The climate in Africa about 3 million years ago was much wetter than today. The conditions favored rich plant and animal diversity. Tall trees allowed primates to live off the ground and eat leaves and berries without fear of predators. When the climate changed and got much dryer, the trees were smaller and sparser. There is some different opinion about the environment in Africa at those times. Warm and wet with many lakes and rivers. It attracted birds and various small aquatic inhabitants. Primates had to go down to the ground and travel short distances, looking for food. Because they had been in the water, searching for food, they had to stand straight up on their hind legs.