Tuesday, April 26, 2005

The origin of life, the universe, and everything...

…seems as good a place to start as any. Specifically, I’ll start with Life, since I’m a biologist, not a physicist or astronomer. (Even making that statement, though, seems limiting. Why pigeonhole yourself? But that’s a discussion for another day.)

Life! All of us, being more or less alive, have a fairly intuitive understanding of what life is. But people have a lot of trouble when it comes down to defining life. They can say, “This frog is alive. This amethyst crystal is not.”

(I’m sure you can find some argumentative types to come down on the side of the crystal, but anyway.)

The currently more or less accepted NASA definition of life is (abbreviated): a chemical system capable of evolution by natural selection. This is a nice compact little statement that implies a whole lot of characteristics of life, most particularly: variation, fitness consequences of variation, and heredity of variation (which in turn implies some template for the variation, i.e. RNA/DNA, and some method of reproduction). However, it also implies the existence of a population. A single individual cannot, in the usual sense, undergo evolution by nature selection, which occurs through differential reproductive success. Some rabbits are more fit than others, thus they produce more offspring, thus over time their genes undergo positive selection. One bunny rabbit, alone in the universe, is not, by this definition, “life”.

Some people are okay with this. Others are troubled. Some rationalize it with a sematic distinction which, I confess, seems a little silly to me: this poor lonely rabbit, while not constituting life, is still “alive”.

Because any definition either states or implies the presence of a number of characteristics about what is being defined, one might argue that a straightforward list of the characteristics of life would be the most accurate definition. But some people—I call them definitionists—argue that a list is not a definition, and we need a definition.

Perhaps it isn’t. But perhaps we don’t.

It’s not hard to come up with a list of properties that we associate with life. One such list might look like this:

metabolism
membranes
template for variation
reproduction
solvent-based (intriguing, no? it’s been postulated that solid-state chemistry could produce life… but its metabolism might proceed at the rate of one or two reactions per millenium)
chemical bonding
etc.

In a functional sense, such a list might be much more useful than a sentence-long definition. Why? Well, what do we want it for, anyway? Partially, of course, to satisfy our basic human drive to understand, nay, to grok. But also because we are looking for it (life) and we want to know when we find it. We are looking for evidence of it in some of the oldest rocks on Earth. We are looking for it on Mars. We want to look for it on Europa, Titan, Triton, and perhaps elsewhere.

And in that case, the easiest thing to do is make specific tests for various properties of life as we know it, and start marking off boxes on our bingo sheet of Life. Some people may feel they’ve gotten Bingo long before others are ever satisfied. But that in itself is a silly analogy, because my next question is why we should have a binary understanding of life in the first place?

As mentioned above, there are a large number of items which the vast majority of people will identify consistently as life or non-life. But there are a number of non-intuitive items as well. Take viruses. A lot of very well-educated people don’t want to call them “life” because they lack the ability to reproduce independently. Viruses reproduce by injecting their template (RNA) into a host cell, along with various enzymes that instigate the host cell’s replicating machinery to build more viruses. In the most basic sense, viruses lack the ability to reproduce.

But many who have suffered through viral infections of one form or another would be eager to testify that viruses are alive—alive and malicious. Leaving moral questions out of the assessment for the moment, however, we’ve still got a quandary. Plenty of “higher” parasites, from single-celled protozoans to macroscopic worms, are obligate parasites. Their life cycle is tied so intimately to their hosts that they will die on their own—and they certainly cannot reproduce in this state. However, because on the most basic level they do their own reproducing, that is, they possess their own replicative machinery, they are deemed alive.

But that’s not really why we call them alive. We call them alive because they’re wiggly little worms, for goodness’ sake.

(Okay, we’ll call this the Introduction to the Origin of Life. Tomorrow, I’ll get to the Origin of Life itself. The juicy stuff. I promise.)




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