The evolution of unusual cognitive capacity is the special feature of
the evolution of enlarged brains in birds and mammals.In mammals
the enlargement is correlated with evolution of the cerebral neocortex
of the forebrain,a structure that is seen only in mammals.The forebrain
in birds does not have the layered structure that defines mammalian
neocortex.It is called hyperstriatum because of its similarity in appear-
ance to the basal ganglia (striatum) in mammals.From its connections
to other brain structures,however,hyperstriatum appears to be func-
tionally homologous to neocortex (Karten 1991),and it is of additional
interest in light of discoveries of plasticity and lateralization of brain and
behavior in the control of bird vocalization (Arnold 1980;Marler,this
volume).Evolutionary Distance
Throughout this volume a good deal of evidence is presented on musical
expression in mammals and birds.Figure 12.1 is a phylogenetic tree (cf.
Carroll 1988) that indicates the relationships among those animals as
well as their evolutionary distance.Mammals and birds are very distant
relatives,and even within the mammals,long periods of independent
evolution separate groups from one another.179 Paleoneurology and the Biology of Music
Figure 12.1
The phylogeny of vertebrates.Note especially the evolutionary distance between birds and
mammals and among groups of mammals.Birds,cetaceans (e.g.,whales),and primates are
the groups emphasized in comparative studies of musicality.Fig.12.1