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.