The mainstream view is that climate change is the main driving force of human biped walking  Millions of years ago, the area of forests in Africa began to shrink and the area of grasslands increased greatly  In an environment with few trees, the significance of biped walking is clear: standing up allows human ancestors to look beyond the lush grass to see predators and prey
The knee joints of early humans are strikingly similar to those of modern humans
 As a result, the massive increase in the area of grasslands has made the ancestors who are the most good at standing more likely to survive and pass on their genes