Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!caip!rutgers!sri-spam!nike!ucbcad!ucbvax!decvax!decwrl!amdcad!cae780!teklds!tektronix!orca!tekecs!mikes From: mikes@tekecs.UUCP Newsgroups: talk.origins,net.bio Subject: Re: What's this LIFE stuff? Message-ID: <7710@tekecs.UUCP> Date: Sat, 4-Oct-86 16:40:07 EDT Article-I.D.: tekecs.7710 Posted: Sat Oct 4 16:40:07 1986 Date-Received: Wed, 8-Oct-86 22:03:06 EDT References: <45500088@uiucdcs> <7670@tekecs.UUCP> <812@hdsvx1.UUCP> Organization: Tektronix, Wilsonville OR Lines: 122 Xref: watmath talk.origins:119 net.bio:661 > In an earlier posting, someone posed the question, "what is life?", with > particular emphasis on how to recognize alien lifeforms as being alive. > > In article <7670@tekecs.UUCP> mikes@tekecs.UUCP (Michael Sellers) writes: > >There are some general common to all those things we say are alive (though > >most if not all of them have a few notable exceptions): I realized when posting my original response that the criteria I suggested were, in some cases, almost too general to be of value; and that they are easily (at face value) confounded, as in the solar-refrigerator-on-wheels scenario. I originally had a more narrow set of criteria, but broadened them to include what might have to be considered in looking for/at extraterrestrial life. > > IT MUST METABOLIZE. All living things use material from their environment > >that, when broken down, is used for the maintenance of the thing itself (or > >for growth or reproduction, etc.). > > Yes, but this is awfully easy to fake. There are many kinds of man-made > systems that take material from their environment, process it, and then > use the material for maintenance. (You might be able to make this defintion > tight enough to exclude, say, an automobile, but I'm sure that someone could > design and build an electo-chemical-mechanical system to duplicate almost > any metabolic process you can come up with). I would lean toward saying that the metabolic process must happen on a micro scale, such as with individual or small sets of carbon chains being broken down for their bond energy in a cell rather than en mass in a car engine. This might not hold for ET life, though. Also, it has been recently suggested (no refs here, sorry) that our classical view of the cell as a very small test tube is essentially flawed; given that, for example, muscle cells have calcium thresholds at concentrations of 1x10-7 to 1x10-9 (-7 and -9 are superscripts), that viewing them as very small test tubes operating only by using the vagaries of diffusion and osmosis (and using active transport only when necessary) is infeasable. In this view, every single chemical reaction and gradient (upwards of 10,000 per minute per cell) is maintained by active guidance of proteins and controller molecules. There is much evidence for this, the muscle cell (above), paramecia contractile vacoule, Golgi complex activity, and ribosomal protein synthesis rates in response to various factors being the few pieces that I can remember. This would seem to be a hallmark of life; it is not so crude as a gasoline engine, and makes use of chemical properties in a directed fashion on a micro scale. What happens when we find life that uses atomic forces instead of chemical? I don't know; with luck I'll be dead by then :-). And, when someone manages to build a molecular machine that uses chemicals in its environment for its continual upkeep, will that be life? If not, then it is awfully, awfully close, and maybe only egotism would prevent us from saying that it was. > > IT MUST REPRODUCE. All living things are able to reproduce others of their > >kind that are or will become fully functional, independent entities. > > Nope. Mules do not reproduce. Furthermore, although a species may posess > the ability to reproduce, it is possible for individual members of that > species not to posess the ability, having lost it through congenital defect > or later damage (e.g. sterilization). True, mules do not reproduce, and yet you bring up the larger issue. They are descended from species that do. I think this requirement, amended, still stands: living things reproduce. Some living things do not, but they are the exception; if they became the rule, there would be no more life. > > IT MUST RESPOND TO ITS ENVIRONMENT. All living things have mechanisms for > >responding to the changes in the environment that are salient to them, as > >for avoidance, food location, life cycle staging, etc. > > Although I believe you are correct here, this is not always a very useful > criterion for judgement. Lots of vegetation may not appear to respond to > the environment, except when observed long and carefully. For example, > moss, or evergreen trees, as long as they remain healthy. On the contrary. All things that are without argument alive can be seen to respond to their environment with just a little observation. Mosses only reproduce if the temperature and humidity are just right; evergreens have a variety of responsive systems (chemical responses to parasites, for example). Some things that are not life also respond to the environment, which makes the puzzle more difficult (lead only agrees to melt if you set the temperature just so). In our experience, only those things that live are seen to responed coherently and with a wide range of activities to different conditions. This is doubtless why we are still startled by robots and intrigued by solar- tracking systems; they mimic life very well, and yet we don't want to believe that they are alive themselves. > > IT MUST BE AN INDEPENDENT ENTITY, not bound up physically in the structure > >of another living thing. All living things are entities unto themselves, > >interdependent but not completely dependent on the structure of another. > > I think this one would be very hard to call for parasitic organisms, highly > symbiotic organisms, and fetal organisms (even hard-core pro-abortionists > might recognize that a non-viable fetus is still alive in some sense). Agreed. I don't know that this invalidates the criterion though. It does become a judgement call as to why mitochondria are not alive by a bacterial parasite found only in humans is... > All of which is to say that I don't think it is possible to tell the > difference between something which is alive, and something which has been > cunningly wrought to seem alive, nor between something which is dead and > something which seems to be dead, without advance notice of the properties > of the life-forms under consideration. But then, why does it matter? Also agreed. I don't think that we are going to be able to find a litmus test that says "A" is alive, "B" is dead, and "C" is non-living. At best we can approximate the central distinguishing marks of life vs. non-life. > [This is reminds me of a biology course I took once where someone asked > the professor how you could tell whether something was a mammal. The > professor replied (essentially) "hair and milk." I said "you mean like > a coconut?" The professor was not amused.] Actually, I think that is an excellent example of the sorts of examples you can expect to encounter when dealing with something like this. I'll have to remember that one. > Richard Hoffman | "They sought it with thimbles, they sought it with care, -- Mike Sellers UUCP: {...your spinal column here...}!tektronix!tekecs!mikes INNING: 1 2 3 4 5 6 7 8 9 TOTAL IDEALISTS 0 0 0 0 0 0 0 0 0 1 REALISTS 1 1 0 4 3 1 2 0 2 0