Why did hominids begin to walk upright and eat meat?

While this is still a point of contention among researchers, there are a few leading theories. First, as primates began to migrate away from forested areas, they needed to adapt to be able to move faster on the ground (else they would become easy prey). Because their dexterous hands were so essential to their survival, evolution toward a four-legged primate would be less likely than the development of more effective upright movement.