1. Cooking came first and then followed by decrease in jaw muscle size and strength. In the earlier days the homo genera are hunters and gatherers which used to eat raw meat, requires high muscle size and strength. As evolution being taken place, started cooking raw food to eat. Cooked food does not require much energy to eat so the muscle size and strength has slowly came down. In olden days inorder to hunt the animals requires high muscle strength later started to trap the animals, fishing and usage of modern weapons (bows and arrows instead of rock material) lessen the muscle size and strength simultaneouly as the genera started to utilize brain than the remaining mammals comparitively increased the size of brain than the jaw according to use and disuse of organs.
2. In the process of evolution nature started to select the improved brain size and decreased jaw muscle strengthed homo genera. Because they started to utilize other organisms by thinking ability and eating of cooked meat enabled them to decrease the jaw size. They started to live in caves and built houses with thatched roofs. So nature selected the the fittest mammal which started to use its brain for survival.
