The Meaning of LifeAug. 29, 2002
by Seth Shostak - Senior Astronomer
"Oh my God, its alive!"
Thats the frequent reaction in sci-fi films when a freshly landed, seemingly inert hunk of gooey ooze from space suddenly shows signs of well, life.
But what, exactly, is a sign of life? Or more precisely, an unmistakable sign of life? This sounds like a tedious, trivial question, but a quick dip into your high school biology text will show that its not. Defining life in a way thats both complete and exclusive is not only hard, it hasnt been done.
This fact has obvious consequences for our efforts to find life elsewhere in the cosmos. If we cant adequately describe what were looking for, how can we be sure that we, or our proxy robots, will recognize it if we do? This is more than mere academic quibbling: a quarter-century ago, NASAs Viking Landers plopped down onto the dry surface of Mars to make a sophisticated reconnaissance for life. The Viking experiments sought signs of microbial metabolism, and one of the on-board tests came up positive.
However, most of the Viking scientists ultimately nixed the idea that the landers had really pawed up some Martian microbes. That was because the probes mass spectrometer failed to find any organic molecules the building blocks of biology. Nonetheless, the biology teams conclusion is still the subject of dispute and debate. So you see, even when we look for life-as-we-know-it (which, of course, Viking did), theres a problem because we havent precisely defined life. If we had such a definition, then presumably we could think of a thoroughly unambiguous test for its presence.
Why is this so tricky? In the past, it was popular to suggest that anything that was alive had a component called rather redundantly vitalism. But this concept, like that of the luminiferous ether, proved slippery and unnecessary. Todays biology books usually describe life by means of a short laundry list of properties: metabolism, reproduction, movement, and so forth. The trouble with this approach is that there are always ambiguous cases (are viruses alive?) and exceptions. Fire has a certain "metabolism," and crystals can reproduce.
These days, a popular definition of what it means to be alive is to say that life is a chemical system that can undergo Darwinian evolution. This sounds pretty good since Darwinian evolution is certainly fundamental to all terrestrial life.
But upon more careful examination, even this tweedy definition falters. In a recent paper in Origins of Life and Evolution of the Biosphere, Carol Cleland (University of Colorado) and Chris Chyba (SETI Institute) note the long standing-objection that "mules cannot reproduce, so they are not capable of Darwinian evolution." In addition, physicist Freeman Dyson has suggested that the earliest stages of life on Earth might have undergone reproduction that wasnt precise replication. In other words, while these early creatures were able to produce more of themselves, the copies werent exact. Therefore, changes caused by genetic mutation werent passed on, and Darwinian evolution didnt happen. And yet, such creatures if they ever existed were surely alive.
Cleland and Chyba suggest that our problem of definition might lie in some fundamental, missing insight into what characterizes life. A laundry list is the wrong approach for objects created by nature. Consider how we describe water. "One could try to define water as being wet, odorless, tasteless, and thirst quenching," the two authors note. However, this "could still allow substances that superficially resemble water to be incorrectly classified as water."
This situation changed radically once we understood molecules and chemistry. Then we could throw out our laundry lists and identify water as H2O. No ambiguity there.
It seems we may be in a similar situation with regard to defining life. Perhaps there is some deeper, meta-understanding of life that eludes us now, but that well ultimately discover. At that point, all the usual textbook definitions of life will seem quaintly nave.
Physicist Paul Davies has suggested in his book, The Fifth Miracle, that the real key to understanding life may lie in the generation of its information content (think of the gigabyte of code in human DNA). How could the apparently non-directed actions of chemical and physical processes produce this information? Clearly, once life is underway, Darwinian evolution can generate new information to add to a cells repertoire. But the question Davies poses is how sufficient information to produce life got organized in the first place. He suggests the possibility of hidden science which we live in "a self-organizing and self-complexifying universe, governed by ingenious laws that encourage matter to evolve towards life and consciousness." Perhaps we could formulate a better definition of life if we understood this process.
The answers still are not in, and its unclear whether they ever will come in. Meanwhile, when it comes to recognizing biology, were at least temporarily stuck with the hoary criterion used for both art and pornography: "Well know it when we see it." Clearly, it would be nice to have a better basis for recognizing our extraterrestrial prey.