I was looking through the SciTech Daily website (a good place to go for new reading in a whole range of science areas) when I saw the link to an article on the evolution of running in Homo. Followed it, read the article – & thought, this is really interesting.
The article describes research on the efficiency of walking and running in humans. It notes that the Achilles tendon linking calf muscles to the heel is essential for energy-efficient running. Chimps and gorillas don’t have this long tendon, and the research team comment that it would be very interesting to know at what point in our evolutionary history the Achilles tendon evolved:
“But if, as seems likely, early humans lacked an Achilles tendon then whilst their ability to walk would be largely unaffected our work suggests running effectiveness would be greatly reduced with top speeds halved and energy costs more than doubled.
“Efficient running would have been essential to allow our ancestors to move from a largely herbivorous diet to the much more familiar hunting activities associated with later humans. What we need to discover now is when in our evolution did we develop an Achilles tendon as knowing this will help unravel the mystery of our origins.”
No Achilles tendon in great apes? Another search took me to a blog entry (scroll down to the posting entitled “Marathon Man”) that includes an image of human & chimp let anatomy (the original is from a paper in Nature). You’ll see that in humans, the Achilles tendon links the calf muscle to the tarsal bone in our heel. But in chimps, the muscle extends right down to the tarsal. Why is this difference significant? To find out, I read the Nature article (reference at the end of this post).
In humans, the Achilles tendon acts as a shock absorber and also stores energy. It stretches as your foot comes down (the braking phase), gaining elastic potential energy. Then, it releases that energy through recoil as you push off from the ground again. That is, the tendon is acting like a spring – one that can save up to 50% of the metabolic cost of running through reducing the work done by the muscles themselves. This makes Homo quite good at endurance running, something that would be essential for active hunting out on the savannah.
It’s not just the Achilles tendon, of course. There’s a whole suite of adaptations that may be related to running as a means of getting around. The question is, did these adaptations arise during the evolution of bipedal walking, or are they specifically related to running? I’ll summarise that discussion in the next post.
Reference: D.M. Bramble & D.E. Lieberman (2004) Endurance running and the evolution of Homo. Nature 432: 345-352