Why did early humans begin to walk on two feet?

Many people ask themselves how and why do we walk on two feet, and when did this transition happen. Like many other parts of history, it will never be known for sure but there are many theories.
One theory is that climate is what made the ancient people walk on two feet, the change in climate in Africe forced the ancient people to walk on two feet. In the time of the ancient people Africa was practically one big forest. But over time the forest became dryer, so in time it became a high grassland. The animals the humans used to hunt, migrated because of their food sourece. Naturally the humans followed their animals. Because of the large amounts of walking they were doing, they saw how walking on two feet is more efficient, faster and requires less energy.
Another theory claims that the hominids changed their diets so their food was high up in tries. Because of this they needed to stand on two feet because their reach would be higher.
There are many other theories, including theories about hominids being able to carry more food that way which relates to the fact that women were attracted by males with lots of food. But we will never know for sure, the most convincing theory for me is the theory that they needed to be faster and follow their food when it migrated because it is scientifically most realistic.