Hunting

Learning to hunt was a decisive step in human evolution. Hunting necessitated communication, planning and the use of tools, all of which demanded a larger brain. At the same time, adding meat to the diet made it possible to develop this larger brain.

However, the side effect of meat eating was reduced weening periods for mothers which meant an increase in reproduction. Population increase and the spread of humankind across the earth was as a direct result of eating meat! 

But at what cost to the plant?