Behavioral reconstruction

Today I will be talking about the behavior of Ardipithecus ramidus, because this is a fossil and not a living animal, behavior will have to be assumed from physical characterizes of the fossil as well as environmental characterizes which I talked about in a pervious blog post. I will specifically be looking at the article: “Behavioral and phylogenetic implications of a narrow allometric study of Ardipithecus ramidus” and critically analyzing it.

The article compared the dimensions of the fossil bones of Ardi to different primates like chimpanzees, baboons, and humans. The authors of the study were hoping to find some correlation in the length on Ardi’s limbs and teeth dimensions to a living primate, which could give some insight into the behavior and lifestyle of Ardipithecus ramidus. They only compared Ardi to primates with weights close to 44.6-58.3kg, which is the estimated weight that Ardi would be if she were living today. One thing that I think would have made the study better is if the researcher had direct access to Ardi, instead they used the measurements given in the initial articles and analysis of the Ardipithecus ramidus fossil, some of which were estimated based on other lengths so they were not direct lengths.

In the discussion section of the article the authors concluded that “Body segment lengths of Ardipithecus showed greatest overlap with those of hominoids,” and not with the monkeys that they included in the study. This makes sense because Ardi is a hominin. All of the measurements were almost exactly in the middle of the chimpanzee and ape lengths and the human lengths. So far none of the findings surprised me because Ardipithecus ramidus is an intermediate hominin and possibly a stem hominin and could have bridged the gap between human’s last common ancestors with non-human apes. One thing that the study said that I do not agree with is that they concluded that Ardi was mostly a palmigrade walker and only practiced habitual bidealism. Multiple other studies about Ardipithecus ramidus’ pelvis have proven that Ardi was bipedal and but retained arboreal qualities. I think because the study only compared limb length is was limited in its analysis of the pelvis and that could be why they came to this conclusion.

Overall the forest environment in which Ardi lived in along with its arboreal and terrestrial abilities suggest that Ardipithecus ramidus would have been arboreal to gather food and may have walked upright short distances while carrying food. This also suggests that Ardi lived in groups, which would have helped them gather enough food to survive. As an intermediate hominin Ardi would have been comfortable in the trees or on the ground but unlike what the study suggests, I believe that Ardipithecus ramidus was fully bipedal, even if it was not quite as functional of an upright walker as modern humans. It is difficult to assume factors of behavior from fossils, however I think that most of the evidence points to Ardi living in at least small groups and gathering and carrying food from the trees to the ground in order to survive.

The image on the left shows how Ardipithecus ramidus may have walked, the image on the left shows and artist's reconstruction of how Ardi may have gathered food from the trees in groups.

Bibliography:

Sarmiento E, Meldrum D. 2011. Behavioral and phylogenetic implications of a narrow allometric study of Ardipithecus ramidus. HOMO - Journal of Comparative Human Biology [Internet] 62:75–108. Available from: http://uw8rw3ad9q.search.serialssolutions.com/?genre=article&issn=0018442x&title=homo - journal of comparative human biology&volume=62&issue=2&date=20110101&atitle=behavioral and phylogenetic implications of a narrow allometric study of ardipithecus ramidus&spage=75&pages=75-108&sid=ebsco:sciencedirect&aulast=sarmiento, e.e.

Image from: http://ardipithecusramidus.yolasite.com/resources/Ardipithecus_ramidus.jpg

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