Evolution of the Miocene Great Apes

The small-bodied ape ran across the top of a tree branch, away from the anger of the dominant male of the group. In haste to get away, however, it had underestimated the thickness and strength of the branch, which gave way. The ape fell to the ground, breaking a forelimb; it yelled in pain. Above, the other apes started a commotion and ran across the branches in anxious movements. The yelling would surely bring carnivores to the small patch of tree cover, which was an island of refuge in the surrounding open country. Soon a lone carnivore appeared and saw its opportunity for an easy kill — it struck swiftly and surely, closing its jaws around the neck of the small ape, crushing its windpipe. Anchoring the head between its jaws, it dragged the ape away from the tree cover. The small apes in the trees could still be heard yelling and thrashing around in the small forest patch.

This sort of thing must occur frequently in tree-dwelling primates. Life in trees can be just as dangerous, in terms of injuries, to those that live in more open habitats. In the mid-1980s a Miocene small-bodied ape fossil was discovered in Kenya and allocated to a new genus, Turkanapithecus. The specimen, KNM-WT 16950, was remarkable for its state of preservation, with most of the facial anatomy retained in detail. Also preserved was a puncture mark, most likely from a carnivore canine, located just below the midface or snout. The death of this individual was surely the result of an encounter with a carnivore.

It was probably the mass extinctions of the dinosaurs around 65 million years ago that enabled the diurnal mammals to occupy "in mass" the now-numerous vacant niches, which had until now been closed to them. Among these mammals would have been the primitive and small-bodied primates (similar in appearance and size to squirrels), which appear during the Paleocene epoch. These archaic primates can be differentiated from other small mammals by their possession of numerous primate specializations, including more convergent orbits (stereoscopic vision), a postorbital bar (i.e., the orbit is closed off and not exposed to the temporalis muscle), increased grasping extremities, and nails as opposed to claws (see Fleagle, 1999). Cartmill (1992) suggests that these characteristics evolved as a result of the predatory nature of the archaic primates, for stereoscopic vision is usually observed in predator species that rely on vision to detect their prey, though undoubtedly increased stereoscopic vision would be an excellent exaptation for leaping behavior in primates (Crompton, 1995), or perhaps it was the other way round, that is, exaptation for predator behavior. Conversely, Cartmill notes that most arboreal nonprimate species are not defined by orbital convergence and, in addition, are defined by having claws on their digits to assist in climbing. In early primates, however, the increased ability to grasp by their digits (associated with nails as opposed to claws) is a likely requirement for grasping prey or other food objects, as opposed to assisting in climbing (see also Fleagle, 1999).

With the demise of the dinosaurs, the primitive primates were able increasingly to become more diurnal, and eventually many species adapted completely to life in the sun, depending on daylight feeding patterns. It is with the coming of the late Eocene/Oligocene transition, around 40-37 million years ago, that we witness within the fossil record the first true anthropoids. Most fossil anthropoid species from this temporal region have been discovered over the last 40 years or so in the Fayum depression of Egypt, southwest of modern-day Cairo (Figure 2.1). While earlier primate-like species have been documented in Europe, Asia, and even the New World, Africa appears to represent the place of origin for the earliest true primate species. It is at this time that we see a reduction in the snout of the primates, which can be correlated with a reduction in the olfactory apparatus (sense of smell). This would also enable a further enhancement of the three-dimensionality of these primates (see Fleagle, 1999; Kingdon, 2003). With the emergence of diurnal primate

Figure 2.1 ► Reconstruction of facial musculature of the Oligocene primate Aegyptopithecus zeuxis from the Fayum depression of Egypt.

species came an increased ability, in some species at least, for predatory behavior (e.g., insects), while in others it likely resulted in an adaptive shift toward larger, often harder fruits; all of which would conceivably result in an increase in body size/weight (Kingdon, 2003). The large number of archaic primate species (and genera) that have been identified so far from the Fayum depression alone supports the extreme range of biological diversity of the primates (and other nonprimate faunal groups). The primates of the Fayum represent the likely anatomical condition of the primates that eventually gave rise to the later hominoids, though whether these primates themselves are the ancestral population seems doubtful.

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