exists.Control of behavior (and experience) involves extensive neural
circuitry in the brain,the function of which is becoming better under-
stood, but we know relatively little about its evolution.The basic
element,the neuron,is perhaps best understood,and like sensory cells,
it must have been remarkably stable in function.Much of our knowledge
of neuronal physiology comes from studies of single nerve cells in inver-
tebrates,which have been separated from vertebrates for more than 500
million years.Fundamental aspects of a neuron’s operation must have
appeared before that separation and were evidently conserved in the
face of other evolutionary developments.
Neural networks,which control complex behavior such as the condi-
tioning of extending or contracting a body part,have been studied in sea
slugs (invertebrate mollusks) in which only a few dozen nerve cells are
involved.This provided important insights into the nature of learning
(Kandel 1967;Byrne 1987),and such learning appears to follow common
principles in sea slugs,locusts,pigeons,and rats.That their neural basis
may be the same encourages one to expect homologous features in the
development of neural control,even of complex behavior (Macphail
1993).Like the neuron,these fundamental adaptations of neural circuitry
for the control of complex activity appeared early in the history of life,
and unless they evolved independently in vertebrates and invertebrates,
they must have been conserved during subsequent evolution.Therefore,
important homologies may exist with respect to behavior and experience
among rather distantly related species.
Important uniformities are observable at still higher levels of analysis.
If one analyzes parts of the brain in mammals to determine how much
their relative sizes were modified in different species,one reaches the
surprising conclusion that some uniformities are impressive across
species.For example,figure 12.2 shows the relationship between two sub-
structures of the brain and the brain as a whole in seventy-six species of
mammals (data from Stephan,Frahm,and Baron 1981).
The upper half of the figure shows the relationship between the size
of the cerebellum and of the whole brain,and the lower half shows the
relationship between the basal ganglia and the whole brain.The sizes of
these two major structures involved in motor performance and in con-
ditioning and learning are related in a very orderly way to the size of the
whole brain.I have named a few species to indicate the diversity for
which a single rule operates for the size of the brain and its parts.Notice
that the homology is with respect to the rule,the equation,that relates
the sizes of parts of the brain to the whole brain.It is easy to imagine
relatively simple genetic instructions concerning growth of the brain and
its parts that determine the rule and that have been conserved during all
of mammalian evolution.183 Paleoneurology and the Biology of Music