Why did early humans begin to walk on two feet?

Over time, people have asked this question many times, and a lot of anthropologists, and archaelogists have different ideas, and opinions about how we evoloved into bipedals.
The first theory is that environmental changes that occured on our planet, caused the early humans to develop certain adaptations, including walking on two feet. One of these changes is forests turning into savannas and dry grasslands. Long ago, Africa was completely covered in forests. However, it kept becoming dryer and dryer, and evntually those forests turned into dry grasslands. The animals that the early humans hunted didn't have enough food in the savannas, so they migrated to other areas. Humans didn't have any other food, so they followed the animals. However, walking on all-fours was not that efficient, so eventually, humans started walking on two feet, because, that way, they could travel further distances, they could go faster, and this way of walking didn't require as much energy. One of the other reasons early humans had to adapt to environmental changes, is so that they could see over the very tall savanna grass. Back then, humans were very short,and if they walked on all-fours they wouldn't see anything in front of them, except the grass. So, they decided to stand up, and find an easier of way of following their food. 
The next theory is that homonids started eating different foods, and therefore needed different adaptations. Some anthropologists suggest that early homonids needed to stand up on two feet to hunt animals, as well as pick up fruits from very high trees.
One of the thoeries that was first thought of by C.Owen Lovejoy, was that walking on two feet helped early hominids find mates. He thinks that male homonids started walking on two feet, so that they could gather food, and therefore attract other female homonids. When males walked on two feet, they could carry more tools, and gather much bigger amounts of food. Also, walking on two feet could have been very good for the males who engaged in social displays.
Overall, no one will ever know for sure how we became bipedals. There will always be more theories and more ways of thinking about how we came to stand upright, today.