Was Lucy better adapted to bipedalism than we are?

In Chapter 7, I review the controversies regarding the reasons we evolved into a species that walks on two legs. The theories include the need to free up hands for carrying infants, to be able to carry food long distances, to appear larger as a defense mechanism, to be more efficient in locomotion, and several others. In my view, none of the explanatory theories were very convincing and the bipedalism mystery persists. But there is a related question that also interests me. Was Lucy better adapted to walking upright than we are?

Lucy is the most famous Australopithecus fossil—maybe the most famous fossil of any kind. She lived about 3.3 million years ago in sub-Saharan Africa at a time when climate change was thinning out the trees. This required her predecessors, the Ardipithecines (whose most famous fossil is nicknamed Ardi) to occasionally leave their natural habitat in the trees and walk to a more distant tree. They were probably the first pre-human bipedal species in our lineage. By the time of Lucy, those distances became greater and there were numerous speculated reasons that the Australopithecines continued to advance bipedalism. Lucy’s foot was more “human-like” than Ardi’s in that it did not have an opposable big toe. Nonetheless, Lucy was still an ape, about 4 feet tall with a small brain who didn’t use tools. When asked about Lucy, her discoverer, Donald Johanson, said “Oh yes, she walked erect. She walked as well as you do.”

Although we don’t know for sure the exact line of evolution from these pre-human species to humans and finally to Homo sapiens, we do know that they were all upright bipedal walkers. We also know that upright posture doesn’t work all that well for Homo sapiens given the widespread back problems we have of all kinds: degenerative arthritis, narrow, bulging and ruptured intervertebral discs, sciatica, spondylolisthesis, spinal stenosis, and others. In short: a lot of back pain.  Did Lucy and her relatives suffer from the same back afflictions?

Probably not. There is one big difference between Lucy and Homo sapiens: our big brain. Lucy’s pelvis was rotated compared to an ape in just the right way to allow the appropriate muscle attachments needed for upright walking. But it was still a small pelvis that easily accommodated a small newborn head at childbirth. That pelvis had to get a lot bigger to accommodate a human newborn head. That is what caused the problems we have today.

C. Owen Lovejoy is the guru regarding the anatomy of locomotion in both living and extinct species. He states the following: “In one respect Lucy seems to have been even better designed for bipedal­ity than we are.”  That respect related to a pelvic design that became compromised as the human brain enlarged. This required changes in the size and shape of the birth canal, which had a negative impact on the mechanics of upright locomotion. Although these changes in the male are less pronounced than the female, even our male anatomy is less adapted to upright posture than Lucy’s—particularly in view of the larger bulk of a male. Lovejoy goes on to say, “The difficulty of accommodating in the same pelvis an effective bipedal hip joint and an adequate passage for a large infant brain remains acute, how­ever, and the human birth process is one of the most difficult in the animal kingdom.”

Since upright posture preceded the enlargement of the brain, perhaps had the sequence been reversed, humans would be quadrupedal today.