The Side Chain and the Evolution of Vertebrate Head
In a previous (2nd) posting I argued that there is a cost for increasing the
length of the ontogenic chain. This point can be understood rather
intuitively. In any multi-level control system where the success of one
level depends on the success at the previous level, the probability of
failure of the control system would increase rapidly with the number of
levels in the system. There is no reason why the genetic system controlling
ontogenic development should not be viewed as such a control system, with
Group 1 to Group N genes viewed as levels.
There is an interesting feature of any multi-level control system in which
the successful completion of events at one level depends on successful
completion of previous levels. In terms of the ontogenic chain, we can label
the probability of successful completion of developmental processes
controlled by Group 1 genes as P1, the probability of successful completion
of developmental processes controlled by Group 2 genes as P1*P2, ..., and
the probability of successful completion of developmental processes
controlled by Group N genes as
P1*P2*...*P(n)*...*P(N).
Thus, the probability of successful completion of developmental processes
decreases with n.
This generates an important prediction in developmental biology. That is,
developmental processes responsible for the most important structure should
be initiated very early rather than late (i.e., controlled by Group 1 or 2
genes rather than by Group N genes).
At the end of the previous posting I asked the possibility of increasing
organic complexity by creating side chains. I will defer the answer to the
question and digress a bit into the evolution of the vertebrate head.
The vertebrate head seems to have jumped very suddenly out of evolutionary
history. A lot of bones appeared simultaneously without any precursors. How
should such a head come into being? Should its origin be of terminal
addition, or should it arise from a side chain?
It is obvious to most of us that the head, with the brain housed within, is
an important structure. It is itself the centre of a major control system
in the vertebrate body. Therefore it has to be very reliable. The
reliability can be improved in several ways:
1. it is made with the best possible material
2. the completion of its construction is not dependent on many processes
that occur before its construction.
Just these two items precludes the possibility of evolution of the
vertebrate head by terminal addition. First, which cell is of the best
genetic quality during ontogeny? It is no one but the zygote and its
immediate desendents because they are free of deleterious somatic mutations,
which accumulate gradually during ontogeny. Thus, the construction of the
head has to be initiated rather early. Secondly, the early initiation of the
construction of head also reduces the probability that the construction
process is perturbed by other developmental processes not directly related
to head construction.
(The same argument can also account for the early formation of germ line
cells, whose reliability is apparently important for the continuation of
life.)
One should remember that we are using "terminal addition" in the Haeckalian
sense, i.e. adding genes at the Group N-terminal. If we broaden the terminal
addition to include adding genes at the Group 1-terminal, then the above
arguments do not reject the terminal addition hypothesis of head formation
because the genes responsible for head can simply be added to the Group 1-
terminal to ensure the early initiation of head construction. This, however,
is against empirical evidence because early ontogeny of pre-vertebrates and
vertebrates is very similar.
Can we now conclude that the vertebrate head evolved from a side chain? We
have emphasized on rejecting the alternative, but is there positive evidence
demonstrating that the vertebrate head can indeed be traced back to a side
chain?
Xuhua Xia
Univ of Manitoba
(to be continued)
P.S. So far I have received only 7 e-mails encouraging me to continue
my random thoughts. I wonder if I should continue to post in the newsgroup
or should I send my writings directly to the 7 interested biologists.
