• New World monkeys, such as the marmosets, spider monkeys, and howler monkeys

Figure 10.4 Primate dentition: Teeth are particularly
important in the reconstruction of primate phylogeny, for two
reasons. First, their extreme hardness means that they are the most
common item recovered from the fossil record, and hence provide 
a disproportionate amount of information about fossil species.
Second, teeth give very clear information about dietary habits
because the shape is strongly influenced by the type of food eaten.
By convention, dental formula is written as shown in the diagram.
This species (a siamang) possesses two incisors, one canine, two
premolars, and three molars (a common scheme in higher
primates). (Courtesy of John Fleagle.)

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onalack the short face, close-set eyes, reduced olfactory
apparatus, and large brains that arboreal life supposedly
favored.”
The British anthropologists valiantly defended their theory,
invoking ingenious and often inconsistent lines of argument.
In any case, the arboreal theory was modified and extended
in the 1950s by another British researcher, the eminent 
Sir Wilfrid Le Gros Clark. It continued to thrive for another
two decades, until Cartmill felled it in 1972.
In reassessing the arboreal theory in the early 1970s,
Cartmill applied biologists’ most powerful toolacomparative
analysis. “If progressive adaptation to living in trees trans-
formed a tree shrew-like ancestor into a higher primate, then
primate-like traits must be better adapted to arboreal loco-
motion and foraging than their antecedents,” reasoned
Cartmill. In other words, if primates are truly the ultimate in
adaptation to arboreal life, you would expect that they would
be more skillful aloft than other arboreal creatures. “This
expectation is not borne out by studies of arboreal nonprim-
ates,” he noted. Squirrels, for instance, do exceedingly well
with divergent eyes, a long snout, and no grasping hands 
and feet, often displaying superior arboreal skills to those of
primates. “Clearly, successful arboreal existence is possible
without primate-like adaptations,” concluded Cartmill.
If the close-set eyes and grasping hands and feet were an
adaptation to something other than arboreality, what was it?
Once again Cartmill used the comparative approach to find
an answer that formed the basis of the visual predation