If one defines life as the simplest form capable of displaying the most essential attributes of a living thing, one very quickly realizes that the only real criterion for life is the ability to replicate. only systems that contain nucleic acids are capable of this phenomenon. this sort of reasoning has led to a new definition of organism: an organism is the unit element of a continuous lineage with an individual evolutionary history. consider the new definition of life. if you were to argue that a virus or a single mitochondria were, in fact, alive based on this definition, you would be ignoring the main tenet of a) cell theory. b) endosymbiosis. c) nucleosynthesis. d) natural selection.