Defining Life

There is life on the Earth, and a question of great interest is whether there is life elsewhere in the Universe. Before we can address the issue in any intelligent way, we must first define what we mean by life. As is evident from contemporary social issues, the definition of life, and of human life in particular, is a difficult problem. We shall as a working hypothesis identify the following characteristics with life:

1. Life is a series of processes with the primary goal being survival of the individual and of the species. Living things modify their environment to make their own survival and that of their offspring more likely.

2. Life as we know it is based on the chemistry of the element carbon. We shall not go so far as to say that this is the only possible chemical basis for life, but with our present knowledge of chemistry it is difficult to find another. At any rate, we may presume that life of any complexity requires a chemistry as complex as that of carbon and its compounds.

3. The retrieval, storage, and utilization of information appears to be a key feature of life. Thus, life as we know it requires a chemical unit for controlling information. For life on Earth, that function falls to DNA. We may speculate that other forms of life would require something similar to DNA to manage and transmit information biologically.

Thus, a reasonable first step in understanding the possibility of life elsewhere is to understand in more detail how these features characterize life on Earth.