Once the necessary building blocks were available, how did a living system arise and evolve? Before the appearance of
life, simple molecular systems must have existed that subsequently evolved into the complex chemical systems that are
characteristic of organisms. To address how this evolution occurred, we need to consider the process of evolution. There
are several basic principles common to evolving systems, whether they are simple collections of molecules or competing
populations of organisms.
First, the most fundamental property of evolving systems is their ability to replicate or
reproduce. Without this ability of reproduction, each "species" of molecule that might appear is doomed to extinction as
soon as all its individual molecules degrade. For example, individual molecules of biological polymers such as
ribonucleic acid are degraded by hydrolysis reactions and other processes. However, molecules that can replicate will
continue to be represented in the population even if the lifetime of each individual molecule remains short.
A second principle fundamental to evolution is variation. The replicating systems must undergo changes. After all, if a
system always replicates perfectly, the replicated molecule will always be the same as the parent molecule. Evolution
A third basic principle of evolution is competition. Replicating molecules compete with one another for available
resources such as chemical precursors, and the competition allows the process of evolution by natural selection to occur.
Variation will produce differing populations of molecules. Some variant offspring may, by chance, be better suited for
survival and replication under the prevailing conditions than are their parent molecules. The prevailing conditions exert a
selective pressure that gives an advantage to one of the variants. Those molecules that are best able to survive and to
replicate themselves will increase in relative concentration. Thus, new molecules arise that are better able to replicate
under the conditions of their environment. The same principles hold true for modern organisms. Organisms reproduce,
show variation among individual organisms, and compete for resources; those variants with a selective advantage will
reproduce more successfully. The changes leading to variation still take place at the molecular level, but the selective
advantage is manifest at the organismal level.