by Mr Skeptic
Consecutive post/s merged.