Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!linus!philabs!cmcl2!seismo!lll-crg!nike!ll-xn!mit-amt!mit-eddie!genrad!decvax!tektronix!orca!tekecs!mikes From: mikes@tekecs.UUCP (Michael Sellers) Newsgroups: talk.origins,net.bio Subject: Re: What's this LIFE stuff? Message-ID: <7670@tekecs.UUCP> Date: Wed, 24-Sep-86 13:25:41 EDT Article-I.D.: tekecs.7670 Posted: Wed Sep 24 13:25:41 1986 Date-Received: Mon, 29-Sep-86 00:29:27 EDT References: <45500088@uiucdcs> Organization: Tektronix, Wilsonville OR Lines: 86 Xref: linus talk.origins:104 net.bio:486 > So, I challenge anyone who reads this notesfile to the following > challenge. The challenge is: > > What is LIFE? > > If we want to discuss the origin of life...we should know what this > `life' stuff is. Think conceptually and abstractly, in the sense that if > (a definite hypothetical example) we go (someday) to other planets both > in this and other solar systems, how will we recognize primitive life forms? > Some things are definitely alive (I am, and you presumably are if > you are reading this). But how do we definitely draw the line between life > and non-life? What properties does a system have to have to be A LIVING > ORGANISM? > What is LIFE? (Post all responses here) 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): 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.). IT MUST REPRODUCE. All living things are able to reproduce others of their kind that are or will become fully functional, independent entities. 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. 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. There may be more to this list, but I believe it is fairly complete. Now, many things are clearly life or non-life: I am alive (except before 10:00 am Monday :-), you are alive, your shoes (we hope) are not alive. There are at least two types of things that may or may not be alive, or may be right on the edge. The first are things such as viruses. Viruses can be crystallized, like a complex but non-living chemical compound, and kept in crystalline form indefinitely. Most of them have no metablic or reproductive functions, relying on their hosts for these functions, nor do they possess mechanisms for responding to the environment. However, they do seem to be independent entities, and they are able to reproduce after a fashion; they successfully pass their genetic code on, though only by usurping a target cell's machinery. So they possess some characteristics of things both living and non-living. No one ever said this was going to be clear-cut (in fact, a case could be made that evolutionary theory would predict such fence-sitters, since this is a viable niche to be filled, and since Life in general must have once passed through such a stage in going from non-living to living). The second group of things that may or may not be "alive" are things such as mitochondria (and possibly ribosomes) in eukaryotic cells. Mitochondria possess their own genetic code, reproduce themselves, have their own metabolic structures (in fact it is from these that the rest of the cell gets its power), and may have small structures and "procedures" for responding to the environ- ment. All animals have them in our cells; we could not live without them. They appear to be life, being almost identical to many prokaryotic (bacterial) cells that are definitely alive. (Their analogues in plants, chloroplasts, are much the same, being similar to bacterial cells containing photosynthetic material instead of or in addition to having a respiratory layer like other bacteria and mitochondria.) While these cell organelles show many of the aspects of Life, they are not normally thought to be alive because they are very highly specialized and not independent entities. One hypothesis of the origin of mitochondria says that they were (possibly parasitic or symbiotic) bacterial cells that became highly specialized for their niche inside of the eukaryotic cell, to the point that they became an integral part of it; another hypothesis (also with good evidence) states that they never were independent living things, but resulted from cell membrane invaginations, much like lysosomes and other structures (Golgi complex, vacoules). Either way, they show many of the aspects of living things, and yet we do not think of them as being so (even though the cell they are in and the body it is in may be alive). It is clear that simply enumerating a list of requirements is not enough to say whether something is alive; the various factors must be weighed according to non-obvious rules and heuristics. It is not that different from asking if something (say, Rich Rosen) is conscious or not. The thing may exhibit what we call conscious behavior in some cases and not in others, so our final decision will be a judgement call. This is one reason biologists are not a dying breed (:-), and also, as I'm sure you can see, one reason the abortion debate has gone on so long. -- 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