Is he an anthropologist or something? What a pointless question. It’s much more important to understand the differences in root canal morphology in molars or variations of root configuration than why molars have 2 and 3 roots. But the answer is just millions of years of evolution and chewing. Whatever an organism needs to do to survive, the body evolves to support those actions. Anymore details or attempts to answer will just be theories since many things have happened to influence this. But maybe the lecturer was just trying to get you to think