According to Scientific American, “Human molars and jaws responded to the invention of cooking by getting smaller. Fossils thus indicate that controlled-fire cooking probably originated about 1.9 million years ago”.
We all learn in Human Species that molars decrease in size due to the act of cooking food. The cooked food is ultimately softer and easier to chew which leads to the evolution of smaller molars.
Researchers therefore compared the molars and body sizes of extinct hominids with modern humans and other primates. Turns out that Homo erectus and Neanderthals had, and modern humans have, molars that are smaller than would be predicted by looking at general primate evolution. The finding is in the Proceedings of the National Academy of Sciences. [Chris Organ et al, Phylogenetic rate shifts in feeding time during the evolution of Homo]
Edited From Scientific American