In the upper jaw, the third premolar averages in surface area, the fourth premolar , the first and/or second molars (which are difficult to distinguish) , and the third molar . In the lower jaw, the third premolar averages , the fourth premolar , the first/second molars , and the third molar . The molars are the biggest of any known ape. Teeth appear to have increased in size over time. The premolars are high-crowned, and the lower have two tooth roots, whereas the upper have three. The lower molars are low-crowned, long and narrow, and waist at the midline—which is more pronounced in the lower molars—with low-lying and bulbous cusps and rounded-off crests.