The Origin of Primates: A Comparison of Arboreal Adaptation Vs. Insectivorous Traits.
The Le Gros Clark scenario of the origin of primates is characterized by the idea that primate morphology is adapted to an arboreal life style or life in the trees has dictated the development of the features that primates have in common. Clark goes on to say that these characteristics developed independently in a parallel manner from the different mammalian breeding stocks from which primates evolved (Clark, 1960). Conversely, Matt Cartmill has suggested very different ideas of how this set or suite of common primate features has evolved. Cartmill says that the features in question are adaptive for insect hunters, and other behaviors not necessarily linked with an arboreal lifestyle (Cartmill, 1993). Both ideas have merit but both can not be correct. First I will define the order of primates based on the suite of features in common then I will compare the origins of those features as laid out by Clark and Cartmill.
A definition for the order Primates based on phenotypic traits can be found in the morphology that is consistent with all primates. This suite of features is as follows: All primates have a post-orbital ridge and forward facing eyes. These are adaptive for depth perception which is useful for a variety of reasons and the ridge (or bar in Prosimians) seems to be adaptive as it protects the eye, which is by and large a primate’s primary sense. Another feature shared by primates their hand morphology. Primates have divergent thumbs or grasping hands and nails covering the ends of the digits instead of claws (except the callitrichids, which have claws). Yet another feature shared by all is a large brain size to body weight ratio compared to other mammal groups. It is the origin of these traits, which is at the heart of the difference in opinion in primate origins.
Clark’s view of these traits is orthogenic in nature. That is to say he believed that all of these features developed independently of each other in differentiated mammals to accommodate a shift to an arboreal existence. Clark says that it is not the suite of features that defines primates but the tendency to develop these traits (Cartmill, 1993). This is to say; if a species is going to live in the trees, the development of the primate traits is essential to that existence. This is an orthogenic view as it has taken a long time of slow change for each of the decendants of several different kinds of mammals to evolve into primates. Cartmill’s view is one of adaptive radiation. He contends that these traits evolved in a Darwinian sense leaving behind a more familiar phylogenetic map of speciation and that the suite of traits shared by primates is unique to their particular form of adaptation and not predetermined for all mammalian tree dwellers. In keeping with the comparative method, I will compare the two views per each adaptation.
Clark suggests that forward facing eyes is adapted for making travel in a three dimensional and complex intertwining branch system less difficult. Forward facing eyes allows for overlapping or stereotypic vision, which in turn provides primates with depth perception. Clark said this kind of vision is crucial in maneuvering the maze of the canopy. This does seem quite reasonable and was the predominate notion for decades until Cartmill posed the question of why don’t all arboreal mammals have these traits. Creatures such as squirrels have no difficulty in the trees even though they are wall-eyed and have claws. Cartmill says instead, based on the comparative method, that the success of visual based predation seems to be enhanced by forward facing eyes. One has only to look at cats and eagles for the basis of this comparison. In Cartmill’s view ancestral primates where nocturnal insectivores, like present-day Bushbabies. The fossil record supports this assumption. While forward facing eyes may increase depth perception it reduces parallax (Cartmill, 1993). This does not appear to be adaptive for increased awareness in a complex environment but rather more useful when the moment of capture is at hand. A cat doesn’t need well developed depth perception to decide that there are viable prey animals to hunt in the area but a cat does need depth perception to make the decision of when and how far to pounce in order to catch its prey. The same is true for an insectivore.
This same comparison can be applied to a grasping hand. Clark’s argument for an opposable thumb being adaptive to limb grasping is convincing but once again Cartmill invokes the squirrel analogy. Cartmill goes on to say that catching insects in ones hands can be easier facilitated with the addition of grasping hands. He also explains that fingernails instead of claws enhance the stealth capabilities that a primate would need in order to get close enough to catch one. In my opinion this is the weakest comparison of Clarks observations due to the fact that not all insectivores have developed this trait. In fact Marmosets have claws and they are insectivores. It is however in keeping with the overall insect hunting adaptation paradigm. In response to this criticism R.W. Sussman postulates that fingernails allows for better grasping in terminal branches to exploit a niche that an otherwise unequipped mammal might not be able to utilize (Cartmill, 1993). This seems a throw back to the arboreal argument. Even Cartmill suggests that further research is required to verify or deny his own explanation (Cartmill, 1993).
Both arguments rely heavily on the comparative method for their results. It would be easy to assume that Cartmill’s views are more accurate as they are derived of a more complete fossil record and his ideas are forty years newer and unencumbered by outdated ideals such as orthogenesis. However as Cartmill is not able to fully credit all of the features of primate morphology to an insectivorous ancestor and we do know that environment plays a large part in adaptive morphology we can not fully abandon arborealism as an explanation for present day primate features. It would seem that where insect predation may certainly have been an influence, even the major contributing factor to primate morphology as Cartmill would suggest, it stands to reason that the suite of features has been influenced over twenty million years of adaptation by the continued habitation of an arboreal environment.