The first human, who tried to smash a stone with another stone to gain a sharp edge basically and eventually changed the world and its own evolution! Of course that time, it seemed only a fun thing to do or a clever trick in the eyes of others…
But human ancestors actually set off an avalanche… The skilful use of forearms cannot be derived directly from walking on two legs. If we had not started using our hands, they could have simply atrophied over the millions of years, as it happened in the case of many animals (e.g. T-rex). But because our ancestors discovered the advantage of hands, the work done by them made the functioning of the brain more complex as well.
The brain also affected the development of the body; moreover, increased brain activity required more energy, which could be provided by the consumption of more meat (protein). If we need more energy, it will be best gained from fresh meat. And why not stop being a scavenger when we can hunt?