Agriculture began about 10,000 years ago in an area called the Fertile Crescent, in modern-day Iraq, Syria, and Jordan. At the time, there were only about five million people in the world. Humanity had a substantial knowledge of hunter-gatherer techniques such as boatmaking, toolmaking, botany, and so forth, but anticipated changes were taking place: the Ice Age was ending, so ice was disappearing and regions like mountain ranges became traversable, while ocean levels were rising and inland seas drying up. Mass human migration was the result.
Because everyone was moving around, people naturally congregated in the boundaries between continents. The Fertile Crescent was such an area, located between Europe, Asia, and Africa. This is where both civilization and agriculture began.
Agriculture probably began in areas filled with animal dung, because there were a variety of seeds there and ample fertilizer for them to grow. The original crops were nothing like the crops people know today. The plants used were often husky, frail, or otherwise difficult to digest or grow. Only through many generations of selective breeding were their nutritional qualities optimized. Therefore, in a very important sense, all the crops we consider "natural" are actually genetically engineered.
The first basic crop was wheat, which has two main forms that still exist today. The first variety is called emmer, and still grows wild in the region. The second variety is not wild, but probably emerged from the crossing of emmer with another wild grass. This is bread wheat, which is still the most important crop on earth. Other plants cultivated during this time include peas, lentils, barley, linseed, and grapes.
It was not until 5,000 years ago that agriculture expanded outside of this nuclear zone. Farmers in different areas took to different crops. In the cold of Europe, oats flourished alongside wheat. In India, cotton emerged. In China, buckwheat was grown instead of wheat and barley.
Thousands of years ago, farming consumed most of humanity's time. Now, it is largely automated and only a small percentage of farmers are necessary to feed a developed nation.