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

hypothesis. Boldly put, the hypothesis states that the suite
of primate characteristics represents an adaptation by a small
arboreal mammal to stalking insect prey, which are captured
in the hands.
Cartmill sought individual elements of the primate suite in
a range of other species. For instance, chameleons have
grasping hindfeet, which they use to steady themselves when
approaching insect prey on slender branches. Some South
American opossums show similar behavior, capturing their
prey by hand or mouth. And, of course, the convergence of
the eyes is found in many predatory animals that need to be
able accurately to judge distance, such as cats, owls, and
hawks.
“Most of the distinctive primate characteristics can thus 
be explained as convergence with chameleons and small
bush-dwelling marsupials (in the hands and feet) or with cats
(in the visual apparatus),” concluded Cartmill. “This implies
that the last common ancestor would have subsisted much 
as modern tarsiers, the mouse lemur, and some lorises do
today.” These species should not be considered “living fossils”
because, like humans, they are also the products of 60 mil-
lion years of evolution. It is simply that their ecological niche
resembles the niche occupied by their ancestors.
Cartmill’s visual predation hypothesis has recently been
challenged by American primatologist Robert Sussman. He
points out that many primate species locate their prey by
Partly because of the emphasis on vision, primate brains
are larger than those found in other mammalian orders. This
increase also reflects a greater “intelligence.” In this charac-
ter, the lemurs, lorises, and other prosimians are, however,
less well endowed than monkeys and apes. Tied to this
enhanced encephalization is a shift in a series of life-history
factors: animals with large brains for their body size tend to
have a greater longevity and a low potential reproductive
output. For instance, primate gestation is long relative to
maternal body size, litters are small (usually one), and off-
spring precocious; age at first reproduction is late, and inter-
birth interval is long. “Primates are, in short, adapted for slow
reproductive turnover,” observes Martin.
If we think of humans as animals with particular physical
and behavioral habits, this discursive definition describes us
as well, apart from the fact that we do not live in trees. For
instance, a quarterback would not be able to stand behind his
offensive line and accurately throw a deep pass, unless he
were a primate. Hindlimb-dominated locomotion, grasping
and touch-sensitive hands, stereoscopic vision, and intel-
ligenceaall are required in that activity, and all are general
characteristics of primates. More historically, when hominins
first began making stone tools, they were not “inventing 
culture” in the sense that is often used, but merely applying
primate manipulative skills to a new task. Although it is 
true that even by primate standards Homo sapiens is particu-
larly well endowed mentally, our generous encephalization
merely represents an extension of just another primate trait.
Later we will return to some of these and other themes,