One characteristic that all human species shared was a large brain. For example, a pig weighing 133 pounds has a brain size of 200 cc. A modern human weighing the same has a brain size of 1200-1400 cc. The brain is not only awkward to carry around, but it requires a lot of food/energy intake. While only weighing about 2% of the total body weight requires 25% of the total food/energy consumed.
Three million years ago, this axis was changing so that the Earth pointed away from the Sun for longer periods. This caused an overall cooling of the Earth, locking away moisture in ice at the North and South Poles. It also made the climate more seasonal. Temperatures in Africa dropped, and the air became stripped of moisture. Humid woodland shriveled away, leaving wide belts of open terrain in its place. Some animals that relied on their forests for their food died out, but others evolved to exploit other dietary resources.
Our ancestors who had previously relied on forest foods such as soft fruit started springing up all over the continent. In East Africa, a hominid called Paranthropus boisei became specialized so that it could eat tough-to-chew but more abundant plant foods such as nuts, roots and tubers (largely underground vegetables, the potato being a modern example). Another species also living in East Africa Homo hablis had a tougher time as its jaw and teeth were small and couldn’t eat the same tough plant food. But it would eat anything that it could get its hands on especially meat. But it was no hunter and had to wait its turn at a fresh kill until all the predators left. It is speculated that we learned how to specialize in the ability to crack bones using stone tools in order to get the marrow which contains a long chain of fatty acids that are vital for brain growth and development.
As our ancestors turned from a diet of fruit into meat eaters, we developed larger and more complex brains that enabled us to not only survive but become better adapted to a changing landscape.