Once our ancestors began cultivating land for food, they were running on a wheel, but never fast enough. More land provides more food. And more food means more children born and fed. More children provide more help on the farm and more soldiers. But this population growth creates demand for more land, which can be won and held only through conquest and war. Put another way, the shift to agriculture was accelerated by the seemingly irrefutable belief that it's better to take strangers' land (killing them if necessary) than to allow one's own children to die of starvation.