Walking upright with two feet defines the bipedalism among the hominid and make them different from the four legs apes. This theory was given in 1920 when they found a small skull approximately three-million-year-old and it indicated that it held its head erect.
One of the reasons that made them stand in two legs might have been the change in the climate. Climate change made Africa a more seasonal environment, which became difficult for a hominid to find food. By walking with two feet they were able to travel fast and were able to use their hands to collect and carry food from one place to another.
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