Locomotion

This week I will be discussing the postcrania fossils and locomotion of Ardipithecus ramidus. I will also be looking at the significance of Ardi’s locomotion and what it means for human evolution. I will only be looking at one article today “The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking” by Owen Lovejoy, Gen Suwa, Linda Spurlock, Berhane Asfaw, and Tim White. I will be looking at the anatomical evidence they present as well as how they interpret it.

In order to understand the way an ancient homin moved it is absolutely critical that some part of the postcrania is recovered. Fortunately a portion of the pelvis, femur, and foot of Ardipithecus ramidus were preserved, meaning that scientists have a pretty good idea of how Ardi would have moved. Researchers found an almost complete left os coxa, a part of the right ilium, and a fragment of the distal sacrum. All of the pieces were damaged and were carefully reconstructed before they could be analyzed.

One of the biggest questions that the researchers wanted to answer was if Ardipithecus ramidus was bipedal. In order to determine this they compared the reconstructed pelvis of Ardi to modern chimpanzees and to the Australopithicus afarensis fossil “Lucy”. They found that the ilium were very broad mediolaterally and shaped a lot like the ilium of later Australopithecines and humans, however the pelvis is not as broad as Lucy’s pelvis. Something that confused scientists was that the ischium looks very ape-like. One aspect of the pelvis that is very ape-like is the long superior ischial ramus, which is actually longer than found in any known Australopithecine. Combined together with the fact that lordotic recurvature in the lower spine was also enhanced the researchers concluded that Ardipithecus ramidus was bipedal, but still retained some easy movement arborealy. The pieces of femur and foot bones that were found also indicate bipedalism, however the foot has a divergent big toe just like apes. The foot and ankle are not as flexible as ape’s feet and the wrist and hand bones of Ardi are not suited for the knuckle-walking that apes prefer.

This image shows as comparison of pelvis characteristics of Ardipithecus ramidus to other modern and ancient taxa. 

The findings of Ardipithecus ramidus are extremely significant. Not only do they prove that a hominin can be bipedal with a divergent big toe, they also tell a lot about the last common ancestor between humans and apes. Unlike what was previously thought, many of modern ape characteristics are not in fact primitive, but derived. This was determined for example “it had been thought that the lateral spiral pilaster of apes was primitive,” but the Ardipithecus ramidus’ femur shows characteristcs that looks like human’s femur and not ape’s femur as would be expected if the ape morphotype was more primitive.

While the postcrania fossils of Ardi answer a lot of questions about human evolution, they bring up just as many questions. It is still not clear exactly how much Ardipithecus ramidus used its bipedal ability, or why it evolved. While Ardi could walk upright, its physiology was not suited for walking long distances, carrying heavy loads, or running very fast. The scientists concluded that Ardi spent much of its time in the trees, but moved short distances by foot.

The information in this article is very detailed and at times hard to understand. The evidence they present seems clear and prove that Ardipithecus ramidus was undoubtedly bipedal at leas some of the time. The article was only written about on individual, which could be an outlier to the species as a whole. Until more postcrania from other individuals is compared to these findings a more accurate interpretation of the findings would be difficult. Overall I enjoyed the article and the images they provided were very helpful for understanding the complicated topic.

Join me next post when I attempt to create a behavioral reconstruction of Ardipithecus ramidus!

Picture and article from:

Lovejoy CO, Suwa G, Spurlock L, Asfaw B, White TD. 2009. The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science [Internet] 326. Available from: http://science.sciencemag.org/content/326/5949/71.full