I showed you how 6 million years ago the monkey was driven out of the tree by a geological phenomenon and thereby became biped (a small area in East Africa became dry so the trees disappeared)
The monkey was accustomed to eat leaves and fruit from the trees and those were no longer available, at least not enough to stay alive. Because of that same geological phenomenon, an abundance of tumbling stones also became available, for example large gravel grains.
That was perfect material to crack eggs and bones with. The monkey became a scavenger and that was facilitated by the open plains where many ungulates, deer and Buffalo's had come to dwell. Those animals died a natural death or fell victim to predators.
The monkey used those stones to obtain marrow from the bones and that was his main menu. Being an ape, his thumb was quite far away from his fingers and his thumb was also more puny than that of humans nowadays. But by hammering he became more agile in grasping those stones. The thumb became larger and moved towards the fingers and he could apply the baseball grip. This process took 3.5 million years to complete but then he was ready for the next big step: the manufacturing of cutting tools from stone.