I will explain my belief about it in a very layman way. Many eons back, few of the monkeys got the nudge to move from trees to grasslands in search of food, the basic need. On the way and during the course of many years, they formed groups, started communicating in sign language, discovered fire, learnt that cooked food gives more energy and is easy to chew , chipped stones to make elementary weapons and we had our ancestors. All the rest who were still on the trees are still eating bananas and eating lice.