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