This week sees the FINAL UPDATE in the Long-Standing Debate series – congratulations on sticking around for a whole 6 million years of evolution!
Carrying on from last week, the only aspect of our bipedalism left to discuss is whether there are any fossil features which reliably indicate whether the owner of the bones was a distance runner in real life.
I’ve already discussed the angle of the glenoid of the scapula in terms of function and in the archaic and the transitional hominins. In Homo erectus and our own species H. sapiens the glenoid faces laterally (sideways) rather than being tilted superiorly (upward). This makes it much harder for us to hold our arms above our heads for long periods of time, or to generate powerful movements while doing so. A large contributing factor to this difference may be down to the fact that we lack a shoulder muscle when compared to chimpanzees (fig. 1).
The absence of the atlantoclavicularis muscle in humans means we have much greater freedom of movement at the shoulder, something which seems to be very important in balance when running, and even walking – just think how when the right foot swings forward, it is the left arm which swings forward at the same time.
Another anatomical feature also constributes to this flexibility in humans and that is our narrow waist. A shorter pelvis and a longer spine in the lumbar region help to separate our ribcage from our pelvis, and a ribcage which gets narrower towards the bottom all help increase flexibility to aid balance. We humans also have much larger attachment areas for our spinal erector muscles and our gluteal maximus (11 and 12 in fig. 1c), both of which are critical in stopping our body tilting forward too far when we run. Again, this makes it easier for us to balance.
Not just a balancing act
The swinging motion of our upper body may oppose the swinging motion of our legs and hips and therefore help us balance, but it also helps to preserve forward momentum at higher speeds. Preserving momentum is all about saving energy from one step and using it to propel ourselves forward into the next step, which has obvious benefits in terms of the amount of energy we use when running.
However, the swinging motion of our upper body on its own does not have a large enough effect on efficiency to make running really favourable in terms of energy – what we need are tendons.
Tendon is the tough but slightly elastic tissue that connects muscle to bone, and the most important in terms of running efficiency is the famous and very large Achilles tendon in the ankle. The Achilles tendon is stretched when we plant our foot during running, and stores a huge amount of energy with each step. When we tense our calf muscles for the next step, we stretch the tendon even more. When we begin to relax the muscle, the energy stored in the Achilles tendon is released far more quickly than the muscle could ever contract. In effect, the elasticity lets us move more quickly than we would be able to if tendons were completely inelastic, and saves energy to boot.
A well-heeled species
The Achilles tendon inserts into the calcaneus, or heel-bone, which has to resist the high forces described above, and withstand repeated slamming against the ground during walking and running. Naturally, you’ll remember reading in earlier posts (particularly concerning the archaic hominins) that certain foot bones are either more ape-like or human like depending on which part of the hominin tree you analyse, and one of these bones is the calcaneus. The modern human calcaneus is relatively very large to withstand all the punishment it gets. Other aspects such a flat face towards the rear of the bone and protruding bits of bone on the bottom face are also associated with increased forces from habitual walking and, later, running.
Most important here, though, is the length of the calcaneus compared to the rest of the foot. Fig. 2 shows the length of the calcaneus which is behind the ankle joint (white) and the distance from the Achilles tendon to the ankle joint (red) which allows the calf muscle and Achilles tendon to act as a lever to bend the ankle.
The comparison that Raichlen et al. make is between the calcanei Neanderthals and modern humans to investigate how efficient each was at running.
According to lever principles, the foot is represented by the lever arm (red) and the load arm (the rest of the foot in front of the ankle). A short lever arm will mean that muscle force is transferred inefficiently to the rest of the foot during walking and running. In humans, the red line is shorter compared to rest the of the foot than in Neanderthals. So far, so bad for efficiency – Neanderthals could produce more powerful movements.
But remember what I said earlier about running?
“An elastic Achilles tendon is stretched when we plant our foot during running, and stores a huge amount of energy with each step.”
In this situation, the ‘rest of the foot’ is the lever arm and the red line is the load arm. Suddenly, energy transfer from foot to heel to Achilles tendon is more efficient in modern humans than in Neanderthals, who had a longer red line. This makes for more energy storage in the tendon.
“Aha!” You may have cried, “but that energy needs to get back out, and that’s where Neanderthals are better than us”.
Well remembered, but I purposely only told you half the story. Yes, Neanderthals are built to favour strong movements of the foot by the Achilles tendon, but the modern human setup actually allows faster movements of the foot. Just think of a medieval trebuchet (YouTube). Here, a short lever arm moved by a weight moves a much longer load arm very quickly. In humans, a short lever arm powered by the Achilles tendon moves the long foot very quickly. So we get speed more efficiently, which is far more beneficial when you’re trying to run long distances.
But why were Homo sapiens running long distances when the Neanderthals weren’t? That’ll just have to wait for next week.
In the meantime though, thank you so much for sticking with me through the long-standing debate surrounding the evolution of upright walking.
How many levels of the ‘long-standing’ pun have you spotted? Answers on a postcard, please. Failing that, the comments section will do just fine.
Bramble, D. M., & Lieberman, D. E. (2004). Endurance running and the evolution of Homo. Nature, 432, 345–52. doi:10.1038/nature03052. Freely available here!
Raichlen, D. A, Armstrong, H., & Lieberman, D. E. (2011). Calcaneus length determines running economy: implications for endurance running performance in modern humans and Neandertals. Journal of Human Evolution, 60(3), 299–308. doi:10.1016/j.jhevol.2010.11.002.