Lannergard

I guess you have a point if we pick single organisms and look for reproduction during a part of their life cycle. (I don't know if movement is much better though, there are lots of organisms who do not move all the time either.) In normal use of the criteria (looking at groups - "viruses", "bacteria", "stones" etc) reproduction is still useful. We know that yeast reproduce, wether or not one particular cell buds off if I look at it in a microscope for five minutes.

What I had in mind was that a baby could be said to fulfill the reproduction requirement at the cellular level. But OK, that might a stretch...?

Francois Tremblay

But then that is a confusion of actuality with potentiality. That it might be alive later (according to the strict definition) does not make it alive now. Furthermore, in my baby example, the baby does not even have the capacity to reproduce at all. There is simply no way you can contrive that a baby can have some kind of reproductive actuality.

I don't want to give the impression that I'm blasting this conversation : it's probably a nice definition for biologists who use it in certain useful contexts, but it's not much good for a layman like me.

[ January 31, 2002: Message edited by: Franc28 ]</p>

liquid

I don't know for sure. Anecdotally, I would say yes, and I am sure there are plenty of surveys asking this question. However, the point is not to show that a virus is alive, but that there is considerable uncertainty and disagreement of what life is.

Again, this shows my point that we do not have an acceptable definition for what life is. Ordinarily, we can say that a rock is dead and a baby alive. People expect a clear barrier. But actually look at the cases of viruses, prions, crystals etc, and there appears to be no real distinction at all - at least, no non-arbitrary one. Perhaps we have to accept that life is nothing special, not particuarly objective and something of a continuum.

LOL! Yes, I did write that the wrong way round! How? Well, simply put, it shows that there is no specific barrier or unique property of life - life and non-life are the same stuff, organised differently. This is nothing new; we've known this ever since the atomic theory. Atoms are dead, but we aren't. Therefore, life can't be a material property but must be an organizational one. Life does arise out of non-life every time our nuclei oscillate, in a sense.

[ February 08, 2002: Message edited by: liquid ]</p>

liquid

And now my second answer, to Bill.

Current viruses could not have existed without cells, of course, but there are viruses that could prey on other things (including other viruses, in fact).

No-one does see them as a stepping stone of abiogenesis. However, there could quite easily be similar organisms that were - look up the RNA world hypothesis for an idea of how proteins could exist in a state of semi-life.

We don't expect anything to be around actually. Only analogous organisms like viruses. This is because the passage of time has been so long that all these organisms were outcompeted and outevolved long ago and driven to extinction. And of course, being so small, they leave no fossil trace. The most 'primitive' free-living organisms we have found so far include the prokaryotes in hot springs and the archeobacteria (forget the correct term) that live around ocean vents. Frankly, these are pretty advanced organisms themselves.

But we do see things that might have been similar to early life. Viruses, RNA, mitochondria in our cells, even clays or crystals. So we can make a conjecture over what might have happened.

Shadow Wraith

The question of what the boundary between life and non-life is is a very good one. Personally, I think whatever transition there is, whether it be virus-like things or any other possible pre-procaryotic organisms, is so unnoticeable that there may not even be much of a difference between life and non-life other than our fabricated classical definitions seperating, say, a rock from a bacteria. Life may just be an extremely complex and unusual type of matter, nothing more. For example, even humans on the smallest scale are made up of substances not considered living when seperate. At what exact point is an agglomeration of various non-living substances defined as an organism? This debate may go on for decades to come, but it would be nice (though probably unlikely) to see a concrete seperation between life and non-life on the transitional ("compounds to cells") level.

BTW, liquid. I think the correct term for acheaobacteria you were looking for is eukaryotes.

Bill

Before I begin, I would first like to explain some terms to those who aren't familiar with their precise usages, thanks to <a href="http://www.amazon.com/exec/obidos/ASIN/0674896661/internetinfidelsA/" target="_blank">Mayr's classic: Toward a New Philosophy of Biology : Observations of an Evolutionist</a>:

• teleomatic is "progression toward an end state through physical processes (i.e., inevitable change in a predictable direction, such as the Hubble expansion of the universe or the second law of thermodynamics)" [from <a href="http://www.nexial.org/bmi/autevol/ghw_dis.htm" target="_blank">HERE</a>
• teleonomic is "processes, which are those that are guided by a program (defined by Mayr as coded information controlling an end-directed process)." [from <a href="http://www.nexial.org/bmi/autevol/ghw_dis.htm" target="_blank">HERE</a>
• teleological is thus relegated to the extremely confined arena of cosmic teleology, except when the context implies the broad field which encompasses all of the above.

With that said, I think that a simple definition of what "life" is can be stated this way: <ol type="1">[*]Demonstrates teleonomic processes for the apparent purpose of long-term survival of itself and its descendants; and[*]Does not require volitional input from an indisputably living entity in order to replicate and/or survive.[/list=a]The only purpose for including the second characteristic is to exclude adaptive computer software and other "human-designed" mechanisms from being defined as "alive."

In this sense, then, a "virus" is clearly "alive," because it clearly "demonstrates teleonomic processes for the apparent purpose of long-term survival of itself and its descendants," and it exists frequently against the volitional wishes of its host organisms, therefore passing the second prong of the test, for at least "natural" viruses.

But what about "human designed" viruses? Well, the second prong will allow those, so long as those "human designed" viruses continue to replicate and survive on their own.

=====

Anyway, I offer this up as a suggested starting point for continued discussion.....

== Bill

liquid

No, 'eukaryotes' is not the term I was looking for. They are actually more advanced that prokaryotes, and indeed are believed to have evolved from agglomerations of prokaryotes in many instances.

There are several major differences, such as eukaryotes having organelles, and prokaryotes not. Organelles are little structures in the cell analogous to our organs. An example is the mitochodria that provide respiration in our [eukaryotic] cells. These are believed to have been free-living prokaryotes at one time, till we swallowed them up and became indistinguishable!

Archaeobacteria are fairly recently discovered and totally different creatures. They were found in ocean smokestacks, and have a fundamentally different body chemistry based on deriving energy from chemosynthesis, not photosynthesis/respiration. It turns out there are billions of them all over the place, underground, inside rocks etc. They are a completely different kingdom to all other forms of life.