We do not know the definitive explanation for how language ability
arose, but there are plausible hypotheses. Intermediate stages
may be reached by gradual changes. For example:
The larger brain size of primates arose before language; it may have
come from adaptation for functioning in social groups or for finding
food.
Human brain size has evolved to be unusually large through neoteny;
the rapid brain growth of childhood is maintained for a longer
time. It may have arisen in conjunction with the evolution of
language or before it.
Before language, some communication already was done via gestures
(which itself probably arose as mimetic imitation) and via
vocalizations. Spoken language ability may have built on those
vocalizations, or it may have been transferred from gestural
language, or both.
Once vocal protolanguage began, the vocal tract and neural
connections for producing and controlling speech would have begun
evolving. Speech probably began in Homo erectus more than a
million years ago.
Once language becomes a central part of human behavior, language
acquisition comes more readily because of the Baldwin effect; the
plasticity of learning allows natural selection for language ability
to proceed more quickly (Baldwin 1896).
Language is obviously useful, so it is not hard to see how it could
have provided selective advantages. In fact, the bigger problem is
explaining why chimpanzees did not evolve language. Language was
probably primarily an adaptation to a social structure that was (and
is) far more complex than that of other primates.
Deacon, Terence W., 1998. The Symbolic Species: The co-evolution of
language and the brain. New York: W.W. Norton.
Johansson, Sverker, 2002. The evolution of human language capacity.
Master's Thesis, University of Lund.
http://home.hj.se/~lsj/langevod.pdf
(esp. pp. 65ff)
Pinker, S., 1994. The Language Instinct. Harmondsworth: Penguin.
Tattersall, Ian, 2001. How we came to be human. Scientific American
285(6) (Dec.):
56-63. Excerpted from The Monkey in the Mirror, Harcourt, 2002.