When 10 million years ago in East Africa, the forests changed by a natural phenomenon into Savannas, our living predecessors who, like the current chimpanzees still lived in trees, had to adapt their lives. They were forced to search for their living together in a different way.
They walked more upright all the time and started to look like a human and therefore are called hominid (= anthropomorphic).
The Savannas were populated by buffalo and deer. They were hunted by lions. The gnawed corpses were left but ungulates also died a natural death.
Sometimes those bones were cracked by the jaws of the lion or hyena and the hominid soon discovered that there was nutritious stuff inside, i.e. marrow. He started to winkle out that with his fingers but soon discovered that he could crack a bone between two stones (do you remember? He could devise new manipulations indeed, according to the rule that you have read here before).
So the new-fangled biped became scavenger and benefitted very well from this.
He became bigger and stronger and was able to make cutting tools from the same boulders in the long term.
© Max Farjon 2018