Answer:
Humans were nomads. Their wandering was mainly in search of food and shelter. When they found that certain places had an abundance of food – plant food or animals for hunting, they stayed on. They observed the plants, their growth and when these food sources diminished, they grew plants themselves, from the seeds they collected. This is how agriculture came about.
When humans stopped their nomadic ways and stayed together, it paved the way for formation of the human social structure. Hierarchies were established in due course.