About 6.5438+million years ago ~ 2? Ten million years ago, apes were mainly distributed in tropical and subtropical forests. Like chimpanzees today, they live a arboreal life. Later, as the climate gradually changed to dry and cold, large forests became savanna, which greatly changed the living environment of apes. They were separated because of their different adaptability to the environment. Most apes are extinct. Some apes retreated from the edge of the forest to the depths, continued to live a arboreal life, and gradually evolved into modern apes. Others were forced to descend from trees to the ground, where they lived and eventually evolved into humans. In the process of living on the ground, they can't run fast to avoid the enemy because they don't have fangs and claws to attack and get food. Under such conditions, apes often walk only on their hind legs, but they liberate their forelimbs and gradually.
So, the answer is: the forest has disappeared, and the apes have to live underground. Because of the change of environment and self-form, generation after generation develops in the direction of walking upright.