April 2005 Archives

This idea of being able to draw a line in the sand to say, everything on this side is life, and everything on the other side isn't, is either arbitrary or impossible with our current understanding.

This is because the most parsimonious explanation for the origin of life is abiotic. That means, sometime in the history of our planet, things that weren't life gave rise to things that were. Earth chemistry somehow became biochemistry, as organic molecules were formed, reacted with each other, enclosed the reactions in some finite space, and started making more of themselves.

It might turn out in the end that life has some other origin entirely. However, even supposing life hitched a ride to Eath on meteors, or that aliens planted it here, that only throws the question of life's origin into an environment we know very little to nothing about--space, or other planets. So if we're going to theorize about the abiotic origins of life, we might as well make it easier on ourselves and stick with our home planet. (To get away from an abiotic origin of life, you pretty much have to suppose a Creator without its own abiotic origin, which is fine with me but gets away from the point. I'm willing to talk about it later.)

So, assume an abiotic origin of life on Earth. The development of what we can agree is life (say, a frog) from what we can agree is not life (say, a rock) was probably not instantaneous. The rock did not jump up one day and say "Hey guys, check it out! I'm a frog!" Molecule after organic molecule had to be created from other, simpler chemicals. For decades people have been doing lab experiments with conditions similar to the imagined early earth, three or four billion years ago, to see what kind of molecules they can make.

The textbook example is the Miller-Urey experiment, in which a couple of guys stuck together a bunch of glass tubes and beakers and threw in some gases that everyone's pretty sure were around on the early Earth. Then they hit it with a lightening equivalent and observed the accumulation of very organic-looking brown goop. Turns out you can make a lot of amino acids this way--the building blocks of proteins. Very promising.

Since then, people have come up with more elaborate scenarios and done nifty things with chemistry. The point is, there's no reason to think there was a definitive timepoint which marked life's beginning. But, quite understandably, everyone always wants to know when it is anyway. The whole thing is frustrating--like not knowing your birthday.

Most textbooks, somewhere a page or two before or after the Miller-Urey experiment, put the first evidence of life on Earth about 3.5 billion years ago, based on putative* fossils from rocks in Warrawoona, Australia. However, there has been a LOT of controversy over those fossils. It seems like they're pretty definitely some kind of organic remains, but papers have been published arguing that the structure and content of these remains could be produced abiotically--in fact, people have even produced similar-looking fossils in the lab.

If you're going, "Hunh? So what's the controversy?" right now, we're on the same page. If not, let me explain. Everyone thinks life is supposed to have an abiotic origin, right? So shouldn't the earliest fossils also have an abiotic origin? Shouldn't this paper proving that such fossils can be produced abiotically in the lab be used as more evidence for how life could have evolved, not for shooting down other people's arguments about early fossils?

Scientists, just like everyone else, like to argue a lot.

That said, and to be fair, the fossil record is so poor in general, and so extremely poor that far back in history, it's unlikely that the earliest life actually left us any traces by which to find it. It's quite probable that by the time we're finding fossils, life had reached the point where, had we seen whatever made the fossils, we would all be able to agree, as with wiggly little worms, that it was really life.


* This is a killer word. It makes your ideas sound a lot surer and more scientific than if you just say, "supposed."

…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.)