104 Chapter 14
of the crucial features of such catalysis have now been discovered in
laboratories. Proteins may well have appeared later, with their struc-
tures coded for by RNA. The present day genetic code is the same from
bacteria to man, so it is very likely that there was only one origin to the
genetic code. Also, almost all the individual components of proteins,
the amino acids, can exist in two forms either with a right-handed
structure, or a left-handed structure, with identical atomic components.
(When amino acids are synthesised chemically, they consist of equal
mixtures of the right and left-handed forms). All proteins today contain
only left-handed amino acids, which again strongly indicates that there
was a single origin of living organisms.
Classical physics tells us that large numbers of molecules will always
achieve a steady state of maximum disorder. It is a law of physics, but
it is violated by the evolution of living organisms. The availability of
energy is an essential ingredient for the formation of ordered structures
capable of replication, and this energy could have come from light,
from organic molecules in the primeval soup, or in some cases from
inorganic chemicals. These primitive organisms, and all their more
complex descendents, were based on the properties of rather few types
of atoms, namely, carbon, nitrogen, oxygen, hydrogen, phosphorus and
sulphur, some common salts and trace amounts of certain metals. The
ingredients were much simpler than the wide range of atoms and their
compounds in the non-organic world. Nevertheless, the simple basic
ingredients were gradually built up into the ever increasing complexity
of the organic world.
These early organisms would have contained nucleic acid molecules
(the primitive genes) capable of replicating themselves, proteins and
probably membranes, which enclosed all the constituents. There would
have been many mistakes in the replication of genes and the correct
assembly of proteins. This probably meant that reproduction was
followed in many cases by death of the organism, because the
molecular order which is essential for life could not be sustained.
Obviously, in early evolution accuracy would be an advantage, so
natural selection gradually increased the precision of gene replication
and protein synthesis. Natural selection of the fittest organisms is
always accompanied by death of the less fit. The cost of evolution
is very high: to generate the few survivors with advantageous traits,
many many organisms must die.
At some stage during the early evolution of organisms, the RNA
genetic information was replaced by DNA, with the same genetic code.
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