A while back, Glenn claimed that information on Broca's area
obtained from fossil endocasts constituted powerful evidence of human
evolution. Reading through the Cambridge Encyclopedia of Human
Evolution (1992), I was interested to discover the following:
The sulcal markings associated with Broca's language area (see
main text) have recently been demonstrated in Homo habilis and
H. erectus endocasts. These folds do not appear on endocasts
of australopithecine brains. Does this mean that Broca's area
first appeared in the brains of early homo? Yes, if we define
it only in morphological terms. But if we are interested in the
language functions of Broca's area, the endocast evidence is
less compelling for several reasons. There is variability in the
presence and the position of these sulci in human brains and some
inconsistency in the correspondence of the landmarks with the
locations of language functions. More importantly, an homologous
area has been demonstrated in monkey brains, so the appearance of
these gyri and sulci in fossils does not indicate the appearance
of a totally new structure (p. 117)
Surprisingly enough, these studies show that the spatial distribution
of language functions vary from one persons brain to another. (p.120)
Language could not have been the result of the addition of a new
structure because it is controlled NOT BY A SINGLE STRUCTURE OF THE
BRAIN BUT BY A NETWORK OF INTERDEPENDENT CORTICAL AREAS, each
contributing a particular function. (p. 123 EMPHASIS mine)
So language is controlled by a number of different brain areas that
vary from person to person. Also, structures homologous to the Broca's area
structures (not found in australopithecines) are found in the brains of
monkeys not considered to be human ancestors.
How is this evidence for human evolution?
In Christ,
robert van de water
associate researcher
UCLA