Continuing the Captain Caveman series…
3.7 million years ago, two Australopithecines walked across the mud at Laetoli in Tanzania leaving their footprints behind. These hominids were clearly bipedal.
The most famous of this type of hominid is “Lucy,” whose remains were found in Ethiopia and are 3.18 million years old. Lucy was 3 and a half feet tall and could both walk and climb. She is famously held to be the “missing link” between apes and humans, a “bipedal ape,” although as with all such things, there is plenty of disagreement.
Why did apes decide to come out of the trees and walk? It may be that an Ice Age in northern latitudes caused cooling and drying in Africa, with vast swathes of jungle turning into savanna. Exposed on the open plain, hominids found that walking made them better able to see and combat danger. Enlarging brains allowed them to devise survival strategies in this harsh environment; free hands allowed them to fling objects. Natural selection meant that the smarter hominids with better bipedalism survived. Mix all of this in a million year pot and you eventually have homo.
Bipedalism led to the “curse of Lucy.” The bipedal hominid’s pelvis had to bear a stronger load, which led to a narrowing of the birth canal. This meant that birthing became more painful and much riskier. Also, to fit through the birth canal, the baby’s head (brain) had to be small, which is why human babies are such mewling, puking incompetents. Protection of mother and baby in this environment necessitated male-female bonding. And here we are.