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Design features for language and stone toolmaking (not tool use) involve similar if not homologous cognitive processes. Both are arbitrary transformations of internal “intrinsic” symbolization, whereas non-human tool using is mostly an iconic transformation. The major discontinuity between humans and non-humans (chimpanzees) is language. The presence of stone tools made to standardized patterns suggests communicative and social control skills that involved language.
In commemoration of P. V. Tobias’s 80th birthday, and the 80th year since Dart published his paper on Taung, newer studies on chimpanzee brains and newer fossil discoveries (e.g. Hadar 162–28, StW 505) make it appropriate to re-assess Dart’s original contribution. The matter of where to place the lunate sulcus has been a controversial issue, but the contrast between the Taung child’s endocast morphology, and that of pongids, particularly the chimpanzee, as well as the clear-cut lunate sulcus in a posterior position of StW 505 indicate that Dart was indeed right, and that the brain of australopithecines was reorganised prior to any size increase in endocranial volume. This reorganisation involved a relative increase in posterior parietal association cortex, the flip-side, as it were, of a relative reduction in primary visual striate cortex. It is most probable that the changes in cerebral organisation were an adaptive response to changing and expanding ecological opportunities for these early hominids. The latter development of any major size increase of the whole brain had to await the evolution of Homo lineages. Clearly, from at least three million years ago, the evolution of the hominid brain has been a mosaic process of both size increases, allometric and non-allometric, as well as critical reorganisational changes, of which the posterior placement of the lunate sulcus is proof of that change, which Dart had already appreciated in 1924–5.
Given the primate propensity to make noise, it is unclear why a manual gestural stage would have been necessary in the development of either language or right-handedness. Cortical asymmetries are present in australopithecines but become clearly human-like with the appearance of Homo about two million years ago, including Broca's cap regions. Stone tool-making is still our only empirical entry into past cognitive processes.
Although this author's (R.L.H.) disagreements with Harry Jerison are legion (e.g. Holloway, 1966,1974,1979), I have always found his ideas stimulating and thus of great value to my own work regarding human brain evolution. I believe we best honor Harry Jerison by taking his ideas seriously, whether or not we agree with them.
There do not appear to be any serious disagreements that the brain became reorganized as well as enlarged during hominid evolution, but there is considerable controversy as to when reorganization, particularly that relating to the reduction of primary visual striate cortex, Brodmann's area 17, had taken place. (Reviews of these questions can be found in Holloway, 1995, 1996.) Since the only way we will ever know for absolutely certain when this process occurred requires travel with a time machine and some histological sectioning of australopithecine brains, one might wonder why we are writing this paper. It is already apparent from the literature on early hominid brain evolution that a major controversy exists regarding the fossil australopithecine endocasts and their interpretation regarding that infamous landmark, the lunate sulcus. Falk (1983, 1985, 1986) interprets the paleoneurological evidence from the Taung and Hadar (AL162–28) endocasts as indicating that the lunate sulcus was in an anterior pongid-like position. Holloway (1981, 1983, 1984) interprets the evidence as suggesting a posterior, more modern-human like position.
Despite the ontogenetic allometric size effects that explain much of phyletic variation in brain components, the residuals of some structures indicates that mosaic brain evolution was an important factor in hominid evolution, and that reorganization of the hominid brain may have occurred as early as 3+ MY. Finlay et al.'s allometric technique masks residual variation around allometric trends, and the patterns of residuals relevant to species-specific departures from strict allometric trends.
Data do exist to support the fact that the corpus callosum
is relatively larger in women than in men. The corpus callosum is
an integral part of the brain, and contrary to Fitch & Denenberg's examples of “pseudostatistics,”
is not an extrinsic structure when determining its relative
If POT (parieto-occipital-temporal junction) reorganization came earlier in australopithecines than in Homo, it is likely that the selective pressures were different, and not necessarily directed toward language. The brain endocast evidence for the POT in A. afarensis is actually better than it is for early Homo.