[Unpublished introduction to comparison between genetic and neural codes]
In this transcribed audio recording from one of Nirenberg's requests to his lab, he discusses the evolution of genetic
and neural codes. He suggests that while the genetic codes could have arisen only once or could have been selected from a
number of precursors, the neural codes almost certainly were selected from a large population of precursors. This is significant
because if the genetic code arose de novo it might influence the course of its subsequent evolution.
Number of Image Pages:
3 (162,368 Bytes)
1967-10-10 (October 10, 1967)
Nirenberg, Marshall W.
Reproduced with permission of Marshall W. Nirenberg.
The genetic code probably evolved in conjunction with the transition from a cellular to cellular kind of organization; that
is, during the evolution of single cells. The code probably became fixed at a relatively early date as suggested by Hinegarden,
et al., because soon after much information had been acquired, further modification of the code probably was restricted to
those which would not prevent the information which had been acquired, from being expressed. The earliest fossil bacteria,
estimated 3,000 x 10^6 years old; however, the fossil record first becomes abundant approximately 600 x 10^6 years ago. Virtually
all of the invertebrate phyla and the first vertebrates had evolved 500 x l0^6 years ago.
The genetic code evolved as the original cells evolved. Fossil bacteria, one to three billion years of age, have been reported;
however, the fossil record first becomes abundant about 600 million years ago. Neurons must have originated as multicellular
forms appeared and became more highly differentiated. Therefore, the genetic code is older than neural codes. Since single-cell
organisms, such as bacteria, are highly sophisticated, biochemically, it seems probable that the early neural mechanisms employed
extremely sophisticated enzymatic mechanisms. The mechanisms, almost surely, were based on mechanisms that are operative
in single cell organisms or simple multicellular organisms; however, problems involving selective expression of genetic information
and basic mechanisms required for differentiation were available. Basic mechanisms involving cell-cell contact undoubtedly
had evolved. The cells probably were mobile. Hormones probably had evolved in simultaneous attempts to integrate the activities
of multicellular organisms. Probably directional cell migration and the formation of highly specific cell interactions were
One, possibly major, difference between the evolution of the genetic code and the evolution of neural codes should be mentioned
though. One must distinguish between the origin and the evolution of each kind of code. If the genetic code arose only once
then the nature of the de novo code may greatly restrict the course of its subsequent evolution. Alternatively, the genetic
code may have been selected from a large population of precursor codes. Therefore, the genetic code may have evolved from
only one precursor rather than from the population of precursors, and so the influence of possible non-random origin may still
be apparent. In contrast, neural codes almost certainly were selected from a large population of precursors. Basic biochemical
mechanisms, upon which the neural codes were based, almost surely evolved at a very early stage by a process of selection,
that is, from a statistical event -- not from a single event.
Biochemistry in general and certainly the biochemistry of the genetic language demonstrates that molecular complexity is achieved
by combining relatively few kinds of molecules in different sequences. The rather obvious examples are nucleic acids and
proteins. Polysaccharides may also be cited.