What nonhuman primates can, and cannot, tell us about the evolution of speech

Julia Fischer

Max-Planck-Institute for Evolutionary Anthropology, Leipzig, Germany

fischer@eva.mpg.de

This paper will provide an overview over some of the key issues that
have been studied in the vocal communication in nonhuman primates with
regard to the evolution of speech.  I will use Hockett's design features
of speech (Hockett 1960, _Scientific American_, 203, 88-96) as a
starting point to compare the communicative abilities of nonhuman
primates and humans.  A number of studies have addressed the question of
'meaning' in primate communication and have specifically examined
whether primate vocalizations refer to objects or events in their
surroundings.  Present findings suggest that listeners use calls as
predictors of events in their surrounding, while there is less evidence
that the callers intend to provide this information.  This form of
'functionally referential' communication has now been described for a
number of animal species, including birds, rodents, and carnivores. 
Although nonhuman primates may voluntarily control the _onset_ of
vocalizations, they lack the neural projections from the primary motor
cortex to the laryngeal motor neurons.  Consequently, they have little
control over the _structure_ of their calls.  Unlike songbirds or some
cetaceans, nonhuman primates appear unable to mimic novel sounds or
incorporate new sounds into their repertoire.  The apparent inability to
modify call structure also poses a barrier for the arbitrary pairing of
sound and meaning, one of the key features of human speech.  Nonhuman
primates also lack a combinatorial system to generate new meanings from
reordering the units of their communication, despite the fact that a
number of studies have shown that listeners' responses to calls may vary
with regard to the combination of different call types.  In sum, there
is a wide gap between the verbal production of human speech and the
vocalizations of nonhuman primates.  The findings highlight the
importance of evolutionary constraints that apparently operate on the
vocal communication of primates, a great deal of which can be attributed
to differences in the neural substrate underlying the production of
sounds.  However, current reports about the gestural communication in
nonhuman primates do not support the view that it is the lack of motor
control alone that prevents nonhuman primates from more elaborate
communication.  I will argue instead that limitations in social
cognition also play an important role in constraining the evolution of
speech.