Biology is the scientific study of life. Other disciplines study life, and life-forms, from other viewpoints, but biology employs the scientific method.

Biology has been transformed by the emergence of evolutionary science. Many of the characteristics of organisms make little sense in terms of operational design or adaptation to their environments and can only be understood in terms of evolutionary ancestry. This applies to some of their structural and functional characteristics and particularly to much of the DNA. This is what evolutionary geneticist Theodosius Dobzhansky meant by his famous statement, "Nothing in biology makes sense except in the light of evolution". Furthermore, evolution has given an organizing principle to biology, which might otherwise have continued to be a cataloguing of types of organisms and their structures and functions.

Although biologists study life, they cannot precisely define it. Ernst Mayr, perhaps the most prominent biologist of modern times, indicated that a lifeform must have the following capacities:

- For metabolism, in which energy is bound and released, for example, to digest food molecules

- For self-regulation, whereby the chemical reactions of metabolism are kept under control and in homeostasis, for example, to maintain relatively constant internal conditions of temperature, or moisture, or chemical composition

- To respond to environmental stimuli, for example, moving toward or away from light

- To store genetic information that determines the chemical reactions that occur in the organism, for example, DNA, and to use this information to bring about changes in the organism

- For growth

- For differentiation, for example to develop from an embryo into a juvenile into an adult

- For reproduction

- To undergo genetic change which, in a population, allows evolution to occur.

While most of these characteristics may not be much in dispute, it is impossible for scientists to imagine all the possible forms that they could take. In addition, no life-form carries out all of these activities all of the time or under all conditions. These considerations become important in two respects. First, would it be possible to recognize a life-form on another planet? Scientists may not be able to witness putative life-forms carrying out metabolic and other activities. Second, at what point might scientists be able to construct a mechanical life-form? Although computerized robots cannot grow or reproduce themselves, they can construct new components and whole new robots. Many computer algorithms already utilize natural selection to generate improvements in structure and function. Should robots, or even computer programs, be considered life-forms? . . .

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