Overview

Experimental Design

The following five basic characteristics are used as the definition of life.  Life
1. Shows evidence of growth and replication;
2. Shows evidence of purposeful energy transfer;
3. Responds to stimuli;
4. Acts in such a way as to ensure self-preservation;
5. Is significantly different from the surrounding environment.

We have chosen to define life as having these five characteristics in order to try to accommodate all forms of life, including those which are vastly different from Earth life.  This definition, based on energy properties of living systems, rather than chemical properties, attempts to avoid biases introduced by human experiences with one particular chemical form of life on Earth.  Each of these properties, however, is fully met by every organism on Earth, so it does not exclude objects widely accepted as living.

The generality of our definition leads to great difficulties in implementation, however.  Growth and replication take time to measure, and may require prior knowledge of appropriate growth media and nutrients.  Purposeful energy transfer is difficult to treat quantitatively, since the moniker of “purposeful” is subject to many different interpretations.  It is difficult to know what kinds of stimuli a candidate organism might respond to, making it almost impossible to find and recognize a response.  Self-preservation is also a subjective term, and very difficult to apply to microorganisms.  Finally, differences with the surrounding environment are very difficult to detect in a systematic fashion, and the level of difference necessary for an object to qualify as alive is difficult to determine.

Due to these problems, our practical definition of life must be much more restrictive.  In effect, our experiments have been designed to look for carbon-based life, and for its chemical signatures.  While other chemical forms of life are not covered by our working definition, it may be possible to detect and characterize such entities.  Unfortunately, energy characteristics are simply too general and too difficult to measure to be the basis of experimentation.

Definition of Past Life

Unfortunately, the properties that characterize fossils of Earth life are somewhat less developed than those characterizing living organisms.  Obviously, the chemical composition will be radically different in a fossil, and no metabolism or growth can be measured.  Thus, the definition of life must be modified and restricted further in order to be sufficient to describe fossils.  We are using the following definition, from Schopf (1999).

Fossils are
1. Composed of organic material;
2. Show evidence of complex structure, especially filamentous and globular colonies;
3. Are part of a larger sample of similar fossil-like objects;
4. Show small individual variations consistent with genetic variation;
5. Show evidence of cell division;
6. Are found in rock which formed under conditions favorable to life;
7. Bear carbon and/or sulfur isotope ratios consistent with metabolism.

References
Cradle of Life: the Discovery of Earth’s Earliest Fossils.  Schopf, J. William.  1999.  Princeton, NJ: Princeton U. Press.  Harvard MCZ call #QH325.S384.