I like to run. I occasionally take Eli out on runs, which we thoroughly enjoy. There are some differences in our running styles that is a reminder that we are, in fact, different species and our most recent common ancestor lived a very long time ago and probably resembled some rodent-shrew hybrid. I often think about the anatomy of running and one of the key unusual features about human anatomy is how our ankle is arranged. Eli’s leg is, in fact, more representative of living mammals right now and my leg is the unusual one. If you look at his leg, you notice that his femur is relatively small compared to humans, and his tibia & fibula are also shorter than a humans. The major difference, which is what gives us the stability to balance on two legs while propelling is the angle that the tibia & fibula meet the calcaneus & talus. On him, they are elongated and give him torque and spring. On me, they are short, load and impact-bearing bones. For me to “walk” like Eli, it would be like walking on my toes. I tried it this morning and tripped after two steps (despite popular belief, I am not a ballerina). Some runners promote running on one’s toes, saying that it is optimal and less injury-prone, but I am not convinced. I think the loss of stability and reduction in efficiency by using so many different muscles to absorb impact rather than using our well-adapted heel is significantly less efficient.
Anyhow, I’ve often wondered what adaptations and selective pressures led to primates developing such an unusual ankle. I came across a Ph.D. dissertation by Jeremy DeSilva (from University of Michigan) that described the how the angle of the tibia/fibula meets calcaneus/tarsus in living chimps and gorillas are different and pose a hypothesis related to tree-climbing. The hypothesis seems to be that tree-climbing selects for smaller angles and presumably the load-bearing probably selects for an optimal length of calcaneus as well. I think humans are still quite distinct because our calcaneus is impact-bearing as well. Chimps run with front knuckles on the ground, and sometimes they walk upright; and when they do, they put their calcaneus on the ground like us — though it is awkward and slow compared to running.
I think about these things because, as a long distance runners, we need to optimize our form to minimize risk of injury and maximize energy usage. So we need to think about the sources of “spring” in our running steps. Certainly Eli gets a lot more spring from his calcaneus than I get from mine. Each joint and tendon is a source of spring and there is an optimal stride length and form to store energy in elongated tendons that is released on the step — stretching after running helps strengthen the tendon, but I noticed that I loose “spring” if I stretch before a run. Also — It takes me a lot longer to get up to speed than it does Eli. However, once I get going, I can go at running speed for a lot longer. It takes me a lot less energy to maintain speed; the stability lent by smashing more bones into the ground means I can maintain forward momentum through my hips without having to transfer as much energy across as many different joints (and losing it) at each step. We also have vastly different metabolic storage/release systems and temperature control mechanisms. But I was mostly thinking about our ankles because of publication of a recent discovery.
In a collaboration among the Chinese Academy of Sciences, the American Museum of Natural History (in NY), Northwestern University, Northern Illinois University (Chicago), and Carnegie Museum of Natural History (Pittsburgh), archaeologists and evolutionary biologists have exquisitely described the oldest fossil of an extinct primate and named it Archicebus achilles. The species name is a shout-out to the morphology of its ankle. While the rest of the skeleton puts it in a clade with the living Tarsus (pictured is one of only 7 living species), the shape of its ankle sufficiently resembles that of Anthropoidea (chimps, apes, and hominids). This, among loads of other data, put it at the base of the Tarsiiformes branch. So this places the newly discovered ancient little guy at the very base of his clade’s branch, making his most recent common ancestor with all the apes and chimps be the same common ancestor as ours. I wish I could re-post the picture of the cladogram from Nature, but it’s copyrighted, and I encourage anyone with a spare afternoon to skim through the Supplementary Data and look at their amazing Figure 14.
I suppose I shouldn’t be surprised that the website “Answers from Genesis” has taken this new discovery and spun it bizarrely as evidence in support of a single creation event of all species and that “evolutionists” are wildly adjusting theories to fit this in. The fact is that this discovery fits in well with the currently accepted models. They seems to think that we will find a transition species, as if we could confidently say that an individual specimen actual is the a common ancestor between two branches. Because of the vast time, distance, and number of generations represented in the fossil record, all science can actually say is that we share a common ancestor. They don’t seem to grasp this concept. They seem to think that modern chimps led to modern man, when evidence really says that chimps are the living species with which we share a common ancestor that lived more recently than any other living species. There are lots of extinct species with whom we share more recent ancestors, but unfortunately they aren’t around. I often wonder what civilization would be like if some of them had survived and created cultures. Would we compete with them in running races?“Answers with Genesis” actually reproduced this artist’s rendering from the Nature News article without crediting Nature Publishing Group, which (as of June 25), would charge a non-profit, academic website $138 USD to reproduce. I inquired earlier whether answerswithgenesis.com paid for a license. Update (June 26) – the copyright holder of the artist’s rendering is the article’s author Xijun Ni, not NPG.