Answer-a way for early humans to reduce heat stress and regulate their body temperature. By standing upright, early humans would have exposed less of their bodies to the sun, reducing their overall heat load. Additionally, walking on two legs instead of four would have allowed for greater airflow around the body, further reducing heat stress. This hypothesis is supported by the fact that many other animals that live in hot environments, such as kangaroos and ostriches, are also bipedal. However, other factors such as the need for efficient long-distance travel and the ability to carry objects may have also played a role in the evolution of bipedalism.