Humans are a species obsessed with classification. This isn't a bad thing, as sorting objects and experiences into categories is how we make sense of our world. For instance, while we clearly recognize that every domestic cat is unique, we have no problem tucking all of them, despite their differences, into a single category: cat. In some cases, we broaden that category even further to include cat-things which aren't house cats. To put these diverse creatures into one envelope together, we essentially ignore their differences and focus on their similarities, and the ways that they are all different from all other animals. This is a very vital part of our survival mechanism.
Among the many, many classification systems we use in our lives, one of the most fundamental is the "life" question itself: just what is alive and what is not alive? This is a question which, at first consideration, seems pretty silly to worry about. In most cases, it's pretty easy to tell the difference between something that's alive and something that isn't alive. A dog is alive; a rock isn't. Easy. But there really are a lot of cases in which this determination isn't so easy. Is a mushroom alive? What about the lichen that may be on the surface of that rock? What about bacteria? Viruses? There was a time when it was claimed that bacteria couldn't possibly be alive, because they were "obviously too small to have had parents." So does that mean that having parents is the key requirement for being alive? As happens so often, the more closely you examine this issue, the more difficult things become.
In the less demanding past, issues like this were typically solved by resorting supernatural explanations. Make sure you read the essay Vitalism and Mechanism to consider a rather traditional example of the attempt to answer this question. But these days this kind of solution is basically considered to be cheating.
Today, scientists have thoroughly discarded the notion of vitalism to explain the nature of life. But that leaves us back at the point of trying to "define" what it means to be alive. There has been very little success in this attempt. Frankly, we still can't define life.
What we can do is describe it. This is the form that almost all "definitions of life" take: a list of those features which we consider typical of anything that we think is alive. Though these lists vary in some particulars of description, in the end they all say pretty much the same sorts of things. Here's a representative example:
This is a very nice little list, and it would seem that using this list would make it possible to neatly sort all of the objects we encounter into two neat little boxes--"alive" and "not alive."
Sadly, it isn't that easy. Or maybe it isn't so sad, as this is one of the really exciting thing about looking at the universe realistically and exploring its mysteries.
It turns out that there are things in our world which possess some of these features, but not all of them. The most obvious example is a virus. Though they can be fascinating in their tiny design, viruses are not cellular. They are all far, far simpler in construction than even the simplest cell. They perform no metabolism: no respiration, no homeostasis. They are assembled more than they grow and develop. They have no real irritability. However, they certainly reproduce with great abandon, they have a hereditary system that works just like ours--in fact, a lot of what we know about how genes and DNA behave we learned by studying viruses--and they certainly evolve. So are viruses alive, or are they not alive?
Don't worry if you can't really answer this question--neither can I. The best answer to this puzzle is that viruses are "sort of" alive. They aren't really alive, but they aren't really not alive either.
And here we learn a very great lesson. This is a lesson which will be reinforced a number of times this quarter. We are driven to categorize in order to make sense of our world. But the universe isn't required to cooperate, and frequently doesn't. In the end, our categories are all artificial--we invent them, not the universe. So very few of our categorical boxes have sharply defined boundaries. There are almost always things which fall into the fuzzy areas between the boxes, things which are impossible to force into our classification system. One of the most striking ways we encounter this reality in biology is in the attempt to come to terms with the definition of "species." This is the most specific category of our taxonomic system, but we can't devise a definition which will allow us to define the boundaries of all species. Again, that's because the entire concept of species is really our invention. Species are screen captures from a very long moving picture. In some cases, the current snapshots don't display the distinctive separation necessary to truly delineate the boundaries of species, so we are stuck ;^)
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Updated 25 September 2004