The answer is
Longitudinal waves.
The particle displacement in a longitudinal wave is
parallel to the direction of wave propagation. The ground particles simply oscillate
back and forth their equilibrium positions unlike transverse waves that do not
move along with the waves but rather oscillate up and down their individual
equilibrium positions.
A vert short answer is: people had time.
Before this, people were focused on not starving and everyone's attention was turned to gathering food. Noone could afford doing anything else than gathering food for themselves (and maybe their offspring) because then they would starve. But, with the surplus of food, some people could focus on innovations and those innovations (such as writing) gave them a higher status, which ultimately led to a stratification.
