[dr_dshiv]

The steppe people evolved to raid and invade agricultural communities; and those communities evolved to grow larger to fend off the raids.

"pressure from the steppe selected for the unification and scaling-up of agricultural societies into larger groups to more effectively counteract these incursions. This in turn would select for greater size in pastoralist communities, and also other neighbouring agricultural groups who were now relatively smaller and at a competitive disadvantage with their neighbours. This effect would be amplified by the diffusion of such military technology from the steppe"

From this awesome modeling paper https://www.nature.com/articles/s41599-020-0516-2