Following on from last week’s installment about the pelvis, I’m going to take a look at the adaptations of the thigh bone (femur) to upright walking.
One of the most important requirements of being able to walk efficiently is keeping your body’s centre of mass (CoM) above your base of support (essentially the size of the area of ground your feet enclose when standing; fig. 1). This is especially important in upright walking since the base of support is so much smaller than for a quadrupedal animal.
Here’s a fun(?) diagram of the femur so everyone’s on the same page with anatomical terms:
The femur helps keep our CoM efficiently within our base of support through a property known as its bicondylar angle, which is illustrated in fig 2.
The bicondylar angle is created by altering the angle of attachment of the shaft of the bone to the femoral condyles, which articulate with the tibia to form the knee joint. The presence of a bicondylar angle in australopithecines indicate that they were efficient bipeds as it brought their feet closer to the midline of their bodies meaning they wouldn’t have had to sway from side to side while walking to keep their CoM within their base of support as modern chimps have to.
Hopefully all of you will have noticed that the australopithecine bicondylar angle is even more pronounced than in modern humans. Does this mean that they were more efficient at walking than we are? Probably not. What we have to remember is that australopithecines were shorter than we were (the famous ‘Lucy‘ fossil of Australopithecus afarensis from around 3.2 million years ago was only 3’6” or only a little over 1 metre tall [Jungers, 1988]). This means that to get their feet close to the midline, their shorter femurs would have to have been more angled than ours are.
This ties in with a feature of the australopithecine pelvis that I mentioned last week; namely that Lucy had more highly flared iliac blades than we do, giving a greater lever length to the small gluteal muscles aid balance. To me, these features don’t suggest that Lucy was better at walking and standing than we are, simply that the greater lever length (more flared iliac blades) was necessary to support a femur which was further from the vertical. Without a greater lever length, Lucy’s muscles would have had to work harder than ours to hold an upright posture or balance on one leg during walking.
Keeping with the theme of the bicondylar angle for a while, we’ll look at the effects of upright walking on the femoral condyles (mentioned above). In chimps and other non-bipedal animals the lateral condyle is quite small as it doesn’t bear a significant amount of weight in normal locomotion. In us and our upright ancestors, the lateral condyle is relatively quite a bit bigger than in modern chimpanzees (fig. 4).
The bicondylar angle comes into this as it essentially creates a weight-bearing column of bone which runs from the pelvis, through the top of the femur and into the tibia through the lateral condyle (fig. 5).
Figure 4 also shows that the condyles of the femur are more closely associated (have greater congruence) with the articular plateau of the tibia below it which contributes to stability of the knee joint.
It may have struck you that once our ancestors made the switch to upright ground-based walking their leg bones would have had to put up with carrying a lot more weight than their more arboreal predecessors. This fact was not lost on evolution either, and adaptations can also be seen in the anatomy of the femur where it articulates with the knee and with the hip.
Greater congruence is also a feature of the femoral head which is much larger in modern humans than it is in chimpanzees (fig. 6)
Greater congruence is advantageous to a weight-bearing role in two ways. In a very mobile joint such as the hip, the socket (acetabulum) of the pelvis wraps around the head to help prevent dislocation. The other role is to spread the weight over a larger area, thereby reducing the stress (force per unit area) experienced by the femur thereby reducing the chance of fracture.
So there you have it: one bone can tell us a lot about how our ancestors moved, and because it is by far the heftiest bone in our body, it’s quite common to find chunks of femur in the fossil record. But that’s for later posts.
Next week: something’s afoot.
Jungers, W. (1988). Lucy’s length: Stature reconstruction in Australopithecus afarensis (AL 288–1) with implications for other small bodied hominids. American Journal of Physical Anthropology, 231, 227–231. Retrieved from http://onlinelibrary.wiley.com/doi/10.1002/ajpa.1330760211/abstract
Neter, F. Mont Alto: The Pennsylvania State University. Available from: http://www.ma.psu.edu/ [Accessed: 21/02/14].
Shefelbine, S. J., Tardieu, C., & Carter, D. R. (2002). Development of the femoral bicondylar angle in hominid bipedalism. Bone, 30(5), 765–70. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/11996917