- Human ancestors have been consistently refining walking on two legs since the inception of their lineage 7.0 to 6.0 million years ago through numerous modifications to their entire skeletal anatomy at various phases of their evolutionary history.
- 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).
Highlights
Walking on two legs is a unique mode of locomotion that only humans use to efficiently move from place to place. This mode of movement is one of the hallmarks that distinguishes humans from all other mammals, including from their genetically closest primate relatives (chimpanzees and gorillas). Human ancestors have been consistently refining walking on two legs since the inception of their lineage 7.0 to 6.0 million years ago through numerous modifications to their entire skeletal anatomy at various phases of their evolutionary history.
One of the oldest fossils ever discovered dated at 3.2 million years old, “Lucy” (Australopithecus afarensis) provided scientists evidence of bipedal, upright walking by human ancestors. In this animation, see what similarities Lucy shares with modern humans.
Video courtesy California Academy of Sciences
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.
Nearly 3.7 million years ago, a group of Lucy’s species— Australopithecus afarensis—walked across volcanic ash, which was preserved and discovered in 1976. They show clear evidence of how modern humans walk today—the arched foot and push off by the big toe.
Image courtesy ASU Institute of Human Origins
While there are other explanations as to why bipedality evolved, it is possible that bipedality evolved for a combination of a variety of reasons including those mentioned above.
Written by Yohannes Haile-Selassie PhD
