It's Life Jim........

[peter]

I have noted elsewhere, my belief that life is a property emergent from the complex arrangement of the 92 odd different forms that matter takes - comparable if you like to the emergence of meaning from the ordering of the 26 letters of the alphabet (a relationship that holds true in terms of complexity as well as at a simple present or absent level)......

But it is on this last point - the point at which something can be deemed to actually, definitely, be alive, and the grey area that precedes it, that I want to think about today.

In biology you occasionally encounter the use of the phrase "ontogeny versus phylogeny" to describe a comparison between the (now - all of a sudden I realize I have to be careful here...... this is not as simple as I thought and I have to explain what I think the phrase means, but I might be wrong) developmental processes that from the initial fertilized egg to the fully formed organism and the evolutionary process through which countless successive generations have passed, leading upwards to the form which the organism now takes - the place it has now arrived at following countless reproductions since the dawn of time.

This process of change can be occasionally glimpsed in the patterns of development of the embryo; take a human one as an example and during the process by which it moves from an undifferentiated ball of cells to a fully formed fetus, we see at various times gills appear and disappear (reflective of the piscine stage of our evolutionary development), a tail forms and regresses (reflective of the pre-anthropid stage) etc. But, and I might be wrong here and I'd like to include this this idea anyway, you also might use the phrase to consider how the various forms of life we see arrayed before us today, ranging from the most simple to the most complex, amoeba to man to put it simply reflect (horizontally if you like) the evolutionary history of the development of the various forms of life, the orders, families, genus and species when taken in vertical or temporal development through time.

Now I wonder to what extent, the emergence of life as we see it today, horizontally as it were, across that grey area I mentioned above, is reflective of the development of life vertically through time passing from the primordial soup of hydrocarbons speculated by investigators as having existed, to the point where life can definitely said to be given the thumbs up as being existent. To consider this we have to look step by step backwards at what we have today, and consider its place in the temporal development, as a possible link in the chain, passing from definitely not life to definitely being life.........

(I'm going for breakfast - I'll be back shortly....?)

[peter]

(Continued.....)

It might be apposite at this point to cosider a definition of what we think actually constitutes life. Like energy, life tends to be defined in terms of what it does rather than what it is. I've sort of defined it as 'an emergent property', the significant word being property in terms of what we are talking about here, so what is a property in this context - what does it actually mean? Steel has the property of 'hardness', ice the property of 'coldness'. Complex arrangements of multiple different types of atoms have the property of life......does this make sense? Is it the same kind of thing as these former examples? Well, passing over that lets cheat a bit and take the simple way out just by giving it a different word, say a property is an attribute - a feature that is characteristic of an object or entity. In this sense we can say that life has five or so attributes, properties, that collectively allow us the luxury of certainly that something is alive. Living things grow. They reproduce themselves. They respire, they feed and excrete, they respond to stimuli. That's about it. Those five activities, collectively allow us to say with a degree of certainty "this thing is alive".

So what happens when we apply our yardstick to organisms of increasing simplicity (tricky in itself that one) that we see extant in our world today. Starting at the single celled organisms, the protozoa and protista, we have no problem.; all of the characteristics are there - they are alive. What about the viruses? Now we start to encounter problems in that some of our defining activities are not there. They do reproduce - but only by means of coopting the reproductive machinery of host cells into which they have insinuated themselves. Do they grow? Not really. Do they respire, feed and excrete? Not so much. Do they respond to stimuli. Certainly not in the sense that higher organisms do. So we are into definite grey area territory here in respect of being alive.

So are they candidates on the vertical ladder from the primordial hydrocarbons to life? Well it's tricky that one, because if you think about it, using as they do the nuclear machinery of higher level organisms to effect their reproduction, this surely necessitates that the higher level organisms were around first in order for the viruses to utilize them. Thus is it speculated that viruses, far from being the primetive life form that some would have them, are in fact a highly specialised life form that has simply evolved by shedding all of those attributes, those activities that are superfluous to the actual process of reproduction of a string of DNA or RNA, if those activities can be filched from other host organisms. Seen this way, the viruses are far from the degenerate organisms some would have them to be, but are rather highly developed minimalists akin to an individual who owns nothing, does nothing, but exists by simply taking everything at need from others around themselves. On this thinking, the viruses can not be seen as a precursor stage in the evolutionary development of life as properly seen by our five point criteria.

Going even further away (I say away rather than down deliberately here) from our understanding of life, but still eerily similar in some indefinable way we have the prion protein group. Exclusively disease causing organisms, these bad boys seem to buck the trend of 'how to do life' altogether. They eschew the prosaic business of nucleic acids and all that stuff and simply cut to the chase, protein to protein as it were without even the need for a transcription and translation phase at all. That the two aforementioned stages form the central dogma upon which our entire understanding of cell biology rests is of no concern to them. It's like doing physics without quantum mechanics and relativity. It shouldn't be done. DNA to RNA to protein is how it works. Protein as enzyme reflecting a given gene - and pushing a biochemical reaction in a given direction, building bodies to, well...... make more DNA. Not bent protein, see protein, bend it to make, bent protein. It's like an altogether alternative way to do life - just one that hasn't quite found its measure yet. So no, I don't think our prion protein group can sit on the ladder between not-life and life either.

If there are other, more simple things that sit in the horizontal scale (that is the current extant line) between the non-living inert matter and the definitely alive then I don't know them, but on this brief and grantedly amateur assesment it seems to me that little is to be learnt about the temporal vertical, generation to generation development of life from the inert to the motivated, by looking at the gradation from simple to complex that we have before us.