Defining Life

A question that continues to pester science is: What is life? One of my dictionaries offers this definition: “the condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, and continual change preceding death.” That seems to cover the topic quite well, but it has a problem: it may work well for distinguishing life on this planet, but how about other worlds in space? It also assumes that we know what an animal and a plant is—as well as being able to tell what “functional activity” means; and finally, death. These definitions depend on what we know about life on Earth; leading us to think that we will recognize life elsewhere when we see it, wherever it is.

But as we look out toward other bodies in the universe and wonder if we're alone or not—if life may have emerged elsewhere—could we even recognize it on alien worlds? In recent decades scientists have discovered here on Earth several species of what are now called “extremophiles”: microbes that live in habitats that we once thought were impossible for life; such as next to hot and toxic undersea volcanic vents; highly acidic pools of water; and deep underground locations, far from the sun and any source of plant nutrients. So our definition of life—and the conditions under which it can survive—has had to be widened.

Now we are on the verge of exploring other worlds in our solar system, upon which we know some of these extreme conditions exist: Mars, Jupiter's moon Europa, Saturn's moon Enceladus, and others. Scientists are eager to check them out, and we may well be able to do so, in another decade or so. An even greater stretch of our ability to detect life may come when we develop better instruments to evaluate the properties of planets orbiting nearby stars.

Our ability to tell whether life exists “out there” hinges on the adequacy (or breadth) of our definition of it. Researchers keep offering updated definitions, but for every one put forth, other scientists quickly come up with a counter example even here on Earth that nullifies it. Take the reproduction part of the definition of life given above: What if we found some entity on the moon Europa that seemed to be alive, but showed no evidence of reproducing? Maybe it just has a very long reproductive cycle. How long do we wait, in order to say yes or no? What might death mean on Enceladus? How do we judge whether something has “functional activity” on an alien world?

Our current understanding of biology is still a little too primitive to come up with a clean definition of life here on Earth, let alone on some outer-space planet. For example, in the late 1970s the two Viking spacecraft that landed on Mars had the explicit objective of determining if life could be found there. The limited design of the Viking experiments, however, did not allow a decisive answer. NASA may have even conducted the wrong kinds of tests.

We'd better keep our minds open and not try to nail down the definition of life, until we are able to get to an alien world and observe what conditions prevail there. It just might show us something beyond our wildest expectations, and even beyond our current ability to wrap our heads around.