An understanding of the fl factor of the Drake Equation rests on the notion of a planet suitable for the development of life. The goal of this chapter is to present the transformation of this notion as it has evolved over the last two centuries. Three periods can be distinguished. The first period, which begins toward the end of the nineteenth century, is characterized by the conviction that life is present throughout the entire universe. Some scientists imagine other worlds, often comparable to our earth. Others develop the theory of panspermia and the dispersion of germs of life to all of the new planets. The second period, from the 1920s to the 1950s, is marked by the formulation of complete interdisciplinary hypotheses about the origins of life on earth. These hypotheses were notably focused on understanding the conditions in which life appeared on earth, the only known example of the development of life. The third period, from the 1950s to the present, has studied the possible probiotic chemical pathways that may have pertained on earth. There has been broad scientific acceptance of the idea that life may exist elsewhere in the universe, though the idea has been contested. However, such opposition has been based more on philosophical conception than on scientific argument.