The concept of a stem cell was first proposed by the German biologist Ernst Haeckel to describe how multicellular organisms are derived from a unicellular organism. In the almost 150 years that have passed since Haeckel's initial proposal, the works of McCulloch and Till, Evans and Kauffman, Martin, Thompson, Takahashi and Yamanaka, and many others too numerous to list here, have significantly advanced the scientific and clinical applications of stem cells. As such, stem cells have transformed the manner in which scientists study human development and develop therapies to treat human disease and injury.