The characterization of the universe as finely tuned suggests that the occurrence of life in the universe is very sensitive to the values of certain fundamental physical constants and that the observed values are, for some reason, improbable. If the values of any of certain free parameters in contemporary physical theories had differed only slightly from those observed, the evolution of the Universe would have proceeded very differently and life as it is understood may not have been possible.
In 1913, the chemist Lawrence Joseph Henderson wrote The Fitness of the Environment, one of the first books to explore fine tuning in the universe. Henderson discusses the importance of water and the environment to living things, pointing out that life depends entirely on earth's very specific environmental conditions, especially the prevalence and properties of water.
In 1961, physicist Robert H. Dicke claimed that certain forces in physics, such as gravity and electromagnetism, must be perfectly fine-tuned for life to exist in the universe. Fred Hoyle also argued for a fine-tuned universe in his 1984 book The Intelligent Universe. "The list of anthropic properties, apparent accidents of a non-biological nature without which carbon-based and hence human life could not exist, is large and impressive", Hoyle wrote.