Opinions differ on whether viruses are a form of life, or organic structures that interact with living organisms. They have been described as “organisms at the edge of life”,[8] since they resemble organisms in that they possess genes and evolve by natural selection,[64] and reproduce by creating multiple copies of themselves through self-assembly. Although they have genes, they do not have a cellular structure, which is often seen as the basic unit of life. Viruses do not have their own metabolism, and require a host cell to make new products. They therefore cannot naturally reproduce outside a host cell[65] – although bacterial species such as rickettsia and chlamydia are considered living organisms despite the same limitation.[66][67] Accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. They differ from autonomous growth of crystals as they inherit genetic mutations while being subject to natural selection. Virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.[1]