Imagine a “primordial soup” on some planet somewhere from which there occasionally emerges a thing that could locomote, and as it locomoted around it would scrape up some of the dust that lay around on the planet, and occasionally eat other things that had come out of the “primordial soup” and it would thus grow. Eventually it would wear out as its molecules, put together by some chemical process of abiogenecis in the aforementioned soup, and thusly worn out, molecules broken down by ultraviolet rays from the nearby star, it would eventually stop moving and remain exposed to the elements and dry out and become part of the dust, to be scraped up and consumed by other things.
Imagine that dozens of shallow seas of primordial soup on this planet each produced a range of such things, and they moved around on the planet, some staying in the soup, some going onto land, interacting, competing, cooperating, eating each other, sliding past each other, being born of the soup and dying, the dust sometimes being blown back into the soupy seas or being scraped up by other things.
The things are alive, right?
What if there was a form of thing on some other planet that had crawled out of the ooze and over time evolved, changed, varied, but over even longer periods of time, a self replicating version of this thing, or set of things, developed a way of perfectly identifying copies of itself that were not perfect, and destroying them. Say this emerged in several lineages of things, and this invariance gave some advantage to the things that did this. All other things, the ones that vary and change over generational time, are out-competed and those lineages disappear. So eventually, there are dozens of lineages of distinct but invariant things walking, sliding, coasting, flying, around on the surface of this planet, replicating but always duplicating perfectly, for hundreds of thousands of generations.
These things are alive, right?
Not according to Edward Trifonov, who defines life as:
“self-reproduction with variations.”
Carl Zimmer has written up a nice piece on the definition of life and in particular the work done by Trifonov (Can A Scientist Define “Life”? By Carl Zimmer). Go read it.
In essence, and you can get the details from Carl’s writeup, Trifonov has done a linguistic analysis of the hundreds of definitions of life previously available and distilled them down to their essence to produce the most functional and applicable definition of life.
Except it isn’t, for three reasons.
1) The search to understand what life is, and possibly find it elsewhere (or forms on earth of which we are currently unfamiliar) is not about finding something we know about, but rather, it is about finding something we don’t know about. By looking only within the stated definitions, we might find a nice linguistic analysis but we’ve limited our definition to a set of criteria less than that which has already been considered. This definition systematically turns its back on the newness that is almost always out there in exploratory science;
2) The simplest distilled definition that is, essentially, the most overlapped part of a set of complex interconnected Venn diagrams, is probably going to leave out details we’ve already thought of, that we were not sure at the time were important, and that may end up being very important, and may even end up being the key nuanced thing that makes both life itself and the definition thereof work; and
3) I just disproved the definition by imagining two universes that you would be hard pressed to disprove exist. I know, I know, I might as well have suggested “Unicorns are born of rainbows” or something, but still… this is a big and long-lived Universe and something that we would call life that does not vary in it’s replication, or that emerges from something we don’t call life such as a Droid or Robot might emerge but where the robot-maker is not a high tech company but rather a puddle, are not outrageously impossible. OK, so maybe I didn’t disprove it, but I made it look weak, don’t you think?
Zimmer also makes the point in his piece that some have questioned the smarts behind NASA going to Mars to find life using an Earth based model, whereby life will exist in relation to water and carbon. I agree with him that life in the Universe may ignore such Earthly rules. However, it is true that Mars is fairly Earth-like, and there is water there, so given the limits on what we can do with remote control robots on other planets, testing the hypothesis that “Mars has or had a roughly Earth-like life” is not too unreasonable. Personally, I think they’ll find it. After all, the only other Earth-like planet we’ve looked at so far has it!