Adjusting to life on two legs
What changed when we began to walk on two legs?
The switch to modern human locomotion goes along with a set of changes in our skeleton, tendons, ligaments and muscles. We are adapted for walking, but we are also adapted for upright running. Compared to other running animals, humans are poor sprinters but outstanding long-distance runners. We stay cooler and tire less quickly than quite a few animals that are prey for hunter–gatherer tribes. Some tribes, including those in the Kalahari Desert, still catch their meat by running down animals such as deer and antelope.
One of our bipedal ancestors was Australopithecus afarensis, which lived between two and four million years ago. Modern humans’ anatomy has changed, making running easier for us than it would have been for Australopithecus. These modifications include changes to the head, shoulders and spine, a longer trunk and legs, shorter forearms and larger, more muscular buttocks. We also have a longer, more elastic Achilles tendon and have undergone changes to the heel bone and big toe.
There was also a complex series of changes in the bones of the pelvis, including it becoming narrower, which probably gave increased running efficiency. Because babies pass through the pelvis, humans had to enter the world earlier and earlier in gestation as their brains increased in size. Otherwise, birth would have been too risky for mother and child. Our newborn helplessness, and long dependency as infants, may come from the shift to an upright stance.Lead image: