The Human Brain
Through the use of molecular biology it is thought that the hominidae family first appeared about 5 million years ago. Based on this time frame it is believed that an African Hominoid lineage was present shortly before that time, approximately 10 myp, which contained the common ancestor to both the chimpanzee and human. The split into proto-chimpanzee and proto-human occurred during the last million years of the Miocene epoch. (Changeux and Chavaillon pg. 61). The fossils, especially those of the skull, from this time frame are limited. This leads to difficulty in proving differences in brain formation.
Fossils of the hominoid cranium are not available until 2 million years after the proto-human lineage begins. The lack of cranial fossils for 2 million years is a problem. We do not know what took place during this time. The first available cranial fossils are those of A. afarensis. The mean endo cranial capacity was 413.5 cm3, which means that its brain size was that of today?s African great apes (Changeux and Chavaillon pg. 65, table 4.1). With the limited fossils available and the apparent brain size of today?s African great apes there is no proof of significant differences in brain functionality.
One study done by Ralph Holloway on the endocaste of the Hadar AL 162-68 skull fragment lead to a different theory regarding significant differences in brain functionality. Hadar ?concluded that the lunate sulcus would have had to be, according to the position of the interparietal sulcus, in a more posterior position than in Pan brains?. This means that A. afarensis ?had an expansion of the parietal association cortex and consequently a brain reorganization (Changeux and Chavaillon pg.106)?. Brain reorganization implies that A. afarensis had a significant difference in brain functionality.
The cranial capacity of A. africanus has a mean of 440 cm3, A. robustus a mean capacity of 530 cm3 and A. boisei a mean capacity of 463.3 (Changeux and Chavaillon pg.65, table 4.1). When the limited sample size is taken into account, is there a significant difference? ?The modern human inter-racial mean is of the order of 1350 cm3, which is 3.52 times that of the chimpanzee, 2.68 times that of the gorilla value, and 3.33 times the orang-utan value, the comparative inter-hominoid index values for Australopithecus species are seen to have hardly increased at all (Changeux and Chavaillon pg. 67).? If we compare the modern chimpanzee, as modern human?s closest living ancestor, to the australopithecines we find that they ?show a small but definite advance over the chimpanzee in both absolute and relative brain size (Changeux and Chavaillon pg. 67)?.
With the advance of Homo habilis, 2.3 to 1.6 myp, we see significant advances in the brain. The mean capacity of H. habilis is 640 cm3. This is a 45% increase over A. africanus. The typically enlarged brain size of modern man, that is the ratio of brain to body mass that is a hallmark off the human species is present in H. habilis (Changeux and Chavaillon pg. 74&75). H. habilis indicates an enlargement of the cerebral hemisphere and ?detectable impressions of the superamarginal and angular gyri? are present for the first time in the hominid lineage (Changeux and Chavaillon pg.76)?. This ensures that two of the three basic neurological needs for language are present. The third need, the superior speech cortex, is in the superomedial surface of the cerebral hemisphere. This can not be detected on an endocast (Changeux and Chavaillon pg.76). H. habilis may have been the first to have language capabilities.
Using brain mass or cranial capacity as a measure of brain evolution is not universally accepted. Holloway states that it is not a useful measure only a statistic for estimating parameters (Jerison pg.388). Studying endocasts as if they are a brain and extracting and applying this information to possible brain development is also debated. It is debated that ?endocasts and brains do not correspond perfectly in any animal and correspond rather poorly to one another in all respects except size in most large-brained mammals, including man (Jerison pg.388)?. Another complaint about the use of brain mass as a measure is correlating mass with function. The idea being that you can not make conclusions about the structure and function of the brain by weighing it.
Though we have come along way in understanding the structure of the brain and its functions we still by no means have a perfect understanding of this complex organ. Using endocasts, particularly fragments of endocasts, to gain information on the structure and function of the brain is a risky proposition. But until we find more complete specimens or a more reliable measure, it is the only measurement we have. Science is speculation and is not perfect. The truth of science is always changing. We change or modify our theories or truths when new information becomes available. Mass implies more room for development a more complex thinking structure. These may be false implications or assumptions but until new information becomes available it is all we have.