On Our Two Feet

Why do early human began to walk on two feet?

There are a lot of reasons why they started walking on two feet.
One reasons was that they have to stand up when they want to use their tools because they have to hold it into their hands.
 Another reason is that they have to stand up to look over the high grass, the grass in Africa was 5 inches high( that's very high) to see what is there or to go somewhere or when they want to catch some animals for their food.