[Nateman]

I think warfare is the answer. Like many other animals people are territorial and once one group was fighting another over land they had to feed the warriors. After the local game gave out they had to innovate or die, like war usually does. Some of the clever members figured out how to grow what they were already eating. Like most secrets generated by war , enemy spies got word and the secret spread like wildfire.

[SauronOfMordor]

Agriculture allowed supporting more people per square mile of territory. A more numerous tribe of farmers or herders can displace a less numerous tribe of hunter gatherers.