However, the full suite of morphological—or form and structure—of changes related to walking on two legs in modern human ways was not complete until the time of Homo erectus about 1.8 million years ago.
One of the most provocative questions has always been why walking on two legs (bipedality) evolved only in the human lineage while all other mammals continued walking on all four (quadrupedality). A number of hypotheses have been proposed to answer this question. However, there is broad consensus that a selective pressure toward freeing the hands from locomotion and using them to carry food was probably the reason. Another possible reason is the decrease in forest and woodland habitats and expansion of more open grasslands making available resources extremely patchy and far apart from each other. This required traveling for longer distances between patches and walking on two legs for longer distances is energetically more efficient than walking on four legs.