Knowledge of the microbial world is essential to understanding the evolution of life on Earth. The characteristics of microorganisms — small size, rapid reproduction, mobility, and facility in exchanging genetic information — allow them to adapt rapidly to environmental influences. In microbiology, the validity of evolutionary principles is supported by  readily demonstrated mutation, recombination and selection, which are the fundamental mechanisms of evolution;  comparisons based on genomic data that support a common ancestry of life; and  observable rates of genetic change and the extent of genomic diversity which indicate that divergence has occurred over a very long scale of geologic time, and testify to the great antiquity of life on Earth. Thus, microorganisms illustrate evolution in action, and microbiologists have been able to make use of the microbes' evolutionary capacity in the development of life-improving and life-saving innovations in medicine, agriculture, and for the environment. By contrast, proposed alternatives to evolution, such as intelligent design and other forms of creationism, are not scientific, in part because they fail to provide a framework for useful, testable predictions. The use of the supposed "irreducible complexity" of the bacterial flagellum as an argument to endow nonscientific concepts with what appears to be legitimacy, is spurious and not based on fact. Evolution is not mere conjecture, but a conclusive discovery supported by a coherent body of integrated evidence. Overwhelmingly, the scientific community, regardless of religious belief, accepts evolution as central to an understanding of life and the life sciences. A fundamental aspect of the practice of science is to separate one's personal beliefs from the pursuit of understanding of the natural world. It is important that society and future generations recognize the legitimacy of testable, verified, fact-based learning about the origins and diversity of life.