The earliest hominids survived on a diet based largely in plant foods. Most plants fall into one of two categores, which represent two different substances invovled in photosynthesis. These substances relate to enzymes contained within the plant that allow the plant to absorb and utilize Carbon Dioxide from the air.
C3 plants make up 95% of plants on earth and include rice, cassava, most trees, beans, potatoes, most temperate crops. C3 plants favor moist, cool, cloudy climates. C4 plants include some dometic crop plants like sugarcane, corn, and most grasses. C4 plants favor hot, dry, sunny climates.
As climate change caused the African continent to become hotter and drier, C4 plants began to dominate the landscape and humans therefore adapted and began to consume more of these plants. Another type of organism that widely consumes C4 plants are large, grazing herbivores. In addition to becoming C4 eaters, it was this shift that likely increased the amount of meat consumed in hominid diets as well as developed skills for hunting game.
Arjamaa, O and Vuorisalo, T. 2010. Gene-culture co-evolution and human diet. American Scientist 98: 140 – 147.