Evolution And History

130 CHAPTER 6 | Macroevolution and the Early Primates

Continental Drift

and Geologic Time

As described in Chapter 5, context and dating are vital for
the interpretation of fossils. Because primate evolution ex-
tends so far back in time, paleoanthropologists reconstruct
this evolution in conjunction with information about the
geologic history of the earth.
The geologic time scale is unfamiliar because few peo-
ple deal with hundreds of millions of anything, let alone
years, on a regular basis. To understand this type of scale,
astronomer Carl Sagan correlated the geologic time scale
for the history of the earth to a single calendar year. In this
“cosmic calendar,” the earth itself originates on January 1,
the first organisms appear approximately 9 months later
around September 25, followed by the earliest vertebrates
around December 20, mammals on December 25, pri-
mates on December 29, hominoids at 10:15 am on New
Year’s Eve, bipeds at 9:30 pm, with our species appearing in
the last minutes before midnight. In this chapter, we will
consider human evolutionary history beginning with the
December 25 appearance of the mammals in the Mesozoic
era, roughly 245 million years ago.
Over such vast amounts of time, the earth itself has
changed considerably. During the past 200 million years,
the position of the continents has shifted through a pro-
cess called continental drift, which accounts for the rear-
rangement of adjacent landmasses through the theory of
plate tectonics. According to this theory, the continents,
embedded in platelike segments of the earth, move their
positions as the edges of the underlying plates are cre-
ated or destroyed (Figure 6.3). Plate movements are also
responsible for geologic phenomena such as earthquakes,
volcanic activity, and mountain formation. Continental
drift is important for understanding the distribution of
fossil primate groups whose history we will now explore.

Early Mammals

By 190 million years ago—the end of what geologists call
the Triassic period—true mammals were on the scene.
Mammals from the Triassic, Jurassic (135–190 mya), and
Cretaceous (65–135 mya) periods are largely known from
hundreds of fossils, especially teeth and jaw parts. Because teeth are the hardest, most durable structures, they often
outlast other parts of an animal’s skeleton. Fortunately,
investigators often are able to infer a good deal about the
total animal on the basis of only a few teeth found lying in
the earth.
For example, as described in Chapter 3, unlike the rel-
atively homogeneous teeth of reptiles, mammals possess

PA

NG

AEA

ANTARCTICA

AFRICA

EURASIA

INDIA

AUSTRALIA

SOUTH

AMERICA

NORTH

AMERICA

65 million

years ago

250 million

years ago

Present

EURASIA

AFRICA

SOUTH

AMERICA

NORTH

AMERICA

INDIA

AUSTRALIA

ANTARCTICA

Figure 6.3 Continental drift is illustrated by the position of

the continents during several geologic periods. At the time

of the extinction of the dinosaurs 65 million years ago, the

seas opened up by continental drift, creating isolating barriers

between major landmasses. About 23 million years ago, at the

start of the time period known as the Miocene epoch, African

and Eurasian landmasses reconnected.

continental drift According to the theory of plate tectonics,

the movement of continents embedded in underlying plates on

the earth’s surface in relation to one another over the history of

life on earth.