Figure 3.

Left hemispheres of a chimpanzee and human brain illustrating different sulcal patterns. The chimpanzee has a lunate sulcus (L) at the rostral border of the primary visual cortex, which was lost during human evolution. The chimpanzee also has a fronto-orbital sulcus (fo) that delimits so-called “Broca's cap” (BA 44 and sometimes part of BA 45). Most of fo became buried deep within the brain as hominin brains enlarged. Consequently, the bulge on human brains that appears to occupy the same location as the orbitolateral swelling in chimpanzee frontal lobes, contains BA 47 and BA 45, in addition to two new sulci (the horizontal and ascending limbs of the anterior Sylvian fissure) that form two sides of the pars triangularis (BA 45). The fo, L, and sulci bordering BA 45 are exaggerated and reddened here.