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Facts of the Matter Richard Brill |
Life on Mars?
It depends on how
you define ‘life’What is the meaning of "life"? It depends on whom you ask. Is there life after death, or life after 40, or does Honolulu really have a night life? The questions that concern biologists today have to do with the definition and recognition of life.
Multi-megadollar machines are orbiting Mars and sampling its surface, looking for signs of life past or present, seeking conditions that might support it or hard evidence that such conditions might have existed in the past.
How would they know if they could not both define life and recognize it and the evidence it leaves behind?
ASSOCIATED PRESS
This image from the Mars Exploration Rover Spirit's rear hazard avoidance camera showed the rover's hind view of the lander platform, showing tracks left on the martian soil by the rover's wheels. The rover is approximately three feet in front of the airbag-cushioned lander, facing northwest.
At first it might seem like an easy thing to do, but coming to a workable definition of life is not so easy. It's like defining "matter" or "time": When we don't think about it, we know what they are, but when we think about it, we don't know.
We might think we know what is meant when we say that something is alive, but that, too, is not as easy as one might think. There is a "know it when I see it" approach that a child can apply when deciding that a goldfish is "alive" while a tuna sandwich is not. This is an easy way but not adequate to the task in any but the most obvious of cases.
Whereas physicists agree upon a definition of matter, biologists have not been able to reach an agreement concerning a definition of life. Physics has been successful in describing and recognizing matter and its behavior from the smallest quarks to the largest quasars. Life is much more complex in general and more difficult to categorize.
Defining and recognizing life is not only important to scientists and philosophers. There are social concerns as well, comprising important moral and legal issues, such as determining when life begins in the womb and when it ends on the deathbed. Scientists are often called upon to make those decisions or to provide guidance to those who must make them. Science is not much good for that, however, and should not be used to decide social and moral issues -- even if the boundaries between science and philosophy are mutable and debatable.
The definition of life has proved to be a provocative problem that has been deliberated upon by many great thinkers since Aristotle pondered it in the fifth century B.C. But it has led to no generally agreed-upon solution even after 2,500 years of debate.
Some would argue that life is simply a name we give to matter that has reached some high level of organization and complexity, but this is a nonanswer since it remains unclear at what level of complexity a thing should be called "alive."
From the point of view of traditional biology, the cell is the fundamental unit of life. Cells have several characteristics that define the life process. These are order, complexity, self-replication, protein-building instructions, growth, metabolism, energy processing and conversion, homeostasis, movement, adaptation, acts of self-preservation, significant differences from surrounding environment and response to stimuli. Different life systems at all scales exhibit a different set of the characteristics, and to varying degrees, which is what makes it hard to determine what is biologically alive and what isn't.
There are numerous examples that lie on the fringes of this broad definition. The fringes are of two kinds. There are systems that we consider nonliving but that exhibit most of the characteristics, and systems that we consider to be alive but that do not exhibit all of the characteristics.
To illustrate the first type of fringe systems, consider fire.
Fire is not biologically alive, yet it fulfills most of the criteria for life. Accordingly, people have ritualized and worshipped it, probably beginning with the first time a humanoid capable of abstract thought stared into dancing and swirling flames hundreds of thousands of years ago.
The important characteristic missing from fire is its lack of protein-building instructions, which is increasingly being valued by scientists and philosophers alike as the most important single characteristic in determining what is alive, although it doesn't help much in detecting signs of past life.
Viruses fall into the second of the two fringe categories, although there is some debate about whether viruses are truly alive.
Viruses essentially are pure genetic material, not cellular organisms like bacteria. They are biological pirates, genetic blueprints for taking over some cell's machinery and using it to make more viruses, which then invade and commandeer other cells, causing a cascade of pirate viruses looking for cells to plunder. The virus spreads until it is overwhelmed by the immune system or the invaded organism dies.
Another example of the second fringe type are pores, seeds and some insects that can remain dormant for years even if dried and frozen, and which exhibit none of the characteristics of life otherwise. There is virtually no way to predict with certainty which seeds will grow and which will not, except to plant them and see which ones sprout. Nonetheless we consider the ones that will not sprout to be alive until their failure to sprout proves otherwise.
Then there are the self-replicating proteins known as "prions," which are biological but are not really alive.
Abnormal varieties of prion proteins can stimulate their own formation by changing other proteins, without the presence of nucleic acids, RNA and DNA, which are present in cells and viruses. These abnormal forms are called infectious prions although they don't really "infect" in the traditional way.
All mammals have a gene to make a prion protein, but the normal prion is a different shape than the infectious prion. Abnormal prions somehow cause normal prions to refold in some cases but not in others. Some researchers think that the transition requires a particular kind of RNA that is present in the brain being infected. Whatever the process, normal proteins are modified into their abnormal counterparts, which accumulate exponentially in the brain until death. How the abnormalities began in the first place is not understood, but some researchers believe that prions are essential in the evolutionary process by providing feedback from the environment to genes, which can be modified as a result. Research into prions is sparse, but it is a growing field and researchers believe the field holds great promise for unraveling many mysteries of life and life processes.
Prions have been a big item in the news because they are responsible for mad cow disease, known to epidemiologists as bovine spongiform encephalopathy (BSE). The sponginess comes from the lumpy masses of protein that have been "infected" by prions. These masses are similar in structure to the fibrous masses that characterize Alzheimer's.
Infectious prions only infect nerve tissue, causing malfunctions in the central nervous system, mostly in the brain. In cows it causes the erratic behavior that characterizes the disease and gives it the "mad cow" name.
Prions that caused the BSE were transmitted to cows in England, where the disease originated, by food that contained ground waste that included brain and nerve tissue from sheep that had scrapie.
Scrapie is one of the widely known animal prion diseases, found in goats and sheep. It results in loss of coordination, incapacitation and itching, the latter of which caused the sheep to scrape against trees and fence posts.
Recognizing life might be difficult at the other end of the size scale as well. Planet Earth exhibits most of the characteristics of living cells, as noted in the Gaia theory of James Lovelock, but many biologists aren't ready to accept Earth as an organism even if it does fit the criteria, albeit in unique fashion.
One wonders whether a microbe, were it capable of such thought, might not have similar difficulty conceiving of a complex organism such as ourselves as anything more than a collection of single cells!
The interactions and interdependence that makes us so much more than the sum of our cellular parts might not be evident to a microbiotic philosopher any more than we can comprehend the immense complexity and interrelationships that constitute the planetary ecology that supports and nourishes us.
Biologists who research the origin of life need to know what life is and how to recognize it as well. Their studies are central to the search for life on Mars and elsewhere, but understanding where life came from is even more difficult than defining and recognizing it since such studies must consider how and when groups of chemicals of ever-increasing complexity become "alive."
Wondering where we came from and what separates the living from the nonliving are both part of the combination of curiosity and spirituality that make us human. They also are the reasons we send machines of metal and silicon to explore distant planets, whether or not we intend to colonize them.
We encounter the same problems and questions whether we are designing the mechanized sensors that look for life on Mars, trying to decide when life begins in the womb, when death occurs or trying to understand and control diseases such as mad cow, ebola or HIV that are caused by "fringe" life forms.
As much as we strive to recognize and understand the biological and physical order that apparently exists ad infinitum on both large and small scales, we find that the diversity and complexity on this planet is unmatched. Without knowing unambiguously what life is or how to recognize it in either present or fossil form, we might very well miss it on Mars or somewhere else. That would be a shame.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be contacted by e-mail at rickb@hcc.hawaii.edu
