What would you look for if you were given the task of searching for life on another planet? This is not a simple question, and the answers turn out to be harder than you might have imagined.
It is almost impossible to define the phenomenon we call "life" in a single sentence, or even a paragraph. There does not seem to be a simple statement that summarizes the full range of properties associated with even the simplest, tiniest creature.
Instead, it is necessary to develop a list of those properties that we find associated with living things on this planet, none of which are exclusive to life itself, but rather inclusive of all the commonalities we find in organisms as diverse as bacteria and humans.
These are the signs of life, some, most, or all of which we might expect to find on another planet if we were ever lucky enough to voyage there in search of creatures we could call "living".
Looking at life on earth one of the first things we notice about life, particularly animal life, is that if we poke it, we provoke a reaction. The type of reaction, however, will differ from organism to organism and will usually be much different to the reaction we would get from a non-living object.
The second obvious property that all living things on this planet share is the property of complexity. Even the simplest creature alive today is a marvelously complex living machine that is built and functions within a set of rules or principles common to all such machines.
Living organisms grow larger and eventually, by a variety of mechanisms, produce more of themselves in the process we call reproduction. Growth and reproduction are not exclusive properties of living organisms, but the manner in which living creatures enlarge and form the next generation is a special feature of life itself.
During the process of reproduction smaller versions of the parental forms are created. In some mechanisms these offspring are almost identical copies of their parents, but when variety is introduced the offspring are the "same" as their parents in many ways, but different in other important ways.
During reproduction, copies of the blueprints and instructions used to create the organism must be copied and passed along.
But even the most perfect creature might run into problems if the world in which it lives begins to change around it. As long as there has been life on this planet, the planet itself has been changing. Life itself must therefore be capable of change, if it could not change and adapt to new circumstances, then life itself would cease to exist the moment the environment became too hostile.
Life must also be capable of resisting change. A sudden drop in temperature, for example, must not be allowed to bring about destructive freezing of cells or tissues in the human body. Special mechanisms exist inside most complex creatures that are designed to compensate for the variability of most environments.
Who pays for all this? Nothing as complicated as the phenomenon we call life could be created without some cost. This cost comes in the form of energy - the energy needed to build and maintain a living cell has to be drawn from outside the cell itself. The way living things manipulate energy to their advantage is one of the major and most important "signs of life".