Why Did Early Humans Start Walking On Two Feet?

There is several reasons why early humans started to walk ton two feet.One reason is that they didn't have balance on two foot. Another one is that they couldn't see the front or around and they needed that to be aware of what was happening near them. The third reason is that they could move faster and escape from danger much more easier. Also a reason is that they collected food this way because they could see where to search and find food.
The reason that I found that was most realistic for me was that they collected food on two feet because they needed food and it was easier on a biopedal position.

How does the use of fire demonstrate prehistoric people's ability to adapt to their environment?

FIRE
 The use of fire demonstrate prehistoric people's ability to adapt to their environment because they show a way to provide light, warm weather so they don't freeze outside, cook their food like meat or animals that they found or hunted when they were scavenging.  
It helps them when they have fire to create light and use it because they can go around search for food and also helps them see their families.
Fire helps prehistoric people to be warm because it gets cold at night and they don't want to freeze to death.
It shows prehistoric people abilities by knowing how to make a fire.Maybe using two sticks, rocks or something else to create a fire. It also shows that they are smart and can find a way to survive in the nature with the wild animals.