Human paleontological evidence relevant to language behavior.

The paleoneurological evidence for human language origins and other cognitive activities is tantalizing, but uncertain given the often incomplete, fragmented, and eroded cranial portions of our fossil ancestors. Nevertheless, both the Taung and A.L. 162-28 endocranial portions, attributed to the earliest-known hominids (i.e. Australopithecus afarensis and africanus) evidence some cerebral organization beyond a typical pongid pattern, in that there appears to be a reduction in primary visual striate cortex, and thereby a relative increase in posterior and inferior parietal cortex. At 1.8-2.0 million years, there is clear fossil evidence for a Homo lineage showing a more modern and enlarged third inferior frontal convolution, expanded brain size (e.g., 750+ ml), and strong cerebral asymmetries identical to those known for modern Homo sapiens. Additional evidence of sexual dimorphism in the modern human corpus callosum, in which the posterior splenial portion is larger in females, taken in conjunction with known clinical and psychological evidence relating to cognitive task specialization, suggests that this dimorphism represents a biological heritage from past selection pressures for a dichotomous but complemental social behavioral set of adaptations to favor a division of sexual labors compatible with nurturing offspring with delayed maturation, prolonged growth, and a longer period of postnatal neural plasticity.