In the beginning, our chimpanzee-like ancestors probably couldn't walk very well on two feet, but they spent most of their time up in the trees.
At night, they slept there and, during the day, they would head back to the trees anytime conditions became dangerous on the ground.
Bipedalism involved major transformations of the body;
•The whole body had to straighten up from the bent and curved posture of the chimpanzees,
•The legs lengthened while the arms shortened,
•The rib cage became smaller while the digestive tract got shorter,
•There were structural changes in our hands and feet; the big toe, for example, had to evolve from a grasping position to become aligned with the foot about 2.5 million years ago.
Some say this was the moment when humans stopped climbing up trees, but I disagree.
I think they still needed the safety of the trees to sleep for 2 million years afterwards.