Eating leaves

Another bout of overactive imagination.

I was wondering why cant humans eat only leaves and survive. And I dont mean spinach and the kindd but simple tree leaves like oak leavers etc. Well Our ancestors i.e monkeys still survive on leaves and fruits. So we can aslo survive on leaves,shouldnt we. There seems to be tribes which eat one particular tree leaves as part of the diet. I kept on googling but I couldnt find any study or report on a person who just eat leaves.

Can I survive only on leaves,maybe if I add as much spices as the leaves.

-Amit