Assortments of Thought

Defining Biological Life

Posted on: July 11th, 2009

It is very difficult to permanently yet precisely define what is meant by biological life, at least in such a way so as to encompass all that we recognize as being alive, yet recognize life not yet discovered or even formed. This partly stems from the sheer diversity of life here on Earth. However, it also stems from the fact that we haven’t even seen all life that exists here, much less any that may exist elsewhere in the universe. Hence, in defining life by considering the fundamental commonalities of all known life, we face the drawback of making an induction from a sample. We can define life so as to include all that we are currently aware of as being alive, but any such definition may fail to account for what we may later discover, or even for something which hasn’t yet evolved. However, I believe that by considering a fundamental, chemistry-inspired difference between life and non-life, we may in fact be able to obtain both a more permanent and yet more precise definition. Rather than relying on an induction, we can instead define two single requirements for something to be considered alive based on chemistry. Namely, that living things contain their own descriptions as genetic material within them, and that they propagate through some form of reproduction and transmission of this material.

Let’s first review the typical biological approach. All that is recognized as life is classified into an elaborate hierarchy, traditionally starting with kingdoms (animals, plants, fungi, protists, archaea, and bacteria) and working all the way down to specific species. This classification is of course done on the basis of similarities and differences between particular life-forms, including their appearances and, more recently, their genetics. In any case, once the great many life-forms here on Earth have been classified into such a hierarchy, life can then be defined on the basis of what all these recognized life-forms share.

To be sure, definitions obtained in this way are quite useful and correct, so long as they reflect nearly all the similarities noted in all that we consider to be alive. Such similarities may include, for instance, reproducing on the basis of and via the propagation of genetic material; taking up and ordering energy to drive ongoing internal processes; taking up nutrients and depositing wastes, again to drive internal processes; and exhibiting adaptability and change in the face of environmental changes. Such things are indeed features of practically everything we consider to be alive, and so definitions using them generally work. We can even be confident that many life-forms discovered in the future will be described by them as well. Still, as such definitions are developed from inductions based on a very incomplete sampling of all life, some life found in the future will undoubtedly not be described by them, and so will challenge and require that they be revised.

Let’s now consider a more chemical approach. Chemically, what is at least one striking thing that distinguishes all life from non-life? That in life-forms, all the information necessary to describe and propagate them is contained in complex molecules within them, whereas with even the most complex chemical mixture, this is not the case. Which is to say, a chemical mixture does not describe itself in any way, and nor does is propagate itself through reproduction. Indeed, to drive this point home, if all instances of a chemical mixture were eliminated from the universe, new ones could very easily and extremely quickly form by a simple remixing of the original constituent compounds. On the other hand, if all life of a particular species goes extinct, unlike even the most complex chemical mixture, that species is likely gone forever, even if one like it comes to evolve over the next thousands of years. Or, put more briefly, unlike non-living things, life-forms have a complexity such that their continuing existence is totally dependent on genetic material, in complex molecules within them, that describes them and that is passed on by them through reproduction to create further life-forms.

Hopefully I just made that very clear, because I believe it can serve as the basis for a whole new, non-induction based definition of life, one that’s perhaps more permanent yet precise than induction-based ones. Consider that all known life has genetic material within it to describe itself, and it passes this material on through reproduction. Further, if this wasn’t the case, the “life” in question would cease to exist with the current generation, not to mention it would never have evolved in the first place, for evolution absolutely requires genetic information being passed on. Thus it seems this new definition can not only describe all currently accepted living things as being alive, but can also account for all life that will ever be discovered in the future as well.

Of course, it may still not be quite that precise, because we still don’t know how life initially forms. If the formation of life involves a jump from non-living chemical mixture to living organism complete with self-descriptive genetic material, then this definition really is that precise and foolproof. On the other hand, if the formation of life involves gradations between clearly non-living and clearly living per this definition, then this definition isn’t completely precise, and cases that challenge it do exist. As we continue to unravel the mystery of how life forms, then, the exact precision and reliability of this non-induction based definition will become clear.

Finally, before I wrap this up, there’s one more thing to consider: this definition only applies to biological, physical life. Which is to say, if some form of post-death life exists or we ever manage to create non-biological life of some sort, this definition will very likely not describe it. Rather, reconciling such life with biological, physical life would require a whole new definition. But for biological, physical life, I believe this chemistry-inspired, non-induction based definition works perfectly.

©2009, D.S. Applemin. All rights reserved.

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