The key to maintaining this necessary degree of flexibility is slightly imperfect reproduction, followed by testing against the environment. Variability plus natural selection generates the process of evolution. This topic has been developed at length here because it represents probably the most important single criterion of life. No nonliving chemical system, no matter what its complexity, has this ability to respond to long-term challenge and to evolve. The development of an imperfect hereditary machinery probably was the most important single step in the evolution of life.

In summary, we can find five hallmarks of living systems that set them apart from all other chemical systems. One need invoke no special properties other than an unusually high level of chemical and spatial organization. There are no vital principles, only chemical principles. A living creature is an elaborate chemical system, which has special properties that arise from its complexity. In this chapter and those that follow we shall be concerned with the most challenging question in chemistry today: What are the chemical bases for these essential activities of living systems? Or in brief: What is the molecular basis of life?

Right: Photos of Biston betularia (peppered moth)