In general, members of the Primate Order possess larger brains for body size than other mammals, with modern humans (Homo sapiens) evolving the largest brains. The Internal Carotid Artery (ICA) provides blood supply to the brain but there are distinct anatomical differences between the primate groups. While vascular and other soft-tissues are very rarely fossilised, evidence of the ICA passage is retained where it was encapsulated in bony tubes or as distinct grooves of the endocranial surface. The ICA evolved in primates from two main pathways in the auditory complex: the promontory artery branched from the cochlear space to supply blood to the brain while the stapedial artery branched from the obturator foramen of the stapes to provide blood to the cerebral meninges and the orbito-facial complex. Primate groups are recognised by ICA anatomy: Within the Strepsirrhini, Lorisiformes and Cheirogaleids both lack the stapedial and promontory arteries with the External Carotid Artery (ECA) supplying blood to the brain instead of the ICA; whereas, non-Cheirogaleids possess the ICA and stapedial artery but often retain a much smaller promontory artery, while the anthropoids (apes and monkeys) lack the stapedial artery entirely retaining only the promontory artery.
The evolution of these distinct differences was examined by comparing living and fossil primates and reconstructing the hypothetical phylogenetic pathways. The size differences between the stapedial and promontory arteries were compared to endocranial volume (ECV) to investigate the influence on brain size. Only the size of the promontory artery had a consistent correlation with brain size. There was no reported correlation between brain size and size of the stapedial artery, with the stapedial only correlating with size of the promontory artery. This suggests that throughout primate evolution, the trend for body and brain size increases caused the stapedial artery to become restricted as head size increased but size of the obturator foramen did not scale equally with the head. Most early primates evolved a reduction in the size of the stapedial artery and quickly accommodated an increase in the promontory artery allowing even greater blood flow to the brain and driving the encephalisation process and the eventual loss of the stapedial artery in anthropoids.