Some organisms consist of a single cell, while others are composed of many cells, often many distinct types of cells. These cells vary in a number of ways and can be extremely specialized (particularly within the context of multicellular organisms), yet they are all clearly related to one another, sharing many molecular and structural details. So why do we consider the organism rather than the cell to be the basic unit of life? The distinction may seem trivial or arbitrary, but it is not. It is a matter of reality versus abstractions. It is organisms, whether single or multicellular, that produce new organisms. As we will discuss in detail when we consider the origins of multicellular organisms, a cell within a multicellular organism normally cannot survive outside the organism nor can it produce a new organism - it depends upon cooperation with the other cells of the organism. In fact, each multicellular organism is an example of a cooperative, highly integrated social system. The cells of a typical multicellular organism are part of a social system in which most cells have given up their ability to reproduce a new organism; their future depends upon the reproductive success of the organism of which they are a part. It is the organism’s success in generating new organisms that underlies evolution’s selective mechanisms. Within the organism, the cells that give rise to the next generation of organism are known as germ cells, those that do not (that is, the cells that die when the organism dies) are known as somatic cells.33 All organisms in the modern world, and for apparently the last ~3.5-3.8 billion years, arise from a pre-existing organism or, in the case of sexually reproducing organisms, from the cooperation of two organisms, an example of social evolution that we will consider in greater detail in Chapter 4. We will also see that breakdowns in such social systems can lead to the death of the organism or the disruption of the social system. Cancer is the most obvious example of an anti-social behavior; in evolutionary terms, it can, initially, be rewarded (more copies of the cancerous cell are produced) but ultimately leads to the extinction of the cancer, and often the death of the organism.34 This is because evolutionary mechanisms are not driven by long term outcomes, but only by immediate ones.
33 If we use words that we do not define and that you do not understand, look them up!
34 Cancer cells as sociopaths: cancer's cheating ways Recently the situation has gotten more complex with the recognition of transmissible cancers and http://www.ncbi.nlm.nih.gov/pubmed/19956175