Quite a while ago, I wrote a monster double post on something called the Chimpanzee Referential Doctrine, over which a two-sided debate continues to rage. The old guard of palaeoanthropology firmly believe that we can tell a lot about our ancestors from observing the anatomy, environment and behaviour of modern chimpanzees, while more and more people are adopting the viewpoint that our earliest ancestors were distinctly un-chimp-like. There’s also a question which constantly does the rounds of people curious about evolution or those trying to push Creationism: “if we evolved from monkeys/chimps then why are there still monkeys/chimps?”
In the next few posts, I hope the answer will become clear: we evolved from something that was neither human-like nor chimp-like, but gave rise to both.
One of the main issues surrounds how upright walking evolved if our ancestors had the same skeletal anatomy as the modern chimpanzee, which can only walk upright for short periods of time and has to use a bent-hip, bent-knee posture (BHBK). Analysis of modern chimpanzees walking upright has shown BHBK requires huge amounts of energy to keep quadriceps, hamstrings and gluteus muscles tensed constantly (Sockol et al., 2007; Wang et al., 2003). Our straight-legged posture doesn’t require this constant muscle activation.
(My other aim for the next few posts is to make the pun in the title of this series a little more obvious – it can actually work on quite a few levels, I’ve realised.)
Upright walking: the anatomy of the pelvis
Figure 1 demonstrates that the top of the pelvis (the iliac blade) is rotated backwards to different extremes in a variety of ape pelvises, and is not (I repeat, NOT) meant to imply an evolutionary sequence from the chimpanzee anatomy to the human anatomy. The backwards rotation (dorsiflexion) of the lower part of the pelvis relative ot the upper helps position our centre of mass above our feet. In addition, it allows our gluteus maximus (discussed below) and hamstrings to fully straighten (fully extend) our legs at the hip, something which chimpanzees cannot do.
Being able to fully extend our hip joint means that when we walk, our supporting foot ends up behind our body just before we push off for the next step. This is important because the backward direction of the push-off (instead of downward) propels us forward (instead of upward), increasing the efficiency of our straight-line walking.
As can be seen in figures 1 and 2, the chimpanzee pelvis is taller, narrower and shorter front-to-back than in australopithecines, humans and some extinct apes (this last point will become more important in later instalments of this series).
The tall pelvis of the chimpanzee is unsuitable for continued upright walking for two further reasons. First, the long pillars of bone connecting the iliac blades to the lower part of the pelvis (from the sacro-iliac joint with the spine to the hip joint, or acetabulum) would accumulate strain when the entire weight of the torso passed through them from the spine at the top to the hip joints at the bottom and would be at high risk of snapping.
Secondly, the iliac blades which are at the top of the pelvis are flat, and the attachments for the gluteal muscles face backwards. In humans and australopithecines the iliac blades are curved towards the front, and the muscle attachments face backwards and sideways. This means that our gluteus maximus muscles (the ones that are the muscular mass of our buttocks) still face backwards while the other two gluteus muscles (medius and minimus) face sideways and lie at the side of your hip. In this position, the medius and minimus can act to pull our leg sideways or ‘abduct our leg at the hip’ (try standing on one leg and moving the other one away from it sideways).
While you’re there, notice that the exact same muscles on the other hip are also tensed, helping you to keep your balance. In fact, it is this role that make the gluteus medius and gluteus minimus so important in upright walking.
In chimpanzees, all three gluteus muscles face backwards, which is great for providing power to straighten the hip when climbing up a near-vertical tree trunk, but useless for stabilising the pelvis in upright walking. So, in addition to using BHBK, chimpanzees also have to sway their bodies from side to side with each step in order to keep their balance.
You may have noticed in fig. 2 that the australopithecine and human iliac blades are also flared outwards. This is a further adaptation to efficiency in upright walking, since it increases the distance between the muscle origin on the iliac blade and its point of action on the femur. Just like a lever, this increases their efficiency meaning less effort is required to balance on one foot while the other is in the air, as occurs when we walk.
The sharper-eyed amongst you may also have spotted that the australopithecine pelvis is even more flared than those of the modern humans. Does this mean that they were better at walking than we are?
Possibly, but that is something to discuss next week!
Lovejoy, C. O. (2005). The natural history of human gait and posture. Part 1. Spine and pelvis. Gait & Posture, 21(1), 95–112. doi:10.1016/j.gaitpost.2004.01.001
Lovejoy, C. O., Suwa, G., Spurlock, L., Asfaw, B., & White, T. D. (2009). The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science, 326(5949), 71–71, 71e1–71e6. doi:10.1126/science.1175831
Sockol, M. D., Raichlen, D. a, & Pontzer, H. (2007). Chimpanzee locomotor energetics and the origin of human bipedalism. Proceedings of the National Academy of Sciences of the United States of America, 104(30), 12265–9. doi:10.1073/pnas.0703267104
Wang, W. J., Crompton, R. H., Li, Y., & Gunther, M. M. (2003). Energy transformation during erect and “bent-hip, bent-knee” walking by humans with implications for the evolution of bipedalism. Journal of Human Evolution, 44(5), 563–579. doi:10.1016/S0047-2484(03)00045-9