Saturday, April 02, 2005

A. afarensis vs. the Apes

All pictures: Top left A. afarensis Top right: P. troglodytes. Bottom left: P. pygmaeus Bottom right: G. gorilla

One of the most aggravating things one can hear, if one has any training in paleoanthropology, is that the australopithicines were nothing but glorified apes. So let's study the issue (hey, I have to justify the name of this blog, okay! Which means more hominids.) The first set of pictures below is a frontal view of A. afarensis, a chimp, an orang and a gorilla. The first thing you witll notice is how robust the apes are. There are strong brow ridges, in the case of ornag and gorilla there are large crests. Notice also the size of the canines in the three apes. The roots of the canines form canine juga which are very pronounced in the apes, less so in A. afarensis. The canine juga fade into anterior pillars. Overall, there are a lot of features related to the size of the teeth and the need for for muscles to work the teeth. However, the anterior pillars, canine juga and large canines are less apparent in A. Afarensis. Note also that in A. afarensis the zygomatics (the cheekbones) start much higher up than in apes. One other thing deserves notice. You will note in A. afarensis that you can see part of the brain case (this is an artifact of photography in the chimp picture). A. Afarensis has an encephalization quotient (a measure of relative brain size) of around 3.1 compared to 2.6 in chimps. Basically this is the beggining of the cranial expansion characteristic of hominid evolution.

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In lateral view quite a few differences become apparent. First, note the steeper line running from the (reduced) supraorbital torus to the top of the skull in A. afarensis. In chimps it is more rounded. In orangs and gorillas it is occupied by a massive saggital crest. Note, also, how robust the supraorbital torus is in chimps, orangs and gorillas. It is somewhat reduced in A. afarensis. Note againe that the zygomatics start higher up on the cheeks than in the apes. The apes have pronounced midfacial prognathism (snout sticking out) - A. afarensis is prognathic but not to the same degree. We are starting to see the reduction in prognathism characteristic of Homo sapiens. You can also see the large canines and canine juga in the apes. Towards the rear of the A. afarensis skull, where the zygomatic (cheekbone) joins the back of the skull you can see a small nuchal crest starting (actually a compound tempornuchal crest but lets not complicate things). The nuchal crest, as well as the sagital crest in orangs and gorillas, are areas of muscle attachment. Basically, the larger the crest the larger the muscle that attached to it. The nuchal area (basically the back area underneath the skull) is long and steep in A. afarensis. The ear (external auditory meatus) is similar in all four species. Finally, the ascending ramus of the mandible is wide and tall in all four species. This, along with the sagital and nuchal crests, as well as the anterior pillars are adaptations for large chewing forces.

lateral view 2 Posted by Hello

In basilar view, the first thing to notice is how long, straight and boxey looking the dental arcade is an chimps and gorillas (unfortunately I could not find a basilar view of an orang without the manible). In A. afarensis the dental arcade is somewhat rounded. Note there is a canine diastema (gap between the second incisor and canine) in A. afarensis and the apes. This is reduced in A. afarensis. Akthough you can't see it to well A. afarensis has the begginings of a bicuspid premolar. Apes don't. Also note that the incisors are more similar in size in A. afarensis (although not as much as in humans. The mandibular fossa (the area where the condyles of the mandible articulate with the skull - see the orang skull with attached manible) is flat in apes but has the begginings of human morphology. Finally, the foramen magnum (where the spinal cord enters the skull) is placed more underneath the skull in A. afarensis. Whereas it is not nearly so in apes. This is a trait characteristic of bipedal locomotion.
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Basically, what we have in A. afarensis is a perfect transitional fossil. Although, really there where several transitional fossils between apes and humans (A better way of saying it is that there is a continua of species leading from a comon ancestor, between apes and humans, to humans). I think it is a common misunderstanding that there is only one transitional fossil between apes and humans. First, becuse apes didn't evolve into humans, rather apes and humans had a common ancestor. The lineage then split with one, or more, lines evolving into apes and the other evolving into humans. That being the case, you can see a continua of species starting from, say, Ardipithicus ramidus and running through the various australopithicines (leaving the robust forms aside for the moment) to early homo and from there to anatomically modern humans. A. afarensis is one of these transitional species and is definatel not an ape.