132 CHAPTER 6 | Macroevolution and the Early PrimatesThe Rise of the Primates
Early primates began to emerge during this time of great
global change at the start of the Paleocene epoch. The dis-
tribution of fossil primates on earth makes sense only when
one understands that the positions of the continents to-
day differ tremendously from what was found in the past
(see Figure 6.3). As noted earlier, during this period North
America and Eurasia were connected in the supercontinent
called Laurasia. South America, Africa, Antarctica, Austra-
lia, and the Indian subcontinent—previously joined together
as the supercontinent Gondwanaland—were beginning to
separate from one another through continental drift. Africa
was separated from Eurasia by a narrow body of water.
On land, dinosaurs had become extinct, and mam-
mals were undergoing the great adaptive radiation that
ultimately led to the development of the diverse forms
with which we are familiar today. At the same time, the
newly evolved grasses, shrubs, and other flowering plants
proliferated enormously. This diversification, along with
a milder climate, favored the spread of dense, lush tropi-
cal and subtropical forests over the earth, including North
and South America and much of Eurasia and Africa. With
the spread of these huge belts of forest, the stage was set
for the movement of some mammals into the trees. For-
ests would provide our early ancestors with the ecologi-
cal niches in which they would flourish. Fossil evidence of
primatelike mammals from the Paleocene forests has been
found in North America and Eurasia. See Figure 6.5 for a
full timeline of primate evolution.
One theory for primate evolution, the arboreal
hypothesis, proposes that life in the trees was responsible
for enhanced visual acuity and manual dexterity in pri-
mates. Misjudgments and errors of coordination, leading toHowever, the mammalian trait of maintaining con-
stant body temperature requires a diet high in calories.
Based on evidence from their teeth, scientists know that
early mammals ate foods such as insects, worms, and eggs.
As animals with nocturnal habits, mammals have well-
developed senses of smell and hearing relative to reptiles.
Although things cannot be seen as well in the dark as they
can in the light, they can still be heard and smelled.
The mammalian pattern also differs from reptiles in
terms of how they care for their young. Compared to rep-
tiles, mammalian species are k-selected. This means that
they produce relatively few offspring at a time, providing
them with considerable parental care. A universal feature
of how mammals care for their young is the production of
food (milk) via the mammary glands. Reptiles are relatively
r-selected, which means that they produce many young at
a time and invest little effort caring for their young after
they are born. Though among mammals some species are
relatively more k- or r-selected, the high energy require-
ments of mammals, entailed by parental investment and
the maintenance of a constant body temperature, demand
more nutrition than required by reptiles. During their
adaptive radiation, the fruits, nuts, and seeds of flowering
plants that became more common in the late Cretaceous
period provided mammals with high-quality nutrition.Though popular media depict the coexistence of humans and
dinosaurs, in reality the extinction of the dinosaurs occurred 65 mya,
while the first bipeds ancestral to humans appeared between 5 and
8 mya.The appearance of the true seed plants (the angiosperms) provided
not only highly nutritious fruit seeds and flowers but also a host of
habitats for numerous edible insects and worms—just the sorts of
food required by mammals with their higher metabolism. For spe-
cies like mammals to continue to survive, a wide diversity of plants,
insects, and even single-celled organisms needs to be maintained.
In ecosystems these organisms are dependent upon one another.The Kobal Collection/Hammer
k-selected Reproduction involving the production of
relatively few offspring with high parental investment in each.
r-selected Reproduction involving the production of large num-
bers of offspring with relatively low parental investment in each.
arboreal hypothesis A theory for primate evolution that
proposes that life in the trees was responsible for enhanced vi-
sual acuity and manual dexterity in primates.© Nishan Bingham