ABSTRACT
There is general agreement that life-forms have been present on Earth for nearly 4 billion years, in other words for most of our planet’s 4.6 billion-year history. In this section we introduce some of the fundamental properties of life. Our justification for this section is, first, that organisms are key components of the environment and, secondly, that life processes have a major influence on the state of the physical environment. It follows that most environmental problems require an understanding of both physical and biotic processes and the interactions between them. A discussion of the environment that omitted the living component would therefore be incomplete.
