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?

Plutarch

(I wonder)" by what accident and in what state of mind the first man touched to his mouth to gore and brought his lips to the flesh of a dead creature, set forth tables of dead, stale bodies, and ventured to call food and nourishment the parts that had little before bellowed and cried, moved and lived. How could eyes endure the slaughter when throats were slit and hides flayed and limbs torn from limb?  How could nose endure the stench? How was it that the pollution did not turn away his taste, which made contact with sores of others and sucked juices and serums from mortal wounds? It is certainly not lions or wolves that we eat out of self-defense; on the contrary, we ignore these and slaughter harmless, tame creatures without stings or teeth to harm us. For the sake of a little flesh we deprive them of sun, of light, of a life to which they are entitled by birth and being."