The story of human evolution is that of a tree-dwelling ape spending more time on the ground. This change in lifestyle presented us with new opportunities and exposed us to new selection pressures, driving the evolution of new traits. One of these was the ability to run quickly and efficiently. Simply running doesn’t require that many changes to an animal which can already walk upright. Even our “primitive” ancestors like Lucy could’ve probably run; the trick is running fast and without exhausting yourself after a few minutes.
3.5 million years after Lucy and her kind had begun to drop down from the trees we have developed that ability and can now run quickly for long periods of time (at least in theory. A fondness for haribo has hampered my running capacity). This is a useful skill, allowing us to escape from predators, catch prey, win medals and so forth. We’re now so good at it that some hunter-gatherers use their endurance to chase after animals for hours until their prey collapses from exhaustion and can be easily killed (Bramble and Lieberman, 2004).
Running is also handy from a scientific perspective, since only an animal that spends a lot of time on the ground (like modern humans) will evolve to be good at it. So when we find our ancestors look like they lived both in the trees and on the ground, identifying that they were good runners can show they were spending a good chunk of time on the ground and thus well on their way to the human way of life.
Unfortunately for those of us curious as to how fast, efficient running arose (or as to when the human ground-based lifestyle emerged) many of our adaptations for running are squishy. We have a suite of muscles, tendons and ligaments that make us good runners, but all of these decay over time. Luckily, there are a few circumstances in which some evidence of this soft tissue can survive.
Tendons connect muscles to bone so that when the muscle contracts it pulls the bone. This is how your body moves: your biceps contract, pulling the forearm upwards. Sometimes the bits of tendon that connect to the bone will actually begin to turn to bone themselves, creating “Sharpey’s fibres”. Since bone does last for millions of years this means that some evidence of tendons can reach us in the form of fossilised Sharpey’s fibres (Sellers et al., 2009).
The main tendon associated with running is the Achilles tendon, which attaches onto your heel (because anatomical terminology are nothing if not creative. The big hole the spine enters/exists the skull through is called the foramen magnum, which is Latin for “big hole”). The Achilles tendon is the anchor point for the biggest calf muscle, called the triceps surae, which is responsible for tilting the foot downwards and lifting the lower leg up off the ground. Both of these movements are important in running (Sellers et al., 2009), but that’s not the only reason the Achilles tendon is critical.
The Achilles tendon
The tendon is also very elastic, so the leg will naturally snap back to its original position. This means you don’t have to spend energy moving the leg with other muscles, making the whole process a lot less energy intensive. The elastic nature of this tendon means that a large, human-like one reduces the energy required to run by over 75% and increases the possible top speed by over 80%! The other great apes (who haven’t evolved to run) don’t have such a large Achilles tendon (Crompten et al., 2010).
The simulation showing that the Achilles tendon improves running ability and the results of that simulation. (A) showing how normal (i.e. elastic) tendons increase speed, (b) showing how normal tendons decrease energy cost
The list of reasons why the Achilles tendon is amazing goes on for some time. For example, the forces associated with running (where you have the entire weight of the body slamming onto the ground repeatedly) mean it has to be strong. In fact, it’s so strong that you could hang one of those cartoon “one tonne” weights off the end of one and it wouldn’t break! However, I think I’ll leave it there: the key point is that running fast and efficiently require a large Achilles tendon. Therefore we need to look for Sharpey’s fibres on the heels of our ancestors to figure out whether they are good at running.
These fibres were found on Australopithecus sediba, which lived in South Africa ~1.9 million years ago (Zipfel et al., 2011). This means that Au. sediba was the first of our ancestors we’ve found with a strong Achilles tendon, and thus were the first decent runners. This also means they were living like humans and spending most of their time on the ground, as well as running like them.
The heel bone of Au. afarensis (right), Au. sediba (middle) and humans (left). The asterisk shows where the Achilles tendon would attach. Note the similarity between Au. sediba and humans.
However, since evolution takes a while to work its magic the ancestors of Au. sediba would have been living on the ground (and running) too. This behavior selected for the development of a large Achilles tendon, which had fully evolved by the time Au. sediba lived. Unfortunately the primitive tendon present in such an ancestral creature would be hard to spot, preventing us from pinpointing the specific species which began jogging towards the modern-human lifestyle.
Further, heel bones are quite rare in the fossil record so it may be that earlier species had Sharpey’s fibres and we simply haven’t found them (Zipfel et al., 2009). The fibres themselves are also small, so they could’ve been damaged over the past few millions of years. These confounding factors means that all we can really say is that we had become good runners by 1.9 million years ago, but it may have developed earlier.
Nonetheless, this does mean that running and a terrestrial lifestyle developed in the ape-like Australopithecines, from which our genus (Homo) is descended. Clearly our “primitive” ancestors aren’t quite as different from us as they first appear. They were living on the ground like us, running like us, even making stone tools like us. It would seem modern humans aren’t as distinct as we previously thought.
References
Bramble, D. M., & Lieberman, D. E. (2004). Endurance running and the evolution of Homo. Nature, 432(7015), 345-352.
Crompton, R. H., Sellers, W. I., & Thorpe, S. K. (2010). Arboreality, terrestriality and bipedalism. Philosophical Transactions of the Royal Society B: Biological Sciences, 365(1556), 3301-3314.
Sellers, W. I., Pataky, T. C., Caravaggi, P., & Crompton, R. H. (2010). Evolutionary robotic approaches in primate gait analysis. International Journal of Primatology, 31(2), 321-338.
Zipfel, B., DeSilva, J. M., & Kidd, R. S. (2009). Earliest complete hominin fifth metatarsal—Implications for the evolution of the lateral column of the foot.American journal of physical anthropology, 140(3), 532-545.
Zipfel, B., DeSilva, J. M., Kidd, R. S., Carlson, K. J., Churchill, S. E., & Berger, L. R. (2011). The foot and ankle of Australopithecus sediba. Science,333(6048), 1417-1420.
