Why is there a difference between the maxillary (jaw) angle, teeth, and palate of chimps and humans?
Sexual selection. Teeth are more important for intimidation purposes in chimps. We smile, but it's not intimidating.
Talking. Our mouth is a compromise between what can bite/chew and what can talk.
The relationship between the evolution of bipedalism and skull shape
Bipedalism allowed the easy carrying of tools, including the carrying of crude spears while hunting. Spears increased success while hunting. Bipedalism did too, with the development of "persistence hunting" in which the prey becomes heat-exhausted more quickly than the hunter.
Increased success in hunting meant the hominins could enjoy better quality food -- meat -- thereby making development of bigger brains possible. The bigger brained hominins were better thinkers, making better weapons and planning better hunts or even raids on other groups of hominins. There was probably a kind of arms race in which the groups with better tacticians were more likely to survive to reproduce, and thus bigger brains proliferated.
The bigger brained better thinkers thought up more uses for tools, and then made the tools. With the ability to slice meat with knapped stone knives, and the ability to mash tough vegetation and seeds, having big tough teeth became less important. Also, with cooperation becoming important in groups, sexual dimorphism diminished -- a displaying of teeth was no longer used to intimidate rivals. Hominins with smaller teeth were able to survive and reproduce, with no disadvantage. Cooking played a role here too. Boiling vegetables increased the nutritional value while decreasing the amount of chewing needed, thus a decrease in mouth size was possible. Molars didn't need to be so big